Solder Joint Performance Research Articles

Forward and Backward Compatibility of Solder Alloys With Component and Board Finishes

The primary objective of the research presented in this paper is to qualify the reliability of mixed assemblies by comparing them to the conventional Sn-Pb assembly and completely Pb-free assembly. The research investigates both forward and backward compatibility in electronic assemblies using a design of experiments (DOE) approach. The investigation utilized a test vehicle containing an area array component (BGA169) and chip components (0603 resistors). Hot air solder leveling (HASL) and organic solderability preservative (OSP) surface finishes were used on the test vehicles to represent Sn-Pb and Pb-free alternatives, respectively. The assembled test vehicles were cut into two panels - one containing a resistor section for isothermal aging and the other containing a BGA and another resistor section for thermal shock. The assemblies were subjected to isothermal aging and thermal shock tests as per Interconnecting and Packaging Electronic Circuits/Joint Electronic Device Engineering Council (IPC/JEDEC) standards. The resistors were sheared in the as-soldered condition, and at various isothermal aging intervals and thermal shock cycles. In order to simulate an intermetallic failure in isothermal aging, a reduced shear height (20 mum) was used for the shear test. The performance of the resistor solder joints were quantified in terms of the shear force. The performance of the ball grid array (BGA) solder joint during thermal shock testing was quantified in terms of the number of cycles to failure. Experimental results from shear analysis of resistor solder joints show that Pb-free alloy assemblies' performance is superior to those assembled using Sn-Pb alloy. Also, the ductile nature of the Sn-Ag-Cu (SAC) alloy provides the joints a better fatigue life. With the area array components, Pb-free assembly joints outlived the traditional Sn-Pb joints when subjected to thermal shock loading. Among the mixed assemblies, backward-compatible assemblies (Pb-free bumps and Sn-Pb solder alloy) showed superior performance. The microstructural analysis of the solder joints indicate a uniform distribution of lead in the solder joint matrix.

Interfacial reactions of Sn-Ag-Cu solders modified by minor Zn alloying addition

The near-ternary eutectic Sn-Ag-Cu alloys have been identified as leading Pb-free solder candidates to replace Pb-bearing solders in microelectronic applications. However, recent investigations on the processing behavior and solder joints reliability assessment have revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag3Sn plates in Sn-Ag-Cu joints, especially when solidified in a relatively slow cooling rate, is one issue of concern. The implications of large Ag3Sn plates on solder joint performance and several methods to control them have been discussed in previous studies. The minor Zn addition was found to be effective in reducing the amount of undercooling required for tin solidification and thereby to suppress the formation of large Ag3Sn plates. The Zn addition also caused the changes in the bulk microstructure as well as the interfacial reaction. In this paper, an in-depth characterization of the interfacial reaction of Zn-added Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported. The effects of a Zn addition on modification of the interfacial IMCs and their growth kinetics are also discussed.

Determination of Measurement Limit for Open Solder Bumps on a Flip-Chip Package Using a Laser Ultrasonic Inspection System

The adoption of surface-mount technology has spurred a rapid decrease in the size of electronic packages. Advanced active component packages, such as flip-chips, chip scale packages (CSPs), and ball grid arrays (BGAs), are attached to a printed wiring board (PWB) with small solder bumps between the bottom of the device and the board. The attachment method reduces package size, but the solder connections are no longer visible for inspection. Although lead frame packages can be inspected using automated camera equipment, other techniques, such as scanning acoustic microscopy (SAM) and automated X-ray inspection (AXI), have been used for screening these advanced packages for missing and shorted solder joints online in a manufacturing facility. However, they have trouble identifying open solder joints, especially when applied on-line. An inspection system based on laser-generated ultrasound and interferometric techniques has been developed. The system uses short pulses of high-intensity infrared energy from an Nd:YAG laser to excite the bulk of a chip into vibration through the principle of laser-generated ultrasound. The vibration response of a reference good chip attached to the substrate is measured and compared with vibration responses from other chips. Changes in the vibration response indicate a change in the quality of the chip or its attachment to the PWB. Previously, this system has been used to identify misalignment in CSPs and flip-chips, to identify missing solder bumps, and for detecting structural cracks in chips. In this paper, the ability of the current inspection system to identify open solder joints is investigated. Peripheral array, 48-bump daisy-chain flip-chips (PB18) without underfill were used as a test vehicle. This flip-chip has a higher I/O count and smaller, more closely spaced bumps than previous flip-chip specimens. Individual solder pads were removed from the PWB design, and test specimens with zero to seven adjacent open solder bumps were created. After exciting the chips with laser pulses, the impulse responses at 48 points on the surface of each chip were recorded. Data analysis is performed to compare the responses from reference chips and chips with open solder joints. Results show that the system has the ability to detect open joints for the flip-chip package, and statistical analysis of the results from repeated comparisons between the good and defective chips is employed to establish a detection threshold.

