Electronic manufacturers throughout the world have a long history of using lead-based solders. These solders have proven to be cost effective and high reliable, and are solidly integrated into manufacturing methods and processes. Driven by legislation primarily in the European Union i.e. WEE, RoHS, EEE Directives, manufacturers are concrete the way for removing lead solder from all electronics assemblies. However, the acceptance of lead-free solder posses many draw backs. Conversion to RoHS (Restriction of Hazardous Substance) compliant parts must include the entire product development and manufacturing process. To electronics manufacturers, one of the most important aspects in the process is ensuring the lead-free solder joint reliability is equal to or greater than that of their current leaded solder compositions.
Manufacturers are working independently to validate reliability for their lead-free electronic components and systems. Likewise, industry organizations and other researchers have developed validation programs that attempt to understand failure mechanism associated with lead-free products, generate reliability acceleration models, and predict the reliability of lead-free solder joints. Several models of lead free solder joint reliability not mature enough yet to enable unique requirement of aerospace and military purpose. Several factors affect solder joint reliability, i.e. part geometry, solder impurities, and external environmental stresses. Rapid thermal cycling has been an effective environmental stimulus capable of inducing fatigue on solder joints, particularly with materials having mismatched thermal expansion coefficients. Mechanically induced vibration is another external stress that effectively evaluates solder joint reliability. Thermal cycling can produce high displacement stresses at low cycle frequencies, whereas mechanical vibration can provide low displacement stresses at high frequencies and high cycle counts. A particular type of test equipment and methodology capable of applying a combination of rapid thermal cycling and vibration is Highly Accelerated Life Testing (HALT).
Circuit boards range from simple single moulded plastic boards with copper conductors on one or both sides to multilayer boards with copper conductors, each layer being separated by a dielectric and interconnected by metal conductors. Minimum line width and spacing between lines is less than 100 µm. The board typically is made from a composite such as an epoxy with layered sheets of woven fibreglass. The dielectric material between layers of conductors is usually a polymer, for example polyimide. To maintain solder ability, the exposed copper may be coated with an inhibitor such as benzotriazole or with a solder overcoat. Components are attached to the board with solder or metal-filled conductive adhesives. Fully assembled boards may be further protected against moisture, contamination, and mechanical damage by a cover coat. (Steinberg D,1988)
1.2 SOLDER JOINT RELIABILITY AND FAILURE
Solder joints widely used in the electronic packaging industry to produce good electrical, thermal, and mechanical connections between the package and the printed circuit board. Twenty percentages of the mechanical failure in airborne and automation electronic cause by vibration and shock. Design appropriate measure to ensure the survival equipment in the shock and vibration environment is necessary to do so. Remaining 80 percentage of mechanical failure related to thermal stresses resulting from high thermal gradients, coefficient of thermal expansion and high coefficient of elasticity.
Solder joint failure occurs in several reasons:
Poor design of the solder joint
A bad solder joint treatment
Solder material
Excessive stress applied to solder joints.
In general, however, the solder joint failures are simply ranked according to the nature of stress that have caused. Most joint failure falls into three major categories:
Fatigue failure due to cyclic stress application
Due to the implementation of a long term or permanent load
The stress is due to overloading in the short term
Reflow profile also has a significant role on solder joint reliablity. Because It also has a high influence micro structure of the solder joint.
Vibration failure of solder joints is assessed for reliability using high accelerated life test, which is represented by a GRMS- time curve. For surface mount microelectronic components, an approximation of printed circuit board (PCB) model analysis can be made by assuming PCB as a bare unpopulated thin plate because the increase in stiffness of PCB due to the mounting of the components is approximately offset by the increase in total mass of the populated PCB . However, this estimate can direct to errors in natural frequency calculation for different package profiles, for flip-chip-on-board (FCOB) and plastic-ball-grid-array (PBGA) assemblies. When the component has small profile, the approximation of PCB assembly as a bare PCB can provide acceptable modal analysis results because the stiffness and mass contribution of small component to PCB assembly is not considerable.
