Increasing global concern about the environment is bringing regulatory laws and consumer pressure on the electronics industry in Europe and in rest of the world to reduce or completely eliminate the use of lead (Pb) in products. On July 2006, Europe forbids the use of lead in electronics. The transition from a technology using SnPb for electronic interconnections which has more than 50 years of experience to a new lead-free technology stood like a challenging and demanding task for the companies. Components have to withstand higher soldering temperatures (typically 20-35°C higher) and the solder joints should have at least the same life time (expressed in number of thermal cycles). The report presented deals with the previous work on lead free solders and its reliability and failure analysis. The work carried out is classified in to different sections. [1]
The purpose of solder joint is to provide both electrical and mechanical bonding between the components and the substrate or the board. The failures mainly occur due to fatigue crack at the solder-package interface under thermal loading. The growth of the formed crack eventually causes the failure of the solder joint. This prediction of failure of solder joint remains still as an important task for every electronics manufacturing company. In solder joint reliability and failure, the bump/pad has a crucial role which can be determined by metallization and interfacial defects. The reasons for failure of solder joints are due to different types of loadings they are subjected to. The different types of loading can be classified as following:
Thermal cycling due to repeated power switching evokes heat related phenomena. In connection, the mismatch in the coefficient of thermal expansion (CTE) between the package components causes cyclic mechanical strains.
As a result of the multi-phase nature of Sn based solder alloys and the thermal anisotropy of β-Sn, internal stresses build up in the solder.
Cyclic thermo mechanical loading evokes creep-fatigue damage or creep rupture.
Flex bending and buckling of the board induces bending moments as well as shear and tensile stresses on the solder joints.
The cracks formed at the solder joint are due to the over loading which may be caused due to applying much force during mounting of solder assembly or mishandling of the components. Solder joint cracks are also caused by creep is due to high temperatures (greater than room temperature) and permanent loading. The cracks caused by fatigue or cyclic loading are mainly due to cyclic failure of joint resulted from stresses implied by temperature and the difference in the coefficient of thermal expansion ( strain in the joint exceeds the plastic limit of the crack). In order to conduct failure analysis, one has to have sound knowledge in electronics manufacturing techniques and different types of lead free solders which are in use and the handling of different equipments like reflow Owen, environmental chamber, SEM (scanning electronics microscope) etc.[2]
A. PROJECT AIMS AND OBJECTIVES
The aim of this project is to carry out failure analysis of lead-free surface mount solder joint by cross sectional study and Fracture surface analysis using SEM (Scanning electronic microscope). By subjecting the solder joint to extreme environments we study the surface cracks using fractographic features and the purpose of doing failure analysis is to determine why the surface fractured. This project would address the reliability and failure issues of electronic components in extreme temperatures. Thermal cycling experiments will be carried out to determine causes of failure and study of thermal, mechanical, creep, fatigue properties.
The objectives / activities that should be carried out include:
Develop test specimens.
To carry out stencil printing and reflow soldering of test boards.
To perform accelerated thermal cycling (ATC).
Analyse failures, determine root causes mad characterize solder joints.
Analyze failure data.
2. ELECTRONICS MANUFACTURING
The history of electronics technology is a story of twists and turns, and peaks and valleys. From the earliest days, electronics has been a technology in a constant state of change and evolution; and from the earliest days electronic engineers and technicians have found it necessary to adjust their thinking and practices accordingly. For the most part, these changes have been the result of innovations based on scientific skills and technical creativity.
The present methods of manufacturing conventional electronic assemblies have essentially reached out their limits as far as cost, weight, volume and reliability. This forced manufacturing companies to adapt for new technologies which can replace the old technologies with added benefits. That change occurred in the form of companies changing from using Through Hole Technology (THT) to Surface Mount Technology (SMT). Trough hole technology is method of inserting electronic components on to the PCB's by drilling holes on the board at required point and soldering them later. THT is still in use especially for heavy components like semiconductors. THT method gives higher mechanical strength when compared to the SMT but it is not suitable for large scale production as it is a time consuming technique. Surface mount technology makes it possible to produce more reliable assemblies at reduced weight, volume and cost. Surface mount technology is a major innovation that is just now completing the historical stage of acceptance and economic shakeout. The cost of implementing this technology is currently dropping and demand is rising very rapidly. [3]
A. SURFACE MOUNT TECHNOLOGY:
Surface Mount Technology (SMT) is used to mount electronic components on the surface of PCB or substrate. SMT concept isn't new. Surface mounting has its roots in relatively old technologies such as flat packs and hybrids. Electronics assembly originally used point-to-point wiring and no substrate at all. In 1950's surface mount devices called flat packs were used in high reliability military applications. They can be considered as the first surface mount packages to be mounted on PCB's. In 1960's flat packs were replaced by DIP's (dual in line packaging), which are easier to insert and can be wave soldered.