Reliability and RF performance of BGA solder joints with plastic-core solder balls in LTCC/PWB assemblies

In this paper board-level reliability of low-temperature co-fired ceramic (LTCC) modules with thermo-mechanically enhanced ball-grid-array (BGA) solder joint structure mounted on a printed wiring board (PWB) was experimentally investigated by thermal cycling tests in the 0–100 °C and −40 to 125 °C temperature ranges. The enhanced joint structure comprised solder mask defined (SMD) AgPt pad metallization, eutectic solder and plastic-core solder balls (PCSB). Similar daisy-chained LTCC modules with non-collapsible 90Pb10Sn solder spheres were used for a reference test set. The reliability of the joint structures was analyzed by resistance measurements, X-ray microscopy, scanning acoustic microscopy (SAM) and SEM/EDS investigation. In addition, a full-wave electromagnetic analysis was performed to study effects of the plastic-core material on the RF performance of the LTCC/BGA package transition up to millimeter-wave frequencies. Thermal cycling results of the modules with PCSBs demonstrated excellent fatigue performance over that of the reference. In the harsher cycling test, Weibull’s shape factor β values of 7.9 and 4.8, and characteristic lifetime θ values of 1378 and 783 were attained for the modules with PCSBs and 90Pb10Sn solder spheres, respectively. The primary failure mode in all test assemblies was fatigue cracking in eutectic solder on the ceramic side.

Environment-Friendly Electronics: Lead-Free Technology

Environment-Friendly Electronics: Lead-Free Technology

Lead-free Solder Joint Reliability – State of the Art and Perspectives

There is an increasing demand for replacing tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry due to health and environmental concerns. The European Union recently passed a law to ban the use of lead in electronic products. The ban will go into effect in July of 2006. The Japanese electronics industry has worked to eliminate lead from consumer electronic products for several years. Although currently there are no specific regulations banning lead in electronics devices in the United States, many companies and consortiums are working on lead-free solder initiatives including Intel, Motorola, Agilent Technologies, General Electric, Boeing, NEMI and many others to avoid a commercial disadvantage. The solder joints reliability not only depends on the solder joint alloys, but also on the component and PCB metallizations. Reflow profile also has significant impact on lead-free solder joint performance because it influences wetting and microstructure of the solder joint. A majority of researchers use temperature cycling for accelerated reliability testing since the solder joint failure mainly comes from thermal stress due to CTE mismatch. A solder joint failure could be caused by crack initiation and growth or by macroscopic solder facture. There are conflicting views of the reliability comparison between lead-free solders and tin-lead solders. This paper first reviews lead-free solder alloys, lead-free component lead finishes, and lead-free PCB surface finishes. The issue of tin whiskers is also discussed. Next, lead-free solder joint testing methods are presented; finite element modeling of lead-free solder joint reliability is reviewed; and experimental data comparing lead-free and tin-lead solder joint reliability are summarized. Finally the paper gives perspectives of transitions to totally lead-free manufacturing.