In this test, varying GRMS-level random vibration test for PCB assemblies were conducted. In order to calculate the reliability of PCB assembly, it is necessary to conduct the dynamic analysis.
1.3 PROJECT PURPOSE
In this modern world due to the causes of health and environmental issues the electronic manufacturing industries facing a challenging problem of necessity to produce reliable solder products in very high density with very low cost.
Solder joints are very important to the reliability of Printed Circuit Boards (PCB). This is a one of the leading factor in transmission of electrical and thermal connections. In case of every PCB even a smaller solder joints are very important.
So this project investigates the Effect of vibration on solder joint reliability in electronics assembly applications. Solder joint of an electronic assembly is very important measurement because of this model based study might help engineers effectively improve the PCB mechanical design and thus improve reliability of electronics attached to the PCB by considering practical uncertainties and adverse vibration environments. This experiment attempted to explore the use of HALT to evaluate the reliability of solder joints. For this test, samples of PCB's arranged and tested on an Environmental testing chamber. The test plan included a response where, as samples become displaced from the printed circuit board (PCB),. In addition, a solder joint life distribution would be generating describing relative performance between Sn95.5/Ag4.0/Cu0.5 vs. Sn42Bi57Ag1 solder paste. The result of this test did not produce the sequence of failures necessary to plan the life data; however the process of testing revealed close by into the application of HALT as a technique to contrast the performance of PCB soldered components. (Steinberg D,1988)
The overall research methodology that has been followed during the project is illustrated in Figure 1.1. There are six distinct steps: Literature Survey, Project Planning, Design of Experiments, Experimental work, Analysis of result and Conclusion, and Documentation and Presentation.
Literature Survey
Design of Experiments
Project planning
Experimental Work
Analysis of Results and Conclusion
Documentation and Presentation
Figure 1.1: Project Methodology
CHAPTER 2
LITERATURE REVIEW
2.1 SINUSOIDAL VIBRATION TESTING
Dynamic deflections of materials caused by vibration can cause problems and malfunctions including failed electrical components, deformed seals, optical and mechanical misalignment, cracked or broken structures, excessive electrical noise, electrical shorts, chafed wiring. Because sine vibration represent mostly a certain fundamental frequency and the harmonics of that fundamental, in its pure state, this type of vibration is generated by a limited significant number of sources. Expressed as amplitude versus frequency, sine vibration is the type of vibration generated in the field by sources such as engine rotational speeds, propeller and turbine blade passage frequencies, rotor blade passage and launch vehicles.
While much of "real world" vibration is random, sine vibration testing accomplishes several important goals in product ability and testing. Much material and completed product was modelled on some type of sine vibration signature. A sine sweep of frequencies will conclude whether the assumptions were correct and if the deviations are considerable enough to cause design changes. In other words, sweep will establish if the imagined frequency has been met and/or discovers the test item fundamental frequency. Similarly, a sweep will help identify the test subject resonance frequencies, which may be the points at which the item experiences particularly stressful deflections. Some of the following tests include fixed frequency at higher levels of the controlling variable (displacement, velocity, acceleration), and random vibration.
Another typical sinusoidal vibration test, sine burst such as the teardrop, goes rapidly to peak pulse and then decays at lower rate (to prevent damage to the unit). The burst test puts a maximum load into an object at a rapid rate and particularly stresses joints and seams to identify workmanship and design issues.
2.2 RANDOM VIBRATION TESTING
The authenticity of random vibrations an effective tool of screening workmanship defects came about during manufacturing. Up until that limited hertz sine was applied during reliability testing. Pure sinusoidal vibration is composed of a single frequency at any given time. Comparisons tests revealed that to equal the effectiveness of random vibration. The test item will have to be subjected to many sine frequencies over a longer period of time, and may by coincidence fatigue the test item. Random vibrations undercover defect faster.