Even though there are many types of equipment required for SMT, the heart of surface mounting is the machine that places the components on to the printed board land prior to soldering- pick and place equipment. SMT is rich in technology and market driven because it satisfies the customers desire in producing faster, smaller electronic circuit boards with less cost. There are three types of SMT boards used:
SMT board with parts on one side or both sides.
Most commonly used type.
Similar to type ΙΙ but also use passive chip surface mount components on secondary side.[4]
3. RELIABILITY:
Reliability is the ability of any system or component to perform to its potential. It gives the assurance by which one can rely on the component. Reliability in manufacturing deals particularly with the ability with which a component can perform under specified conditions for specified period of time. Electronics industry has a very short growth time, everything that is newly introduced is checked for its reliability and is accepted or rejected based on the results it is produced when compared to the older technology that was in use. Every manufacturing company would focus on reliability as an important factor for its success. [5]
A. SOLDER JOINT RELIABILTY
The reliability of solder joint attachments of electronic components surface mounted to circuit board substrate requires explicit attention in design phase as well as in manufacturing. Surface mount solder joints in use can be subjected to many loading conditions and that can lead to permanent failure in inadequate design. Solder joint reliability is the ability of solder joints to remain in conformance to their specifications (electrical and mechanical) over a given period of time under specified operating conditions. Solder joint reliability has two aspects: 1) component level solder joint reliability and 2) Board level solder joint reliability. Component level typically deals with reliability of unmounted parts condition. Board level deals with the reliability of a solder joint after it is mounted on to a board. Board level reliability is the more relevant representative of reliability of a package operating in a field. The traditional way to analyze solder joint reliability is performing solder joint modelling. This is usually done through finite element analysis. The use of modelling at the design stage is more beneficial and is known as Design for Reliability (DFR). Aside from modelling solder joints reliability can also be accessed through reliability testing. Reliability testing includes subjecting selected test vehicles to extreme environments like Accelerated Thermal Cycling (ATC), dye penetration etc to study the failure behaviour of the solder joints. Thermal cycling will be discussed in detail in later sections. By doing reliability testing we can identify the root causes for the failures. If the causes are of the design stage, then the design of the component is optimized by following reliability standards. [6][7]
B. Test vehicles:
The test vehicles which are to be tested are designed such that they are similar to each other to avoid the reliability issues. The substrate, ad size and track thickness should be appropriate to meet the test specifications along with operating temperature, the thermal cycling conditions. There are various factors that can affect the quality of solder joint while reliability testing is done. Solder paste and base material has a long term influence over the reliability where as component used plays a significant role in short term effects which brings short period failures. [8] The design of test vehicle depends upon the type of solder joints. The test vehicles are processed as shown in the flow diagram.
Apply solder paste on component pads
Place components on board
Solder reflow
Wash boards (Electrovert, Aquastorm)
Flow chart for test vehicle assembly process [8]
C. Solder paste:
Solder paste is a specially blended paste that consists of flux medium containing graded powder solder particles. The process of depositing solder paste on to the PCB is known as solder paste printing. It is the first step of test vehicle assembly process. The primary use of solder paste is it acts like an attachment medium between the components and the PCB board. After application of solder paste the components are placed on it and reflowed at high temperature to effect the soldering of devices to the PCB. After this it is allowed to cool down to solidify and attain the necessary mechanical properties to withhold the components on to the PCB. Printing solder paste on to the PCB is known by a technique called stencil printing. [9]
4. Stencil printing
A. Methodology
[10]Stencil printing is used for printing solder paste on to the PCB by using squeegee. Squeegees are generally used to physically deposit and distribute the solder paste evenly across the stencil. By properly rolling the squeegee over the stencil, the solder paste passes through the stencil apertures and gets deposited on designated areas on the PCB. The stencil is then lifted, leaving behind the intended solder paste pattern on the PCB. Squeegees are generally designed to have a very smooth and non-sticking surface with a sharp printing edge. During solder paste printing, the PCB must be held by its support in a locked position that's perfectly parallel to the stencil. The squeegee angle is usually between 45-60 degrees.
