Board-level reliability of Pb-free solder joints of TSOP and various CSPs

To evaluate various Pb-free solder systems for leaded package, thin small outline packages (TSOPs) and chip scale packages (CSPs) including leadframe CSP (LFCSP), fine pitch BGA (FBGA), and wafer level CSP (WLCSP) were characterized in terms of board level and mechanical solder joint reliability. For board level solder joint reliability test of TSOPs, daisy chain samples having pure-Sn were prepared and placed on daisy chain printed circuit board (PCB) with Pb-free solder pastes. For CSPs, the same composition of Pb-free solder balls and solder pastes were used for assembly of daisy chain PCB. The samples were subjected to temperature cycle (T/C) tests (-65/spl deg/C/spl sim/150/spl deg/C, -55/spl deg/C/spl sim/125/spl deg/C, 2 cycles/h). Solder joint lifetime was electrically monitored by resistance measurement and the metallurgical characteristics of solder joint were analyzed by microstructural observation on a cross-section sample. In addition, mechanical tests including shock test, variable frequency vibration test, and four point twisting test were carried out with daisy chain packages too. In order to compare the effect of Pb-free solders with those of Sn-Pb solder, Sn-Pb solder balls and solder paste were included. According to this paper, most Pb-free solder systems were compatible with the conventional Sn-Pb solder with respect to board level and mechanical solder joint reliability. For application of Pb-free solder to WLCSP, Cu diffusion barrier layer is required to block the excessive Cu diffusion, which induced Cu trace failure.     

Effect of solder creep on the reliability of large area die attachment

High temperature solders have been widely used for power device die attachment. One typical solder is Pb92.5In5Ag2.5, which is a ternary eutectic alloy with a eutectic temperature of 310°C. Such a Pb-based solder has a low Young's modulus, a low yield strength, and a high strain prior to failure. So it can be used to attach large size silicon die to mismatched substrates. In this paper, stresses and strains have been studied on a large size power MOSFET attachment using the Pb92.5In5Ag2.5 solders. A commercial finite element analysis software is employed as the simulation tool. Three types of substrates, pure copper, copper–tungsten composite, and pure molybdenum are used in the study, where molybdenum has the closest coefficient of thermal expansion to silicon. In addition to the plastic deformation simulation of the solder, a creep model of the solder was incorporated due to the low melting temperature of the solder alloy. Firstly, stresses and strains are calculated during the cooling cycle after attachment. It is found that the creep strain is the dominant plastic strain at low cooling rate (10°C/min). Also, the maximum Von Mises stress in the Si chip is decreased from 174 to 62.7 MPa after adding creep strain. As expected, the maximum creep strain happens to the die-to-copper substrate attach. Simulation on temperature cycling is done from −55°C to +150°C. The peak Von Mises stress occurs at the low temperature extreme and holds steadily during the soaking period, indicating insignificant contribution from creep. The Von Mises stress at the high temperature extreme is much lower and decreases with holding time. Significant plastic deformation of the solder layer is observed in cooling cycles. For silicon to copper substrate attach, its plastic deformation increases with each cycle. For all three substrates used, considerable solder creep is observed at heating cycles. The creep strain is much larger than the rate-independent plastic strain in the solder alloy for all three types of substrates. It is concluded that solder creep is the dominant factor affecting long term reliability of power semiconductor die attachment.     

Experimental study of WL-CSP reliability subjected to a four-point bend-test

This article deals with the reliability of WL-CSP subjected to a four-point bend-test according to the JEDEC standard and the repeatability of this test. The evaluation of the test repeatability shows a 5% variation on the characteristic Weibull parameter η. Two dedicated failure detection methods are used: with an event detector and with data acquisition and no difference are observed. Parameters considered are pre-load magnitude and loading frequency. PCB strain due to pre-load is measured and its effect on the product reliability is evaluated. Based on these measurements a pre-load value is defined. It is also shown that a frequency of 3 Hz modifies the studied package lifetime.     