Another variation would be a swept sine on random test. In random vibration however, all frequencies occurring simultaneously. Because of this, random vibration analysis usually performed over a large range of frequencies, say from 20Hz to 2000Hz. Researchers are not looking at a specific frequency, specific moment in time or specific anything else; They are statistically looking at a structure response to a given random vibration environment. Definitely be aware about any natural frequencies that cause a large random response at any exposed frequency level, but mostly all the experiments aiming the overall response of the structure.
Random vibration analysis looks at random accelerations or forces over a range of frequencies, which we call the frequency domain. (These random inputs are only sustained over a period of time, but are not time-dependent; i.e., the longer the period of time, the better the statistical sampling in the frequency domain.) The range of frequencies is called a spectrum. Therefore, this is called by Spectral Density. Commonly, if accelerations consider in test, for that Acceleration Spectral Density (ASD) have to use.
2.3 SINE ON RANDOM VIBRATION TESTING
Vibration sine on random testing is performing by superimposing a sine wave on top of a random environment. A sine on random vibration test replaces the combined environment of a spinning helicopter blade with its distinct resonant levels and the rest of the aircraft which generates random engine and aerodynamic induced vibration. Gunfire on board an aircraft causes sine vibration while the rest of the aircraft generates random excitations. These types of tests are duplicating vibration Characterized by dominant peaks (sinusoids) superimposed on a broadband background.
2.4 REAL WORLD SIMULATION.
Most vibration in real world is random for example a vehicle travelling over road feels random vibration from the road irregularities. Ground launched rocket vehicles exposed to non stationary vibration during its flight, the motor ignites the rocket travel through the atmosphere, the motor burn ends and so forth even in wing when subjected to turbulent air flow, undergoes random vibration.
Random vibration is composed of multitude of continues spectrum of frequencies. Movement varies randomly with time. It can be accessible in the domain by a power spectral density function [G2/Hz].
2.5 HIGHLY-ACCELERATED LIFE TESTING (HALT)
HALT involve vibration testing in all three axes with a random mode of frequencies. at last, HALT testing can include the real-time cycling of multiple environmental variables, for example, temperature cycling plus vibration testing. This multi-variable testing approach provides a closer estimation of real-world operating environments. Unlike usual testing, the goal of HALT testing is to break the specimen. When the product fails, the weakest point is recognized, so engineers know closely what requirements to be done to develop product quality. After a product has failed, the weak components are upgrade or toughened. The revised specimen is then subjected to an additional round of HALT testing, with the range of temperature, vibration, or shock further increased, so the product fails again. This identify the next weakest point.
By going through several testing like this, the product can be made quite strong. With
This familiar approach, only the weak spots are identified for improvement. This type of testing provide so much information about the assembly and performance of a product, that it can be quite supportive for newer engineers assigned to a product which they are not entirely familiar. HALT testing must be performed during the design period of a product to construct the basic design is reliable. But it is essential to note that the units being tested are likely to be hand-made engineering prototype. At suggestion, we have found that HALT testing should also be performed on real manufacture units, to ensure that the transition from engineering design to production design has not resulted in a loss of product quality or robustness. Some engineers may consider this approach as scientifically reasonable, but economically impractical. though, the cost of HALT testing is much a lesser amount of than the cost of field failures.
2.6 HIGHLY-ACCELERATED STRESS SCREENING (HASS)
HASS testing is an on-going screening test, performed on usual production units. Here, the idea is not to damage the product, but rather to verify that actual manufacture units continue to operate properly when subjected to the cycling of ecological factors used during the HASS test. The limits used in HASS testing are based on a skilled investigation of the HALT testing parameters. The importance of HASS testing can be suitable when one considers today's typical production scenario. Raw materials are purchased from a vendor who uses materials purchased from other vendors. Components and sub-assemblies are obtained from manufacturers all over the world. The finishing assemblage of the product is performed by a subcontractor. This way that the quality of the final product is a role of the quality of all the components, materials, and processes which are a part of that final product. These components, materials, and processes can and do change over time, in that way affecting the worth and reliability of the final product. The best way to ensure that fabrication units continue to meet reliability objectives is through HASS testing.