B. Reflow soldering:
Solder reflow is accomplished by equipment known as solder reflow oven. Reflow oven employs techniques to expose the board assembly to necessary temperature profile. The reflow soldering is done to ensure good bonding between the components placed on to the PCB and PCB itself. A typical reflow temperature profile consists of following stages:
Preheat
Flux activation
Actual reflow
Cool down
The reflow temperature required by lead free solder assemblies are higher than those required by non lead free solder assemblies, mainly because lead free solder alloys have higher melting point than Pb-Sn solder. [11]
5. THERMAL CYCLING
[12]Thermal cycling experiments are used to estimate the life span of products by subjecting them to thermal ageing. For any product to check for its reliability it has to be tested for its life time period which is not sensible cause it's a time taking process. Every product in order to get success it has to pass the reliability tests which are the bench mark for every manufacturing company. In order to conduct tests on new products thermal cycling is used as a catalyst to speed up the thermal ageing. Thermal cycling experiments speeds up the operating life time of products under dynamic environmental conditions. These experiments were carried out in an environmental chamber. The environmental chambers have the capacity to produce extreme temperatures. The duration depends up on the application and may vary from several hours to days.
6. PREVIOUS RESEARCH ON LEAD FREE SOLDERS
A. Lead free solders:
Tin-lead solder is the most commonly used solder for electronic assembly. However, there are concerns about the use of lead due to its adverse effects on human health. Lead is linked to health hazards such as disorders of nervous and reproductive systems and delayed neurological and physical development. There are laws that control the use of lead. Electronics solder accounts for about 0.5 % of all lead usage. So, electronics industry is looking into lead free solders that can replace the universally accepted and widely used tin-lead solder. Research and development efforts are focused on study of potential alloys that provide physical, mechanical, thermal and electrical properties similar to those of tin-lead eutectic solder. The main factor that should be looked in is the relative cost of the metals that can replace lead; the other is the supply and demand of elements. [13]
B. Challenges:
[14]The main reason for the success of SnPb is that it is a simple material, with a clear failure mode (cycling creep deformation, resulting in mechanical fatigue). Several issues are showing up which are typically for lead-free solder joints and makes simulation and also reliability testing much more complicated
How does lead-free solder materials behave at low temperature? It is known that Ag makes the solder materials more brittle (higher brittleness transition temperature). Even when creep will be lower at the higher temperatures for lead free solder materials, the lower temperatures could be the crack initiator. Dag Andersson et al. [1] found that cracks start to grow even in the first cycles, but only in the case where temperature went down to -55°C.
Formation of new intermetallic systems, which may result in early brittles fractures. A nice example is a SnPb solder joint on a NiAu finish of PCB. The samples failed at 1/3rd of the intrinsic fatigue life of the joint itself. Can we expect similar problems with the lead free solder materials? Or in general, can we have unexpected failures (there is no long term reliability data available for lead free solder materials).
Data for all type of lead-free solder materials in all kind of compositions. SnPb will be not replaced by a single lead-free solder material. Both material data (E modulus, CTE, creep behaviour) and correlation models (e.g. relation between creep strain and life time) must be measured for all these new materials.
C. PROPERTIES:
Among thermal, mechanical, creep, fatigue and other properties, melting point is one of the most important. The compositions of lead free solders are still being optimized to achieve the desired properties. Lead free solders have either much lower melting points or much higher melting points. Special flux is necessary when low temperature solders are used, because standard flux may not be active at lower temperatures. Another problem with the low temperature solders is the reduction in wetting properties caused by the lower fluidity. The fatigue properties of lead free solders are also not good as tin-lead. Ideally the melting point of selected solder should be around 1800C. None of the lead free solders is considered a drop in replacement; however industry is still looking for right lead free solder that can truly be substitute for tin-lead eutectic. It is a challenge.
Some industries have already eliminated lead and have found some suitable alternatives. The below presented are the some alternate lead free solders that are discussed in [15] as a potential replacement to tin lead solder.