Board level solder joint reliability analysis and optimization of pyramidal stacked die BGA packages

Due to requirements of cost-saving and miniaturization, stacked die BGA has recently gained popularity in many applications. However, its board level solder joint reliability during the thermal cycling test is not as well-studied as common single die BGA. In this paper, solder joint fatigue of wirebond stacked die BGA is analyzed in detail. 3D fatigue model is established for stacked die BGA with considerations of detailed pad design, realistic shape of solder ball, and non-linear material properties. The fatigue model applied is based on a modified Darveaux's approach with non-linear viscoplastic analysis of solder joints. The critical solder ball is observed located between the top and bottom dice corner, and failure interface is along the top solder/pad interface. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house TFBGA (thin-profile fine-pitch BGA) thermal cycling test data. Subsequently, design analyses are performed to study the effects of 16 key design variations in package dimensions, material properties, and thermal cycling test conditions. In general, smaller top and bottom dice sizes, thicker top or bottom die, thinner PCB, thicker substrate, higher solder ball standoff, larger solder mask opening size, smaller maximum ball diameter, smaller PCB pad size, smaller thermal cycling temperature range, longer ramp time, and shorter dwell time contribute to longer fatigue life. The effect of number of layers of stacked-die is also investigated. Finally, design optimization is performed based on selected critical design variables.     

Board level solder joint reliability analysis of stacked die mixed flip-chip and wirebond BGA

Stacked die BGA has recently gained popularity in telecommunication applications. However, its board level solder joint reliability during the thermal cycling test is not as well-studied as common single die BGA. In this paper, solder joint fatigue of lead-free stacked die BGA with mixed flip-chip (FC) and wirebond (WB) interconnect is analyzed in detail. 3D fatigue model is established for stacked die BGA with considerations of detailed pad design, realistic shape of solder ball, and non-linear material properties. The fatigue model applied is based on a modified Darveaux’s approach with non-linear viscoplastic analysis of solder joints. Based on the FC–WB stack die configuration, the critical solder ball is observed located between the top and bottom dice corner, and failure interface is along the top solder/pad interface. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house lead-free TFBGA46 (thin-profile fine-pitch BGA) thermal cycling test data. Subsequently, design analyzes are performed to study the effects of 20 key design variations in package dimensions, material properties, and thermal cycling test conditions. In general, thinner PCB and mold compound, thicker substrate, larger top or bottom dice sizes, thicker top die, higher solder ball standoff, larger solder mask opening, smaller PCB pad size, smaller thermal cycling temperature range, longer ramp time, and shorter dwell time contribute to longer fatigue life. SnAgCu is a common lead-free solder, and it has much better board level reliability performance than eutectic solder based on modeling results, especially low stress packages.     

焊点的质量与可靠性

主要介绍了Sn—Pb合金焊接点发生失效的各个失效阶段的各种表现形式，探讨发生失效的各种原因．如热应力与热冲击、金属的溶解、基板和元件过热、超声清洗的损害，以及如何在工艺上进行改进以改善焊点的可靠性，使焊点有良好的可靠性、不易损坏，能够承受变化的负载等，从而提高产品的质量。     

Stacked solder bumping technology for improved solder joint reliability

Solder joint failure is a serious reliability concern in flip-chip and ball grid array packages of integrated-circuit chips. In current industrial practice, the solder joints take on the shape of a spherical segment. Mathematical calculations and finite element modeling have shown that hourglass-shaped solder joints would have the lowest plastic strain and stress during a temperature cycle, thus the longest lifetime. In an effort to improve solder joint reliability, we have developed a stacked solder bumping technique for fabricating triple-stacked hourglass-shaped solder joints. This solder bumping technology can easily control the solder joint shape and height. The structure of triple-stacked solder joints consists of an inner cap, middle ball and outer cap. The triple-stacked solder joints are expected to have greater compliance than conventional solder joints and are able to relax the stresses caused by the coefficient of thermal expansion mismatching between the silicon chips and substrates since it has a greater height. Furthermore, the hourglass-shaped solder joints are to have a much lower stress/strain concentration at the interface between the solder joint and the silicon die as well as at the interface between the solder joint and substrate than barrel-shaped solder joints, especially around the corners of the interfaces. In this paper, the solder bumping process is designed and joint reliability is evaluated. Mechanical tests have been carried out to characterize the adhesion strength of the solder joints. The interfaces of the solder joints are investigated by scanning electron microscopy and energy dispersive X-ray analysis. Temperature cycling results show that the triple-stacked hourglass-shaped solder joints are more reliable than the traditional spherical-shaped solder joints.     