2.7 RELIABILITY
Reliability is definite as the probability that a device will perform its required function under stated conditions for a exact period of time. predict with some degree of Confidence is very dependent on correctly defining a number of parameters. For instance, choosing the sharing that matches the data is of primary importance. If a correct distribution is not chosen, the results will not be reliable. The assurance, which depends on the sample size, must be enough to make correct decisions. Individual element failure rates must be based on a large enough population and important to truly reflect present day normal usages. There are experimental considerations, such as influential the slope of the failure rate and calculating the activation power, as well as ecological factors, such as temperature, humidity, and vibration. finally, there are electrical stressors such as voltage and current. Reliability engineering can be somewhat conceptual in that it involves much statistics; yet it is engineering in its most sensible form. Will the design perform its proposed mission? Product reliability is seen as a evidence to the toughness of the design as well as the integrity of the class and manufacturing commitments of an societies.
One of the basics of understanding a product's reliability requires an understanding
of the estimate of the failure rate. The conventional method of determining a product's failure rate is through the use of accelerated vibration operating life tests perform on a sample of
Devices. The failure rate obtained on the life test sample is then extrapolated to end-use circumstances by means of prearranged numerical models to give an estimation of the failure rate in the field application. even though there are many other stress methods engaged by electronic assembly manufacturers to fully distinguish a product's reliability, the data generated from operational life test sampling is the major method used by the production for estimating the failure rate of a electronic assembly in field service.
Failure Rate (λ)
Measure of failure per unit of time. The useful life failure rate is based on the exponential life distribution. The failure rate typically decreases slightly over early life, then stabilizes until wear-out which shows an increasing failure rate. This should occur beyond useful life.
Failure In Time (FIT)
Measure of failure rate in 109 device hours; e. g. 1 FIT = 1 failure in 109 device hours.
Total Device Hours (TDH)
The summation of the number of units in operation multiplied by the time of operation.
Mean Time between failures (MTBF)
Reliability is quantified as MTBF (Mean Time Between Failures) for repairable product and MTTF (Mean Time To Failure) for non-repairable product. A correct understanding of MTBF is important. A power supply with an MTBF of 40,000 hours does not mean that the
N = Number of units under test.
If the MTBF is known, one can calculate the failure rate as the inverse of the MTBF. The
formula for (λ) is:
where r is the number of failures.
Once a MTBF is calculated, probability can derive from following equation:
R(t) = e-t/MTBF
Confidence Level or Limit (CL)
Probability level at which population failure rate estimates are derived from sample life test. The upper confidence level interval is used.
Acceleration Factor (AF)
A constant derived from experimental data which relates the times to failure at two different stresses. The AF allows extrapolation of failure rates from accelerated test conditions to use conditions.
Since reliability data can be accumulated from a number of different life tests with several different failure mechanisms, a comprehensive failure rate is desired. The failure rate calculation can be complicated if there are more than one failure mechanisms in a life test, since the failure mechanisms are thermally activated at different rates. Equation 1 accounts for these conditions and includes a statistical factor to obtain the confidence level for the resulting failure rate.
The Bathtub Curve
The life of a population of units can be divided into three distinct periods. Figure 1 shows
the reliability "bathtub curve" which models the cradle to grave instantaneous failure
rates vs. time. If we follow the slope from the start to where it begins to flatten out this
can be considered the first period. The first period is characterized by a decreasing failure
rate. It is what occurs during the early life of a population of units. The weaker units die
off leaving a population that is more rigorous. This first period is also called infant
mortality period. The next period is the flat portion of the graph. It is called the normal
life. Failures occur more in a random sequence during this time. It is difficult to predict
which failure mode will manifest, but the rate of failures is predictable. Notice the
constant slope. The third period begins at the point where the slope begins to increase and
extends to the end of the graph. This is what happens when units become old and begin to
fail at an increasing rate.