Reliability Analysis and Design for the Fine-Pitch Flip Chip BGA Packaging

The geometry of solder joints in the flip chip technologies is primarily determined by the associated solder volume and die/substrate-side pad size. In this study, the effect of these parameters on the solder joint reliability of a fine-pitched flip chip ball grid array (FCBGA) package is extensively investigated through finite element (FE) modeling and experimental testing. To facilitate thermal cycling (TC) testing, a simplified FCBGA test vehicle with a very high pin counts (i.e., 2499 FC solder joints) is designed and fabricated. By the vehicle, three different structural designs of flip chip solder joints, each of which consists of a different combination of these design parameters, are involved in the investigation. Furthermore, the associated FE models are constructed based on the predicted geometry of solder joints using a force-balanced analytical approach. By way of the predicted solder joint geometry, a simple design rule is created for readily and qualitatively assessing the reliability performance of solder joints during the initial design stage. The validity of the FE modeling is extensively demonstrated through typical accelerated thermal cycling (ATC) testing. To facilitate the testing, a daisy chain circuit is designed, and fabricated in the package for electrical resistance measurement. Finally, based on the validated FE modeling, parametric design of solder joint reliability is performed associated with a variety of die-side pad sizes. The results show that both the die/substrate-side pad size and underfill do play a significant role in solder joint reliability. The derived results demonstrate the applicability and validity of the proposed simple design rule. It is more surprising to find that the effect of the contact angle in flip chip solder joint reliability is less significant as compared to that of the standoff height when the underfill is included in the package.

Solder joint reliability of TFBGA assemblies with fresh and reworked solder balls

In this article, the solder joint reliability of thin and fine-pitch BGA (TFBGA) with fresh and reworked solder balls is investigated. Both package and board level reliability tests are conducted to compare the solder joint performance of test vehicle with fresh and reworked solder balls. For package level reliability test, ball shear test is performed to evaluate the joint strength of fresh and reworked solder balls. The results show that solder balls with rework process exhibit higher shear strength than the ones without any rework process. The results also exhibit that the different intermetallic compound (IMC) formation at solder joints of fresh and reworked solder balls is the key to degradation of shear strength. For board level reliability tests, temperature cycling and bending cyclic tests are both applied to investigate the fatigue life of solder joint with fresh and reworked solder balls. It is observed that package with reworked solder ball has better fatigue life than the one with fresh solder ball after temperature cyclic test. As for bending cyclic test, in addition to test on as-assembled packages, reworked and fresh samples are subjected to heat treatment at 150 °C for 100 h prior to the bending cyclic test. The purpose is to let Au–Ni–Sn IMC resettle at solder joints of fresh solder ball and examine the influence of Au–Ni–Sn IMC on the fatigue life of solder joints (Au embrittlement effect). The final results confirm that reworked solder balls have better reliability performance than fresh one since Au embrittlement dose exist at fresh solder ball.

Optimisation modelling for flip‐chip solder joint reliability

This paper details and demonstrates integrated optimisation‐reliability modelling for predicting the performance of solder joints in electronic packaging. This integrated modelling approach is used to identify efficiently and quickly the most suitable design parameters for solder joint performance during thermal cycling and is demonstrated on flip‐chip components using “no‐flow” underfills. To implement “optimisation in reliability” approach, the finite element simulation tool – PHYSICA, is coupled with optimisation and statistical tools. This resulting framework is capable of performing design optimisation procedures in an entirely automated and systematic manner.

Mechanical deflection system (MDS) test and methodology for PBGA solder joint reliability

A mechanical deflection system (MDS) was developed for highly accelerated tests to evaluate the solder joint fatigue performance in printed circuit board assemblies. The MDS test system can be used for design verification and qualification tests for solder joint reliability. Cyclic twisting deformation is imposed on an assembled printed circuit board (PCB) at isothermal conditions. The MDS test technique makes a significant contribution to reducing solder joint reliability testing cycle time. Fatigue performance of the PBGA solder joints subjected to the MDS test was investigated by three-dimensional finite element modeling. The solder joint fatigue lives were computed for several different MDS test conditions.

Materials behaviour and the reliability in performance of solder joints

While the implementation of lead‐free solder technology has been the focus of much recent research, the challenge of joint structural integrity should not be overlooked. The paper summarises the significant variability in the mechanical properties of solders, both in terms of the prevailing testing conditions and between the alloys themselves. Using conventional routes to life prediction for an elementary creep situation, it demonstrates the critical importance of understanding the failure processes and utilising materials data that are appropriate.