Vibration fatigue reliability of BGA-IC package with Pb-free solder and Pb–Sn solder

In this paper, a new method is proposed for evaluating the high-cycle fatigue strength of BGA (Ball Grid Array) packages with Pb-free solder and Pb–Sn solder due to vibration. An attached weight induced mixed mode stress in the solder ball of a package was used. To consider the effect of the mixed mode stress that occurred in a solder ball and the frequency to fatigue strength of the solder ball, a test was carried out with the three kinds of weights (σ_n/τ_n = 4, 5, and 6) at various frequencies (10–27 Hz). To clarify the effect of frequency, a nonlinear analysis with a viscoplastic model was carried out within the range of 0.001–3450 Hz. From the continuous observation of the cross-section of the package and finite element method (FEM) analysis results, it was revealed that the maximum principal stress is the driving force to package failure. Although an intermetallic compound in both packages and a Pb-rich region in a Pb–Sn solder based package were confirmed by EDX microprobe analysis, they do not contribute to the initiation of a crack in a solder ball. The fatigue strength of the Pb-free solder and Pb solder was evaluated on the basis of the maximum principal stress calculated by FEM and the experimental results.     

Apple iMac MA199LL台式电脑拆解分析

iMacMA199LL是苹果电脑Mac系列中第一个采用Intel处理器的成员。它采用1440×900宽屏17英寸LCD显示器,内置1.83GHzIntelCoreDuo双核处理器,该处理器带有2MB二级缓存,其FSB(前端总线)频率为667MHz。iMacMA199LL的其他特点包括:512MBRAM、ATIRadeon×1600显卡(带128MBGDDR3内存)、160GB串行ATA硬驱,内置以太网、Wi-Fi和蓝牙接口。下面的拆解统计了不同集成部分在总成本中所占的比例,并在下面的列表中给出了部分器件的一些详细信息,包括产品型号、生产厂商、封装形式、引脚及数据手册的网站链接地址等,希望能给国内的电子…     

Effects of Cu contents in Pb-free solder alloys on interfacial reactions and bump reliability of Pb-free solder bumps on electroless Ni-P under-bump metallurgy

Using the screen-printed solder-bumping technique on the electroless plated Ni-P under-bump metallurgy (UBM) is potentially a good method because of cost effectiveness. As SnAgCu Pb-free solders become popular, demands for understanding of interfacial reactions between electroless Ni-P UBMs and Cu-containing Pb-free solder bumps are increasing. It was found that typical Ni-Sn reactions between the electroless Ni-P UBM and Sn-based solders were substantially changed by adding small amounts of Cu in Sn-based Pb-free solder alloys. In Cu-containing solder bumps, the (Cu,Ni)₆Sn₅ phase formed during initial reflow, followed by (Ni,Cu)₃Sn₄ phase formation during further reflow and aging. The Sn3.5Ag solder bumps showed a much faster electroless Ni-P UBM consumption rate than Cu-containing solder bumps: Sn4.0Ag0.5Cu and Sn0.7Cu. The initial formation of the (Cu,Ni)₆Sn₅ phase in SnAgCu and SnCu solders significantly reduced the consumption of the Ni-P UBM. The more Cu-containing solder showed slower consumption rate of the Ni-P UBM than the less Cu-containing solder below 300°C heat treatments. The growth rate of the (Cu,Ni)₆Sn₅ intermetallic compound (IMC) should be determined by substitution of Ni atoms into the Cu sublattice in the solid (Cu,Ni)₆Sn₅ IMC. The Cu contents in solder alloys only affected the total amount of the (Cu,Ni)₆Sn₅ IMC. More Cu-containing solders were recommended to reduce consumption of the Ni-based UBM. In addition, bump shear strength and failure analysis were performed using bump shear test.     