Figure 2.1 bath tub curve
Reliability Predictions Methods
A lot of time has been spent on developing procedures for estimating reliability of electronic equipment. There are generally two categories: (1) predictions based on individual failure rates, and (2) demonstrated reliability based on operation of equipment over time. Prediction methods are based on component data from a variety of sources: failure analysis, life test data, and device physics. For some calculations (e.g. military application) MIL-HDBK-217 is used, which is considered to be the standard reliability prediction method.
A simple failure rate calculation based on a single life test would follow equation 1.
Î»ï€ = failure rate.
TDH = Total Device Hours = Number of units x hours under stress.
AF = Acceleration factor,
Since reliability data can be accumulated from a number of different life tests with several different failure mechanisms, a comprehensive failure rate is desired. The failure rate calculation can be complicated if there are more than one failure mechanisms in a life test, since the failure mechanisms are thermally activated at different rates. Equation 1 accounts for these conditions and includes a statistical factor to obtain the confidence level for the resulting failure rate
where,
λ = failure rate in FITs (Number fails in 109 device hours)
β = Number of distinct possible failure mechanisms
k = Number of life tests being combined
xi = Number of failures for a given failure mechanism i = 1, 2,... β
TDHj = Total device hours of test time for life test j, j = 1, 2,... k
AFij = Acceleration factor for appropriate failure mechanism,
i = 1, 2,... k
M = Χ2
(α, 2r +2) / 2
where,
Χ2 = chi square factor for 2r + 2 degrees of freedom
r = total number of failures (Σ xi)
α = risk associated with CL between 0 and 1.
2.2 SOLDER PASTE
Role Of Solder Paste In Reflowing
Solder paste is a combination mixture of a flux composition and a highly grinded, powdered solder metal alloy that is normally used in the electronics industry to soldering processes. And also it is call as a attachment medium between the device interconnection features and the PCB itself. The components of a solder paste are specially designed for excellent printing and reflow characteristics.
In normal case of the surface mount soldering process involves placing the substrate and a small amount of solder paste in a printed circuit board. After that the system will be heated until the solder reflows, forms an electrical connection between the solder pad and the electrical contact of electronics part. After this reflow finished it forms both an electrical and mechanical connection between the electronics components and the printed circuit board.
Selection Criteria Of A Solder Paste
Selection of a solder paste is very important factor for reflowing process, reliability & its quality. The following factors are considerable for a good solder paste [6].
The size of the solder alloy particles which are in the solder paste
The tendency to form voids
The properties of the flux medium of the solder paste
Alpha particle emission rate
The design of the stencil to be used for printing
Thermal properties of the solder paste
Electrical properties of the solder paste
CHAPTER 03
MATERIALS AND EXPERIMENT METHODOLOGY
3.1SOLDER PASTE
The details of solder paste used in the experiment are given in the following table
PCB no
ALLOYS
1 &2
Sn95.5Ag4Cu0.5
3&4
Sn42Bi57Ag1
Table 3.1. types of solder paste used in experiment
For this project all above solder paste should be in a container with appropriate labelling and identification on it to distinguish it from the Tin - lead solder paste. The solder paste should be stored in a refrigerator between 35 - 45F. and should be allowed to come room temperature for minimum four hours before doing the solder paste printing. Once it has finished the using solder paste must replace to the refrigerator since it can not be at room temperature over 24 hours. The self life of the lead free solder pastes may be reducing from the typical six month.
The above guidelines are strictly followed in this project. Because it is not only for guarantee the quality of solder paste but also a good way to reduce the errors that may affect the final results of the project.
3.2 SOLDER PASTE PRINTING
Important of Solder Paste Printing
Surface mount technology (SMT) is used extensively in the electronics industry. Surface mount components are potentially more reliable products can be designed and manufactured using the SMT.
The solder paste stencil printing process is very critical and important step in the surface mount manufacting process. Most of all the soldering defects are due to problems dealing with the screening process. Stencil printing processes have major cautions in operation and set up steps. When we are monitoring these factors carefully we can minimize the defects.
The main purpose of printing solder paste on PCB is to supply solder alloy to solder joint to correct amount. That only print must be aligned correctly and can get a perfect component placement.
Printing Process Parameters
Some of the following parameters are very important to printing process.