Sn/Ag (96.5SN/3.5Ag:2210C)
This alloy exhibits adequate wetting behaviour and strength and is used in electronics as well as plumbing. It has good thermal fatigue properties when compared to Sn/Pb. Thermal fatigue damage in solders is accelerated at elevated temperatures. In the Sn/Pb system, the relatively high solid solubilities of Pb in Sn and vice versa, especially at elevated temperatures, lead to microstructural instability due to coarsening mechanisms. These regions are termed as crack initiation sites. It is well-documented that these types of microstructures in Sn/Pb alloys fail by the formation of coarsened band in which fatigue crack grows. By comparison, the Sn/Ag system, has limited solid solubility of Ag in Sn, making it more resistant to coarsening. As a result Sn/3.5 Ag forms a more stable, uniform microstructure that is more reliable.
Although the Sn/3.5Ag alloy itself exhibits good microstructural stability, when soldered to copper base metal, the combination of a higher Sn content (96.5Sn compared to 63Sn) and higher reflow temperature there is a problem of formation of Cu6Sn5 intermetallic compound. To decrease the growth kinetics, alternative surface finishes such as immersion gold (Au over Ni over Cu) may be used. The 2µmNi in the immersion gold coating serves as an effective barrier, limiting the formation of intermetallic compound. Other surface finishes such as immersion in silver (Ag over Cu) and immersion palladium (Pd over Cu) are under investigation.
Sn/Ag/Cu
95.5Sn/4.0Ag/0.5Cu 217-219°C
95.5Sn/3.8Ag/0.7Cu 217-219°C
95.0Sn/4.0Ag/1.0Cu 217-219°C
93.6Sn/4.7Ag/1.7Cu 216-218°C
Because the mechanical stability of the joint is degraded when the melting point is approached, elevated temperature cycling produces more damage for Sn/Pb solder (m.p. 183°C) as compared to higher melting point solders. The melting temperatures of SAC solders make them ideal for high operating temperatures up to 1750C. Regarding wetting properties SAC solders do not wet Cu as well as Sn/Pb using commercial fluxes. The copper dissolution test provides a relative measurement of the solder's tendency to dissolve Cu from the base metal and from the Cu6Sn5 intermetallic compound. For alloys 1-3, the rate of copper dissolution is slower than the Sn/Ag alloy yet faster than the Sn/Pb eutectic. For alloy 4, the high level of Cu in the alloy prevented the dissolution of the copper wire. Industries are now trying to reduce silver to make it cost effective and another problem it has is the less shelf time when compared to tin-lead eutectic.
Sn/Cu (99.3Sn/0.7Cu:227 °C)
This alloy might be also suitable for high temperature applications required by the automotive industry. It is a candidate especially for companies looking for lead and silver free alloys. Preliminary testing conducted on this alloy has shown a significant improvement in creep/fatigue data over standard Sn-Pb alloys.
Sn/AG/Cu/Sb (96.2Sn/2.5Ag/0.8Cu/0.5Sb(known as castin):217-2200C)
This alloy has similar mechanical properties to the SAC alloy.
Sn/Ag/Bi (91.8Sn/3.4Ag/4.8Bi:200-216°C)
In general, bismuth is added to Sn/Ag/X solder alloys in order to depress the melting point. Another benefit of Bi addition is greater joint strength as indicated by ring and plug testing.
This particular alloy was developed by Sandia National Labs. They found no electrical failures on surface mount devices following 10,000 thermal cycles using 68 I/O PLCCs, 24 I/O SOICs, and 1206 chip capacitors on standard FR-4 PCBs. No cracks or deformation were observed on boards cross-sectioned after 5000 thermal cycles. Cross-sectional data on 10,000 cycles is being collected. These results are in good agreement with data collected by the National Centre for Manufacturing Sciences (NCMS) Lead Free Solder Project, which reported very good thermal fatigue resistance on OSP printed circuit boards (Organic Solderability Preservative that protects copper pads and through-holes). This solder is now examined under temperatures up to 160 and 1750C. In combination with Pb from the PCB or component metallisation, a Sn/Bi/Pb ternary compound is formed with a melting point of only 960C. As the trend towards eliminating the lead continues, this alloy may become more attractive.
