Drop test performance is one of the key reliability parameters for characterizing electronic packages such as ball grid array (BGA) under mechanical stresses. It is especially important for a large BGA in a handheld device such as a smart-phone or tablet where the solder joint tends to crack under mechanical stress. This has gained more and more interest as mobile computing becomes dominant for next generation handheld applications. The JEDEC test board is widely used in the industry to calibrate the drop performance of solder joints in BGA and other area array components. One main drawback for this test method is in its test vehicle, which is a 132mmx77mm rectangular board populated a 3x5 array of BGAs. The stress and strain at each component behaves differently dependent to the distance of each component to the center and or screw holes used for mounting on the table. This makes statistical and quantitative analysis of the drop test results impossible unless a large number of sample sizes is used. This paper demonstrated two types of square board with 4 components mounted symmetrically on the top. This significantly improves the efficiency of drop test by making all data points undergo the same stress level so that a consistent drop and a good statistical analysis is feasible.
High reliability at the board level is challenging for a large flip chip ball-grid-array (fcBGA) where large die and stiff substrate are used. For those BGA solder joints, the difficulty is to achieve high reliability in both thermal cycling and mechanical dynamic tests. This paper presents experimental work on an fcBGA with a die size of 25x15mm, a body size of 40x40mm and over 1700 ball count. The reliability tests include thermal cycling from -40C to 85C up to 7,500 cycles, mechanical drop at 1500G and 9-point cyclic bending test run to failure. To develop a good reliability model, the integrity of both the solder joint and substrate copper traces are monitored using in-situ resistance measurement in combination with frequent cross-sectioning and dye-and-pry (DnP) to understand the progression of cracking in the solder joints. Using Finite Element Analysis (FEA), the solder joint failure mechanism was verified and a new failure mode in mechanical reliability testing has been confirmed.
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