Soft solder die bonding of multiple die devices

The aim of the work presented in this paper is to study the soft solder die attach of multiple die devices by a multiple pass process. With a multiple pass process we mean a procedure which needs as many die bonder furnace passes as there are different types of dice to be bonded into a package. The main focus of the investigation is on the effect of the necessary multiple furnace passes on the reliability of the soft solder attachment layer. As this behavior may depend on the specific type of device or package (i.e., solder alloy-substrate combination, die size, package geometry), it is analyzed relative to the one of an identical die processed in one single furnace pass. A real package (TO-220) is used as test vehicle and processed on standard equipment. A detailed analysis of the multiple pass process relative to a single pass process with appropriate equipment is performed. It is concluded that a multiple pass process may be slightly less efficient for throughput. However it gives more process flexibility and allows using standard equipment which is available on the market. The result of this investigation strongly supports the feasibility of multiple die devices with a multiple pass process. No reliability limiting influence of the additional furnace passes causing a repeated re-melting and re-solidifying of the solder layer is found. It is, however, necessary to investigate the capability for any other specific device or package again     

Lead-free solder flip chip-on-laminate assembly and reliability

This paper examines the assembly process for flip chip die with SnAgCu solder bumps and the results of liquid-to-liquid thermal shock testing. The SnAgCu alloy required a thicker dip layer of flux to achieve good wetting compared to the SnPb eutectic alloy. A liquid spray flux yielded more consistent solder wetting with the SnAgCu alloy. With both fluxes, a nitrogen reflow atmosphere was necessary with the SnAgCu alloy. A peak reflow temperature of 246°C was used for the assembly of the SnAgCu thermal shock test vehicles. A lower peak temperature of 235°C did not yield sufficient solder wetting. Liquid-to-liquid thermal shock testing was performed from -40°C to +125°C. The SnPb alloy performed slightly better than the SnAgCu and the dip flux was better that the spray flux. The degree of delamination with the SnAgCu alloy was significantly higher than with the SnPb alloy. Cracks in the underfill between adjacent solder balls were observed. The SnPb alloy extruded into these cracks more readily than the SnAgCu and created electrical shorts     

Comparative studies on solder joint reliability of CTBGA assemblies with various adhesives using the array-based package shear test

This paper discusses the influences of various adhesives on board-level shear strength of ChipArray® Thin Core Ball Grid Array (CTBGA) assemblies through an innovative reliability evaluation approach, i.e. array-based package (ABP) shear test. It is found that the adhesives do enhance the shear strength for all the test categories as compared with the assemblies without adhesives (w/o A), but the degree of improvements between different strategies vary quiet a lot. The specific shear strength is affected by a number of factors, in which dispending patterns and material properties of the adhesives used influences it obviously. In general, the adhesives with high storage modulus and large dispensing volume are preferred, for example, stiff full or partial capillary flow underfills. In order to further understand the failure mechanism of the CTBGA during the ABP shear test, failure analysis on tested devices are also conducted using side view optical microscopy, scanning electron microscope (SEM) and energy dispersive X-ray (EDX), the results indicate that the predominant failure mode changes from PCB pad lift/cratering to fracture at package side intermetallic compound (IMC)/solder interface with increasing dispensing volume and storage modulus, which basically improves the solder joint reliability of CTBGA assemblies.     