STENCIL
Stencils are using for the solder paste slip easily off the aperture edges and thereby secures a uniform print. For this process we using electro formed stencils. Because of these stencils have very shape edge and slightly conic. Generally a stencil is mading from cupper or nickel.
ENVIRONMENTAL
Dust and dirt from the air that will reach the PCBs and stencils can be defects poor wet ability in the reflow soldering process. So PCBs should be stored in sealed packages and cleaned before use.
SOLDER PASTE
Solder paste characterise must be controlled to achieve a maximum production results. Some of the factors are given below [12].
Percent of metal
Viscosity
Slump
Solder balls
Flux activity working life and shelf life
Solder Paste Printing Equipment and Process
Stencil printing parameters are very important factors in printing processes to achieve a best yield. The following parameters must be monitors and controlled in a printing process.
Squeegee pressure = 8kg
Squeegee speed = 20 mm/s
Separation speed = 100%
Printing gap = 0.0 mm
These factors and limit can be adjust for our project purpose
Figure 3.1 DEK 260 stencil printing machine
The DEK 260 stencil printing machine is used to print solder paste on the circuit board. This DEK 260 stencil printing machine has two main functions.
Registers the position of the product screen with in the print head
Positioning the circuit board below the stencil, to ready for the print cycle.
The boards to be print are supported on magnetic tooling and held by vacuum caps arranged on the plate to guarantee the board steady during the printing on to the board. The first step of the experiment is to do the solder paste printing on to the board.
Figure 3.2 Stencil printing by hand
In this project unable to get metal stencil, so circuit boards are printed by hand, below procedure followed to print PCB
Put weights onto the stencil to fix it
roll the squeegee over the stencil
solder paste presses through the aperture onto PCB
separate stencil
Two circuit boards are printed with solder paste for each solder paste types. Totally 4 circuit boards printed.
Pick and Place
Component of PCB placed by pick and place machine (APS Gold-Place L20 ). robotic arms are used to place surface-mount devices onto PCB. Pick and place procedure as follow. pneumatic suction nozzles taken by head of arm, then moved to feeder where the component is loaded. The suction nozzele picks it up using vacuum, and moves to the placement location then centers the component with centering fingers. After justified aligned point, it places the component onto solder paste.
Figure 3.3 APS Gold-Place L20
3.3 SOLDER PASTE REFLOWPROCESS &PROFILE
Figure 3.4 reflow oven
Reflow process is very important to achieve a good reliable solder joint. Novastar 2000 HT convectional reflow oven is used in laboratory for the investigation. When doing the reflow process with lead free solder paste it has to be performed at a minimum peak temperature of 150.It is generally accepted that lead free solders requires a higher reflow temperature up to 230 - 240.reflow oven has six heating zones and cooling zone. (Wu J.2000)
Reflow profile will be affecting the reliability of a solder joint. Because it is a major factor that influence the formation of the intermettallic layers in a solder joint. Intermettalic layer is a critical part of a solder joint. An intermettalic bond thickness should be thin. Therefore a good reflow profile must produce solder bumps with a thin intermetallic layer..( Zhang RR.2001)
PREHEAT ZONE
In this zone indicates how the temperature is changing fast on the printed circuit board. The ramp-up rate is usually between 1-3 per second. If this rate exceeds there will be damage to components from thermal shock. Only In this preheat zone the solder paste begins to evaporate. So if the rise rate is too low the evaporation of flux is not incomplete. This will affect the quality of the solder joint.
THERMAL SOAK ZONE
It is also called the flux activation zone. In this thermal soak zone it will take 60-120 seconds for removal of solder paste and activation of fluxes. Solder spattering and balling will be happen if the temperature is too high or too low. End of this thermal shock zone a thermal equilibrium will complete the entire circuit board.
REFLOW ZONE
In this reflow zone only the maximum temperature will be reached. In this zone we have to consider about the peak temperature that is the maximum allowable temperature of entire process. It is very important to monitor this maximum temperature exceeds the peak temperature in this zone. It may cause damage to the internal dies of SMT components and a block to the growth of intermetalic bonds. we have to consider the profile time also. If time exceeds than the manufactures specification it also affect the circuit board's quality.