The Effect of Solder Paste Volume and Reflow Ambient Atmosphere on Reliability of CBGA Assemblies

To investigate the effect of stencil thickness and reflow ambient atmosphere on the reliability of ceramic ball grid array (CBGA) assemblies, three levels of stencil thickness, 0.10, 0.15, and 0.20 mm, were used to print solder paste on printed circuit board (PCB). After the CBGA modules were placed on PCBs, the specimens were divided into two groups, and reflowed in nitrogen and compressed air separately. Properties of the six groups of assemblies, such as shear strength, bending fatigue life, thermal shock cycles, and vibration fatigue life, were tested to find out the optimum assembling process. The results show that assemblies prepared with a stencil 0.15 mm thick yield maximized performance. And the nitrogen ambient atmosphere demonstrates a remarkable effect on improving the fatigue life. Theoretical models are given to qualitatively explain the relationship between the solder joint volume and performance. This work provides a guideline on how to determine the soldering process parameters of CBGA assemblies.

A universal reliability prediction model for SMD integrated circuits based on field failures

Abstract This paper makes a review of integrated circuit field failures on three types of environment respectively, Ground Benign (GB), Ground Fixed (GF), and Airborne Inhabited Cargo (AIC). It appears that for permanent working GB, there was no package related failures and 2 3 of the failures had EOS origin. For GF and AIC most of the failures were open balls bonds and open solder joints. A universal predicting reliability model is therefore proposed for the die part and for the package part. This study has been carried out in the frame of a working group, to update the French standard UTEC 80810 (ex RDF93 from CNET), and is an alternative guide to the obsolete MIL-HDBK-217F for predicting reliability calculations.

An overview of surface finishes and their role in printed circuit board solderability and solder joint performance

An overview has been presented on the topic of alternative surface finishes for package I/Os and circuit board features. Aspects of processability and solder joint reliability were described for the following coatings: baseline hot‐dipped, plated, and plated‐and‐fused 100Sn and Sn‐Pb coatings; Ni/Au; Pd, Ni/Pd, and Ni/Pd/Au finishes; and the recently marketed immersion Ag coatings. The Ni/Au coatings appear to provide the all‐around best options in terms of solderability protection and wire bondability. Nickel/Pd finishes offer a slightly reduced level of performance in these areas which is most likely due to variable Pd surface conditions. It is necessary to minimize dissolved Au or Pd contents in the solder material to prevent solder joint embrittlement. Ancillary aspects that include thickness measurement techniques; the importance of finish compatibility with conformal coatings and conductive adhesives; and the need for alternative finishes for the processing of non‐Pb bearing solders are discussed.

THERMAL CYCLING OF TEMPERATURE AND STRAIN RATE DEPENDENT SOLDER JOINTS

In a previous paper, i.e., Skipor et al. (1996), 63/37 Sn/Pb eutectic solder was characterized with the viscoplastic model of Bodner and Partom (1975). The material parameters of the isotropic form of the Bodner–Partom model were found for the eutectic solder as functions of temperature. In the present paper, the resulting temperature dependent viscoplastic description for the eutectic is implemented in an infinitesimal strain incrementally linear finite element method. This finite element method is based on an explicit, second-order accurate time-integration scheme, which is presented herein. The finite element method so-obtained is then used to analyze two solder joint configurations under cyclic thermal loading. The results of the analyses show that, along with observed field performance of solder joints, the details of the plastic strain distribution within a joint, i.e., its magnitude and degree of diffuseness, can have large impact on joint reliability. Specifically, it is demonstrated that the geometric details of how solder in a joint connects to harder materials in an electronic package play a significant role.

Crack growth rate measurements in surface-mount solder joints

The effects of cyclic loading conditions (either thermal or mechanical) on the reliability of electronic assemblies are strongly dependent on the performance of solder joints. Most solder joint fatigue models, and the supporting experimental data, do not treat the crack propagation processes that lead to failure. The benefits from a physically based description of crack propagation in solder joints include an accurate representation of the damage produced by cyclic loading and, therefore, a superior basis to evaluate attachment designs and materials. The development of crack growth rate models has been hampered by the lack of experimental capabilities to observe and characterize crack propagation in solder joints and a computational capability to describe the propagation of cracks in solder joints. This paper describes several approaches that have been used to measure crack propagation rates in solders and a new approach to quantify the crack growth rate and its driving force in solder joints by a quantitative analysis of the fracture surface topography.