Interfacial reaction and mechanical reliability of PTH solder joints with different solder/surface finish combinations

The effects of minor Ni addition (0.05 wt.%) on the microstructures and mechanical reliability of the lead-free solder joints used in the pin through hole (PTH) components were carefully investigated using a scanning electron microscope (SEM), a field-emission electron probe x-ray microanalyzer, and a pull tester. The PTH walls (i.e., Cu) of printed circuit boards (PCBs) were coated with organic solderability preservative (OSP) or electroless nickel/immersion gold (ENIG) surface finish before soldering. During soldering, the pins of the electronic components were first inserted into the PTHs deposited with OSP or ENIG, and then joined using a Sn–3Ag–0.5Cu (SAC) solder bath through a typical wave-soldering process. After wave soldering, a rework (the second wave soldering) was performed, where an SAC or Sn–0.7Cu–0.05Ni (SCN) solder bath was employed. The SCN joints were found to possess a higher tensile strength than the SAC ones in the OSP case. The sluggish growth of Cu₃Sn, along with few Kirkendall voids at the solder/Cu interface caused by minor Ni addition into the solder alloy (i.e., SCN), was believed to be the root cause responsible for the increase in the strength value. However, the mechanical strength of the PTH components was revealed to be insensitive to the solder composition in the alternative case where an ENIG was deposited over the PTH walls. The implication of this study revealed that minor addition of Ni into the solder is beneficial for the solder/Cu joints, but for the solder/Ni(P) joints.     

Study of micro-BGA solder joint reliability

A new reflow parameter, heating factor (Q_η), which is defined as the integral of the measured temperature over the dwell time above liquidus, has been proposed in this report. It can suitably represent the combined effect of both temperature and time in usual reflow process. Relationship between reliability of the micro-ball grid array (micro-BGA) package and heating factor has been discussed . The fatigue failure of micro-BGA solder joints reflowed with different heating factor in nitrogen ambient has been investigated using the bending cycle test. The fatigue lifetime of the micro-BGA assemblies firstly increases and then decreases with increasing heating factor. The greatest lifetime happens at Q_η near 500 s °C. The optimal Q_η range is between 300 and 750 s °C. In this range, the lifetime of the micro-BGA assemblies is greater than 4500 cycles. SEM micrographs reveal that cracks always initiate at the point of the acute angle where the solder joint joins the PCB pad.     

Sn－Zn－In软钎料合金初步研究

对Ｓｎ－Ｚｎ－Ｉｎ钎料合金的性能进行了研究，钎料铺展性和剪切强度试验结果表明，在Ｓｎ－９Ｚｎ－Ｉｎ软钎料合金中，随Ｉｎ含量增加，铺展面积增大，钎焊接头剪切强度降低。钎料熔点和接头组织等性能的综合分析结果表明Ｓｎ－９Ｚｎ－１０Ｉｎ的性能已接近或超过传统的Ｓｎ－Ｐｂ共晶     

New lead-free,Sn-Zn-In solder alloys

In view of the need for a lead-free, drop-in replacement for the widely used 40Pb-60Sn near-eutectic solder (m.p. ~183°C), new Sn-Zn-ln based alloys with substantially the same melting point have been developed. It is shown that the alloying additions of In to the Sn-Zn binary system result in a suppression of the melting point to 175-188°C, and at the same time significantly improve the wetting characteristics. While a relatively active flux may be required for good solderability in air atmosphere, the recent manufacturing trend of using inert atmospheres is likely to allow acceptable manufacturability using less active fluxes in the future.     

低银无铅焊膏板级封装工艺和焊点可靠性试验

针对低Ag无铅焊膏的市场需求,研究开发了一种适用于99.0Sn0.3Ag0.7Cu低Ag无铅焊膏用松香型无卤素助焊剂(WTO—LF3000),配制了相应的无铅焊膏(WTO—LF3000—SAC0307),并对其板级封装工艺适应性及焊点可靠性进行了考察,用测试后样品的电气可靠性作为接头可靠性评价条件。结果表明:所开发的低Ag无铅焊膏熔点和润湿性符合产品实际要求。配制的焊膏印刷质量良好,焊点切片观察其孔隙率<25%,满足行业标准IPC—A—610D之要求。样品分别经跌落、震动和温度循环试验后,无焊点脱落等现象,电气功能正常。     

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.