Sn/B (42Sn/58Bi: 1380C)
The low melting point of this alloy makes it suitable for soldering temperature-sensitive components and substrates. If these contain Pb, the SnPbBi ternary eutectic compound may form at 96°C, which in turn adversely affects the thermal fatigue properties. The NCMS Lead Free Solder Project recently reported the results of thermal cycle testing at 0/ 100°C and -55/125°C for over 5000 cycles on OSP boards. The result shows that Sn/Bi outperformed the Sn/Pb at both temperature excursions. The problem lies as the closeness of 1250C to the binary Sn/Bi eutectic at 1380C would cause this alloy to be a poor performer.
Sn/Sb (95Sn/5Sb:232-240°C)
The 95Sn-5Sb solder is a solid solution of antimony in a tin matrix. The relatively high melting point of this alloy makes it suitable for high temperature applications. The antimony imparts strength and hardness. The high strength of this alloy causes the lowest energy crack path to be at the solder/intermetallic interface in the case of thinner intermetallics. As the intermetallic thickens, the crack path is through the intermetallic layer. Formation of the intermetallic compound SbSn is possible at these levels of Sb. This phase has a cubic structure with a high hardness. The wetting behaviour was measured on a wetting balance in air using a standard RMA flux. The wetting force at 2 seconds for 95Sn/5Sb on a Cu wire is significantly less than Sn/37Pb and Sn/3.5Ag. In addition to marginal wetting performance, the toxicity of Sb has also raised concerns. As with bismuth, antimony is also a by-product in the production of lead.
In/Sn (52In/48Sn:118°C)
The melting point of this alloy makes it suitable to low temperature applications. With regard to indium, it displays good oxidation resistance, but is susceptible to corrosion in a humid environment. It is also a very soft metal and has a tendency to cold weld. In addition, the 52In/48Sn alloy displays rather poor high temperature fatigue behaviour, due to its low melting point. The high indium content limits the widespread use of this alloy due to cost and availability constraints.
Sn/Zn (91Sn/9Zn:199°C)
The presence of zinc in solder alloys leads to oxidation and corrosion. Samples of bulk alloys that were steam aged for 8 hours exhibited severe corrosion as evidenced by a purplish colour. In powder form, it reacts rapidly with acids and alkalis and forms a gas. Thus, its compatibility with fluxes and its storage stability is questionable. The reflowed solder joints do not wet as well as other lead-free alloys. When wave soldered, this alloy tends to produce excessive dross. Therefore, manufacturability of this alloy and zinc-bearing alloys in general is a concern.
Au/Sn (80Sn/20Au:280°C)
Au/Sn eutectic solder is a very strong, rigid solder due to the formation of brittle intermetallic compounds. Problems of cracked dies have been seen using Au/Sn eutectic solders in die attach applications. It is not known if the cracks occur from processing or during thermal cycling. The high cost of this alloy restricts its use in many applications where cost is a factor.
Research work that was carried out with candidate lead free alloys indicate a significant improvement in reliability over Sn/Pb. Results from creep tests show that all lead free candidates showing superior creep resistance over Sn/Pb, including Sn/Ag eutectic, Sn/Cu eutectic, and Sn/4Ag/0.5Cu at both room temperature and 100°C. Although the Sn/Cu eutectic outperformed Sn/40Pb, it did not perform as well as the Sn-Ag-X alloys. An aging study of both the 95.5Sn/4Ag/0.5Cu and 96.5Sn/3.5Ag solder alloys was performed in order to evaluate the growth kinetics of the intermetallic layers following extended heat treatment. An understanding of the microstructural evolution that occurs at the solder/copper interface at elevated temperatures is helpful to understand the failure mechanisms that dominate at elevated temperatures. Creep occurs when materials under constant stress, below the tensile stress, slowly deform and finally fracture. The creep rate is dependent on alloy composition and microstructure and is strongly temperature dependent. Because Sn/Ag and Sn/Ag/Cu have similar microstructures, they behave similarly during isothermal aging and creep testing. [15]
7. RELIABILITY AND FAILURE ANALYSIS
[16]This paper provides a comparison of the thermal cyclic reliability and associated failure modes of second level interconnects in lead free, 1.27mm pitch, 256 I/O BGA devices with eutectic tin lead assemblies. The three lead free alloys taken into consideration are: 96.5Sn/3.5Ag, 95.5Sn/3.8Ag/0.7Cu and 95.2Sn/2.5Ag/0.8Cu/0.5Sb with comparisons made to eutectic 63Sn/37Pb solder. In order to evaluate the alloys, identical component carrier substrates for a generic BGA device were acquired. The substrates were bumped in house by printing flux and pre-formed 0.762mm diameter solder spheres. The bumped packages then are separated in to two assemblies TG1 and TG2. The components were assembled to Electroless nickel immersion gold (ENIG) and copper Organic surface protection (OSP) Printed Circuit Boards. The boards are then subjected to a 0/1000C thermal cycle unit failure. Life time and failure analysis are carried out for each test group and compared.