COOLING ZONE
In the reflow process the last zone is cooling zone. A proper cooling inhabits excess intermetallic formation or thermal shock to the components. Generally the cooling zone temperature range is 30 - 100.
In this project, the following temperature profiles were selected. This temperature profile is stranded reflow profile for lead free soldering.
Zone 1 220
Zone 2 180
Zone 3 170
Zone 4 190
Zone5 233
Zone 6 233
Totally 4 circuit boards were printed. Choosing of good reflow profile was not involves any defects or damages in the printed circuit board.
Figure 3.5 Reflow profile
Figure 3.6 printed circuit board after reflow
SET UP EVENT DETECTOR
Figure 3.7 Event detector
The constructed PCB's were connected with event detector by ribbon data cable. Ribbon cable addressed according to `Analysis tech STD series event detectors manual'. pins 1 to 32 function as source terminal and pins 33 to 37 function as ground terminal.
Figure 3.8 Ribbon cable pin address
To obtain closed loop circuit to monitor the behaviour of PCB components, PCB boards 1, 2, 3 and 4 connected to channel 1,2,33 and 34 respectively.
Ribbon cable
After connected ribbon cable with event detector and environment chamber, channels are assigned in "WIN DATA LOG" software which supplied with event detector.
For this test following settings define for data acquisition.
Figure 3.9 set up test data in event detector
3.5 INVESTICATING RELIABILITY OF SOLDER JOINT UNDER VIBRATION CHAMER
Figure 3.10 Design Environmental FS800-70SV
In this study, PCB's were used in Variable Frequency Vibration Test to analyse the dynamic response of PCB assembly subjected to random vibration loading. The PCB specimens were tested at different acceleration levels to assess the solder joint reliability subjected varying G-level vibration loads (G is the gravitational acceleration), respectively. Vibration tests were accomplished by using an electro dynamic Shaker (Design Environmental FS800-70SV) and event detector. One accelerometer was used to determine the dynamic response of the specimen, Daisy chain loops were monitored simultaneously by an Event Detector during vibration test. Any resistance change exceeding a preset threshold with minimum duration of 200ns can be detected by the Event Detector. When a crack is initiated in the solder joint during the vibration test, the resistance will increase. The failure criteria recommended by IPC - 785 standard is defined as daisy chain resistance by thermal or mechanical transients or disturbances in the form of short duration (~ 1 µsec) high resistance spikes (>300 Ω). During thermal changes the solder joints are subject to shear, not tensile, loading; therefore, fracture surfaces of fractured solder joints slide relative to each other producing the characteristic short duration intermittent. Therefore, in this context, the practical definition of solder joint failure is the interruption of electrical continuity (>300 ohms) for periods greater than 1 microsecond.
Figure 3.11 PCB's was mounted inside the Environment chamber in this manner.
Figure 3.12 Above table illustrating test plan.
Figure 3.13 Test plan
Figure 3.14 graphs obtained from chamber
CHAPTER 4
RESULTS
PCB board
Failure rate(%)
Operation time(hours)
Reliability
1
24.0
5
0
2
63.0
13.04
0
3
1.6
0.3305
0.5893
4
2.5
0.5083
0.2806
Table 4.1 reliability of solder paste
The testing began with 1 GRMS. The purpose of this to detects any gross issues in solder joints, as well as with the data acquisition system. No failures were detected at beginning, however, after 1.5GRMS an inspection revealed that both PCBs 3 and 4 were failed. Next, vibration levels were stepped gradually to 6GRMS increments over the period of five hours PCB 1 got failed, In 13.04 hour's period the chamber reached level of approximately 14GRMS, remaining PCB also got failed. However, the failure mechanism was attributed to a vibration acting on the solder connection specific to these parts and was directly related to thermal faults.
