Board Level Reliability Test Research Articles

Exposed Die-Top Encapsulation Molding for an Improved High- Performance Flip Chip BGA Package

The recent advancement in high- performance semiconductor packages has been driven by the need for higher pin count and superior heat dissipation. A one-piece cavity lid flip chip ball grid array (BGA) package with high pin count and targeted reliability has emerged as a popular choice. The flip chip technology can accommodate an I/O count of more than five hundreds500, and the die junction temperature can be reduced to a minimum level by a metal heat spreader attachment. None the less, greater expectations on these high-performance packages arose such as better substrate real estate utilization for multiple chips, ease in handling for thinner core substrates, and improved board- level solder joint reliability. A new design of the flip chip BGA package has been looked into for meeting such requirements. By encapsulating the flip chip with molding compound leaving the die top exposed, a planar top surface can be formed. A, and a flat lid can then be mounted on the planar mold/die top surface. In this manner the direct interaction of the metal lid with the substrate can be removed. The new package is thus less rigid under thermal loading and solder joint reliability enhancement is expected. This paper discusses the process development of the new package and its advantages for improved solder joint fatigue life, and being a multichip package and thin core substrate options. Finite-element simulations have been employed for the study of its structural integrity, thermal, and electrical performances. Detailed package and board-level reliability test results will also be reported

On the difference between thermal cycling and thermal shock testing for board level reliability of soldered interconnections

In this work, the endurance behavior of a ball grid array package is determined at two different temperature cycle conditions: a thermal cycle test and a thermal shock test. The observed failure distributions and failure modes allow deriving a translation factor between the two test conditions. Finite element analyses are carried out to design a predictive simulation model for the fatigue lifetime. Using an isothermal approach, a discrepancy is found between the experimental and simulated results. Incorporating the experimentally measured temperature gradients into a semi-transient model leads to a better match with the observed lifetimes. In particular for the fast thermal shock loading this turns out to be necessary. The results prove that temperature gradients in BGA-packages play an important role in board level reliability testing.

Comprehensive board-level solder joint reliability modeling and testing of QFN and PowerQFN packages

For quad flat non-lead (QFN) packages, board-level solder joint reliability during thermal cycling test is a critical issue. In this paper, a parametric 3D FEA sliced model is established for QFN on board with considerations of detailed pad design, realistic shape of solder joint and solder fillet, and non-linear material properties. It has the capability to predict the fatigue life of solder joint during thermal cycling test within ±34% error. The fatigue model applied is based on a modified Darveaux’s approach with non-linear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during thermal cycling test. For the test vehicles studied, the maximum SED is observed mostly at the top corner of peripheral solder joint. The modeling predicted fatigue life is first correlated to thermal cycling test results using modified correlation constants, curve-fitted from in-house QFN thermal cycling test data. Subsequently, design analysis is performed to study the effects of 17 key package dimensions, material properties, and thermal cycling test condition. Generally, smaller package size, smaller die size, bigger pad size, thinner PCB, higher mold compound CTE, higher solder standoff, and extra soldering at the center pad help to enhance the fatigue life. Comparisons are made with thermal cycling test results to confirm the relative trends of certain effects. Another enhanced QFN design with better solder joint reliability, PowerQFN, is also studied and compared with QFN of the same package size.

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.

Mechanical characterization of Sn–Ag-based lead-free solders

Recently, preventing environmental pollutions, lead-free (Pb-free) solders are about to replace tin–lead (Sn–Pb) eutectic solders. However, the mechanical properties of Pb-free solders have not been clarified. Hence, the following study was conducted; first, a rate-dependent plasticity was characterized to represent the inelastic deformation behavior for Sn–Ag-based lead-free solders. The material parameters in a constitutive model were determined in a direct method combining both rate-dependent and rate-independent plastic strains. The constitutive model unifies both rate-dependent creep behavior and rate-independent plastic behavior occurring concurrently at the same time in the solders. Secondly, the strength of solders with a variety of plating materials was studied. Intermetallic compounds (IMC) between solder and electrical pads are formed during reflow process and gradually grow in service. By using the Cu-plates on which Cu or Ni or Ni/Au plating was deposited, the specimens of solder joints were fabricated with Sn–Ag-based lead-free solders. After aging the specimens in an isothermal chamber, tensile tests were performed. From scanning electron microscope (SEM) microscope observation and EDX microprobe analysis, the growth and components of the IMC layer were also examined. Based on the experimental tests, the relations between solder joint strength and the aging period were discussed. Furthermore, the validation of fracture strength of solder joints resulting from the tensile tests was verified with package-mounted board level reliability tests.

Board level reliability of PBGA using flex substrate

Assurance of board level reliability is necessary and required for adopting any new packages into products. This paper presents board level reliability test results of a flex substrate BGA under thermal and bend cyclic tests. It is well known that solder joint reliability is affected by many factors, such as the size of chip, joint stand-off height, pad design, test board surface finish, substrate gold plating thickness and the utilization of underfill material, etc. However, most of the works have been conducted are BGA on rigid substrates. In this work, thermal cyclic test is performed to re-examine these factors using package housed on a flex substrate. Bending test with two deflections is also performed to investigate solder joint fatigue life and failure modes under mechanically repetitive loading.Two-parameter Weibull model is used to analyze joint fatigue life. Failure analysis is conducted and discussed for each case. Under temperature cycling test, chip size, polyimide thickness and underfill material utilization were found to have significant impacts on joint fatigue life, especially the effect of applying underfill material to the joint. Epoxy thickness was found to have little effect on the joint fatigue life for this case.The effects of test board surface finish and substrate gold plating thickness on the joint fatigue life were found coupled. The term “substrate” here refers to the chip carrier, while the “board” here refers to motherboard, which is the board to assemble test vehicles on. The gold thickness here all refers to the electrolytic gold plating on the substrate. Using organic solderability preservative boards, substrate gold plating thickness affects joint fatigue life slightly, but with Au–Ni test boards, the effect is tremendous. The difference is due to different intermetallic compounds (IMC) formed. In other words, different IMC systems are formed due to different combination of test board surface finish and substrate gold plating thickness. As a result, different IMC induces different failure modes. The joint fatigue life under cyclic bend test with different deflections is also probed and shown. The corresponding failure modes are also discussed.