Table 1: Thermal cycles used [16]
Test group
Dwell time (minutes)
Ramp time (minutes)
Ramp rate (0C/min)
TG1
5
5
20
TG2
5
10
10
The data generated from TG1 and TG2 suggests that all the three lead free solders show greater characteristic life time when compared to eutectic Sn/Pb in 0/1000C temperature cycling. The Sn/Ag alloy showed highest Eta, which was twice as that of Sn/Pb. Early failure was due to interfacial failures between component nickel plating and solder alloy which was observed in all four alloys. Three types of fatigue mechanisms were constantly observed in lead free systems:
Typical component side fatigue.
Multiple crack path solder fatigue and
Vertical cracks.
Test criteria used in this research for event detection was a modified IPC SM-785 requirement for resistant levels. A 300 ohm resistance level and 200 nano second time duration was utilized.
Fig: Failed Sn/Ag/Cu Solder Joint after 7598 cycles(20 min. Cycles)
Fig: Failed Sn/Pb Solder Joint after 6801 cycles (20min. Cycles)
Fig: Sn/Ag/Cu/Sb Solder Joint Vertical crack after 16700 Cycles (20 min. Cycles)
Fig: Failed Sn/Ag Solder Joint on Cu Pad after 10204 Cycles (20 min. Cycles)
FIG [16]
Intermetallic plate formation can improve the reliability of lead free solder joints. The observations showed Sn-Ag plates in all the three lead free alloys and show the evidence of redirecting crack propagation. Dye penetration analysis indicates that predicting reliability using DNP calculation may be invalid for lead free solders. The randomness associated with intermetallic and grain boundary formation has significant effect on reliability of individual joints. The presence of voids may also reduce the reliability.
[17] This journal describes about the research work which was carried out by team named National Electronic Manufacturing Initiative (NEMI). This team performed research on lead free solder joints and their reliability. The team tested various lead free alternatives and suggested Sn/3.9Ag/0.6Cu alloy as a replacement for tin-lead solder. As a part of this study, the NEMI reliability team tested selected solders, components and board finishes using a series of test vehicles to compare the reliability of lead free PCBA's with the standard Tin-lead PCBA's.
Test vehicles used: six different boards were used. One for each component type was used. The boards were eight layered, 0.062-in FR4 with a glass transition temperature (Tg) of approximately 1700C.
Thermal cycling: Accelerated thermal cycle (ATC) testing was performed at six facilities. The temperatures are taken from JEDEC standard JESD22-A104B temperature cycling. The two temperature conditions taken are
-400C (+0, -5) to 1250C (+5, -0).
00C (+0, -5) to 1000C (+5, -0).
Analysis: After ATC, both the failed and survived parts were characterized using several types of analysis:
Visual inspection. (10x to 30x)
Dye penetration.
Cross sectioning to identify failure modes.
Scanning electronic microscope (SEM).
Table: Weibull analysis results [17]
Table: Relative ATC performance [17]
Initial findings from the test vehicles, for all the three material combinations in both ATC conditions, indicated that no pronounced differences in joint geometry occurred. The thermal cycling caused microstructural changes, and failure always occurred first in the solder on the component side. The failure mechanism is observed similar between lead free and tin-lead (bulk solder fatigue). The results show that lead free solders performed equivalent or better than tin-lead benchmark. The team also performed three point bend testing which showed no difference between different combinations.
Although the team did enough research, much work still remains to be done with process optimization, such as flux formulations and reflow conditions. Their success led to follow-on to another project, Advanced Lead-Free Hybrid Assembly and Rework Development.