Failure rate
Time elapsed
Above graph illustrating failure rate vs. Time elapsed for Sn42Bi57Ag1
Failure rate
Time elapsed
Above graph illustrating failure rate vs. Time elapsed for Sn95.5Ag4Cu0.5
CHAPTER 5
RESULT ANALYSIS
From reliability analysis sample 3 has maximum reliability and sample 2 has minimum reliability. Each sample was subjected to nominal supply current and resistance value in accordance with the design and performance specification. Respective test samples continued to vibrate with some performance irregularities observed. The objective of this experiment was to use HALT vibration to establish the relationship between the life of similar parts soldered with different solder compositions.
The results proved inconclusive for the primary purpose of "Sn95.5Ag4Cu0.5" versus "Sn42Bi57Ag1" joint random vibration test. Both compositions demonstrated weak performance and the results may suggest that with respect to this abbreviated random vibration test, solder joint reliability is different between the two compositions. No samples were separated from the test vehicle as a result of the vibration applied.
However, the test process revealed insight into HALT and suggested factors which can affect results when soldered components are being compared.
First, Printed circuit board dynamics would be understood and controlled so that acceleration hot spots and nulls are made more equal in amplitude. A flexible PCB will deflect and produce bending stresses suitable for evaluating stress on solder joints. But with a flexible board acceleration uniformity become difficult to control and make a comparison test less objective. Second, HALT random vibration is a good methodology for generating multi-axis broad spectrum energy for PCBs and their components. (Wu J.2000)
The distinctive method to examine the functionality of a PCB in a vibration environment is to verify the vibration transmissibility. Normally the transmissibility at any location of PCB should be less than a factor of 10, i.e., 1 G input will not produce more than 10 G output at any location of the PCB. To achieve this goal, two methods can be applied to the PCB design. One is to constrain the PCB tightly to the fixture, which can be realized by improving the boundary conditions in the finite element analysis model. The other is to relocate heavy attached components of the PCB board to appropriate optimal locations (i.e., mass re-distribution). (Wu J.2000)
Lead free solder assemblies have rapidly been promote in the micro-packaging development to prevent the environmental pollution, issues on the reliability has not been clearly solved. Several experiments have been conducted on the soundness of alternative lead free soldering technique such as Sn-Ag-Cu solder. During the bumping process, a considerable thermal deformation is occurred and the chemical characteristics of solder and layer qualities are also changed in accord with the surface finishing method such as ET (electric tin), ENEPIG (electro less nickel, electro less palladium and immersion gold) etc. The IMC (intermetallic compound) layer is formed in a micro-scale region between the solder and the surface finishing material under high temperature bonding process. In the field of micro-packaging together with IMC layer, one of the most difficult issues is to measure width interface layers in a solder joints. Since the IMC layers have been estimated as the weakest section in the solder joints, a accurate determination of mechanical properties in the area of this critical region may provide direct and enhanced information to understand the fracture behaviour. • .( Zhang RR.2001)
tightness of mounting bolt of PCB reduced during experiment. Due to this effect the PCB stated to vibrate freely. Applied vibration force was not shared uniformly. From finite element analysis changes in tightness of mountin bolt can be realize.
CHAPTER 6
CONCLUSION
The conclusions resulting from this work are:
.
Interactions between temperature and vibration were found significant and thus had influence on final result.
Stencil printing by hand affect the solder joint reliability.
Better understanding gained about electronic assembly behaviours when subjected to vibration.
Gained knowledge about electronic assembly process and how it is influencing in solder joint reliability.
Gained good experience in handling event detector and environmental testing chamber.
CHAPTER 7
FUTURE WORKS
In this project investigated the effect of vibration of solder joint reliability in electronic assembly. For this investigation two different types solder paste used, finally all specimens were tested in environmental Chamber.
In this project when doing vibration test data acquired by event detector is not accurate value. From data obtained from event detector does not give proper analytical result.
In considering about the future works of this project can be expand based on acquirable data type , as well as finite element analysis because reliability of a solder joint depends on various environmental factors, so result analysis will be more accurate and stand with more international standards.