マイクロジョイニングの接合性評価と信頼性について ６０Ｓｎ／４０Ｐｂはんだの疲労挙動

Recently, reliability assessment on the micro-joining for electronic apparatus attracts major attention of engineers and researchers in this field. Failure problems of the micro-joining have to be investigated from an interdisciplinary point of view, for the failure phenomena are caused not only by a mechanical effect but also by a metallurgical and/or electrochemical effect. For example, construction of brittle intermetallic compounds at Au wire/Al pad interface of ball bonding is a problem of metallurgical aspect, and electro-migration induced between soldering parts on high density printed circuit board is problem of electrochemical aspect. Though there are several micro-joining methods such as thermo-compression, ultrasonic bonding, micro-soldering and so on, a majority of micro-joining applied in electronic apparatus is micro-soldering. Therefore, it is an urgent problem to establish a valid evaluation method for solder joint performance under service conditions. A typical problem on this point is the solder cracking induced by temperature cycling, depending on the mismatch of thermal expansion coefficients of structural elements. In order to clear the failure problem of solder joint, fatigue failure mechanism of solder itself must be systematized.The main purpose of the present study is to investigate the fatigue strength and strain behaviors of 60Sn/40Pb solder plate specimen under several repeated load conditions including impact load carried out at room temperature. And furthermore, room temperature creep tests were also carried out to discuss the correlation between cyclic fatigue and static fatigue of the solder.The results of this study revealed a clear dependence of the fatigue life on the stress patterns; the lower was the frequency of stress cycle the lower was the fatigue strength. And the strength in impact fatigue showed the highest value. Then, the evaluation from the viewpoint of cumulative loading time indicated that the fatigue life of the solder could be well estimated by the cumulative loading time regardless of the type of stress pattern. And furthermore, the relationship of the stress vs. the cumulative loading time in cyclic fatigue well coincided with the creep rupture curve.

Evidence that Visual Inspection Criteria for Soldered Joints are no Indication of Reliability

Exemplary data are given that demonstrate unequivocally the inapplicability of commonly used visual inspection criteria for judging the long‐term service reliability of soldered joints. The data are obtained from a controlled model system. Plated‐through hole (PTH), wave soldered joints are used in this work, having a controlled degree of voiding, blowholing and solder blowout, covering the full range of normal visual assessment, from perfect to very bad, for this particular type of visible fault. The joint performance is assessed in terms of its joint pull strength, thermal shock, mechanical high frequency fatigue, low cycle thermal fatigue and propensity as a corrosion initiation site. No evidence was found that the visible outgassing faults degraded the solder joint performance. Indeed, in all the fatigue tests, the visually unacceptable solder joints performed significantly better. Furthermore, in a related research project, manual reworking of joints has been found to be demonstrably detrimental to solder joint reliability.

The Harmfulness of Blowholes in PTH Soldered Assemblies

One family of defects in soldered electronic assemblies that is almost invariably re‐worked is that due to outgassing, manifested as visible blowholes and solder blow‐out. It is known that re‐working can be very detrimental to the service life of an electronic assembly and should be avoided whenever possible. This paper describes work aimed to determine whether outgassing faults such as blowholes are harmful to service performance or whether more harm will be done by re‐working such faults. Standard test plated‐through‐hole (PTH) assemblies with controlled degrees of outgassing faults have been subjected to mechanical testing, thermal shock testing, mechanical fatigue, low cycle thermal fatigue and corrosion testing. Measurements in all these régimes have been carried out quantitatively with baseline controls. No evidence of significant loss of solder joint performance has been found, even for severe cases of solder loss. On the contrary, in cyclic fatigue testing, solder fillets with outgassing faults exhibit statistically significant performance enhancement. The conspicuous nature of blowholing and solder blow‐out undoubtedly over‐emphasises the problem during visual quality control inspection. Provided the copper barrel has been wet by the solder, outgassing faults should not be re‐worked. These faults should be used as process indicators and to draw attention to processes and the need for process control.