[18] This research describes about the work carried out on reliability of solder joints assembled with lead free solder. The paper includes the study of dynamic mechanical properties of tin-silver-copper and tin-lead solders by subjecting solder samples to twisting cycles. The dynamic mechanical testing is done by procedure shown below. The solder sample was inserted between electric model and an axial stress transducer that was fixed with respect to the motor casting. The power is applied to the motor to subject the solder sample to shear stress and thus the results are noted down in the form of transducer reading. During this solder undergoes both elastic and plastic deformation under stress and the behaviour can be best expressed as dynamic viscoelasticity and the shear modulus (G). The measurements were preformed in a thermal chamber operating under -650c to 1250C.
Fig: Measurement in thermal chamber [18]
The results obtained stated that compared to current tin-lead solder tin-silver-copper solder is harder to deform and has more fatigue life. The combination of lead solder and lead free solders in BGA gives satisfactory results in fatigue estimation.
[19] This paper has investigated the suitability of a number of these techniques to study cracking in lead free solder joints, and hence their use in assessing joint life time. The techniques studied included microsectioning, dye penetration, mechanical tests and thermal conductivity. Test vehicle used is FR4 laminate with 2512-type resistors used in crack assessment study. The lead free solder alloys that were used are 95.5Sn/3.8Ag/0.7Cu and 96.5Sn/3.5Ag. Thermal cycling was conducted with regimes shown below:
Table: Thermal cycle regimes used [19]
This regime is widely used in military application testing, ramp rate is moderate. The relatively low dwell time of 5 minutes is to minimize the cycle period and shorten the test period. Microsectioning is carried out by obtaining images using an optical microscope and cracks were obtained by Scanning Electronic Microscope (SEM) in secondary electron mode. Dye penetration is another destructive technique used for studying the location and extent of cracking. It has the advantages in quantitative analysis of solder joint cracks, and can be used to analyze the whole area of a crack. Shear testing is an established method for evaluating not only the degree of crack propagation and damage to solder joint, but also the strength of the joint. Mechanical tests like pull test, 3-point and 4 point bend tests turned out to be highly technique dependent (based on operators skill) and is hence regarded as immature technique. The techniques which are under development are not used like real time X-ray, inductive ultrasonic imaging which might be value in the future.
[20] This paper deals with the fatigue life estimation of surface mount solder joints. The paper discussed an approach to measuring the fatigue properties of surface mount solder joints.
Fig: experimental procedure [20]
The figure above best describes the procedure that was carried out in the paper with (a) before application of stress, (b0 at maximum load and (c) location of strain gauge for measuring total displacement. The results obtained showed that the technique employed to measure the stress-strain properties of Surface mount solder joint is really helpful. The stiffness index is good source to predict the fatigue status of solder joints. The measurement of total displacement can be recommended as a practical technique to estimate the fatigue life during strain cycling tests for both leaded and lead free solder joints.
8. PROJECT JUSTIFICATION
A. WORK TO BE CARRIED OUT
Due to regulatory rules every electronics manufacturing company is looking forward to a lead free alloy which can be taken as a stand out replacement for current Tin-lead solder. The first note every company looks for, is the reliability of using these alternate alloys in solder joints. According to the current industrial trend SAC (Sn-Ag-Cu) is considered as the best replacement for the tin-lead solder eutectic. Previous researches concentrated mainly on the reliability analysis of leaded and lead free solder joints which also suggested SAC as the best replacement for the tin-lead solder. With the developing technologies there is a need for increased reliability issues in technologies such as 'Ball Grid Arrays (BGA)', 'Quad Flat Packs (QFP)' and 'Chip Scale Packaging'. The ultimate aim of any electronics manufacturing company is to manufacture products which are reliable and it has and will be a key issue.
All the previous researches concentrated on thermal cycling to obtain the failure as it was believed that the major cause for failure is the difference in the coefficient of thermal expansion between the components. This project deals the same methodology by subjecting the test vehicle to thermal cycling and later the failure surfaces are studied under scanning electronic microscope (SEM) and cross sectional study will be carried out. The methodology in which the project deals with can be best understood by following steps:
Understanding the basics by analysing literature review
Planning and scheduling the experiments
Selecting test vehicles
Conducting experiments
Analysing the results.
B. SUMMARY
Reliability of solder joints is the key issue the electronics industry is dealing with. Solder joint failure occurs due to thermal mismatch between solder material and PCB board itself. Although previous researches addressed the problems and analysed the failures there are still some research gap which opens up a scope of taking the challenge and performing it again more efficiently.
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