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版權所有 翻印必究 報 告 人：王禮國 指導老師：林克默 博士 日 期：2011.03.22 2011/3/22 1 版權所有 翻印必究 Introduction • This paper deals with a comparison study between SnPb and SnAgCu solder joint reliability. • The comparison is based on non-linear finite element modelling. Three packages have been selected: silicon CSP, underfilled flip chip and QFN package. Also the effect of thermal cycling conditions has been investigated. • Comparing the induced inelastic strains in the solder joint, the lead-free SnAgCu generally scores better thanks to the lower creep strain rate. • This paper only discusses the reliability of the solder joint making the connection with the FR4-board. 2011/3/22 2 版權所有 翻印必究 Experimental thermal cycling tests of PSGA package • In literature, first results in thermal cycling tests for leadfreeassembled components are published [2–4]. These results depict that there is no general conclusion about the trend in life time from SnPb to SnAgCu. The main conclusion is probably that the trend is very dependent on the package type but also on the applied loading conditions (Tmin, Tmax, dwell and ramp-up time). Lead-free solder materials are more creep resistant at high temperatures resulting in higher life time under similar stress conditions for the solder joint. However, the lead-free solder materials have a higher elastic modulus, which can result for certain packages in much higher stress conditions. 2011/3/22 3 版權所有 翻印必究 2011/3/22 4 版權所有 翻印必究 • This section gives the constitutive model for SnPb and SnAgCu, which will be used for the comparison FEM study. • All trends show that SnAgCu has a higher elastic modulus and a lower creep strain rate than SnPb (Fig. 4). 2011/3/22 5 版權所有 翻印必究 E-modulus SnPb [7]: E =35366 -151T For SnAgCu Darveaux proposes to use the SnAg elastic modulus [7]: E =52400 -193.05T with T in C, E in MPa. 2011/3/22 6 版權所有 翻印必究 • For this study, we selected the model of Wiese, which is based on well-described tests on real size solder joints 2011/3/22 7 版權所有 翻印必究 2011/3/22 8 版權所有 翻印必究 • As depicted in Fig. 10, the inelastic strain is the width of the stress–strain hysteresis loop achieved in each thermal cycle. 2011/3/22 9 版權所有 翻印必究 • The inelastic energy is the area of this hysteresis loop. For SnAgCu, the stresses reach much higher values during the temperature cycling, resulting in higher hysteresis loops. Although the inelastic strain for SnAgCu during LC2 is smaller (= width of the loop), the dissipated energy per cycle (= area in the loop) is higher due to the higher stresses. Figs. 11 and 12 shows the hysteresis loops for one normal and one shear stress/strain component and proves the upper statement. 2011/3/22 10 版權所有 翻印必究 FEM simulation for 56 pins QFN package • The third package that is investigated in this study is the quad flat non-leaded (QFN) package, which is nowadays very popular as it is a thermally enhanced package, in particular when the leadframe is also soldered to the FR4 board (Fig. 14). 2011/3/22 11 版權所有 翻印必究 2011/3/22 12 版權所有 翻印必究 • In this FEM, there is no solder applied in the area between the lead-frame and the PCB. • When applying SnAgCu, almost no inelastic strains were induced in the solder. The same conclusion is true when analysing the energy density. The main reason is that SnAgCu can support much higher stresses before creep occurs, and it seems that these higher stresses are sufficient to compensate the thermal mismatch between the package and board. For this package, it seems that the lead-free SnAgCu gives a much higher solder reliability than its SnPb alternative. 2011/3/22 13 版權所有 翻印必究 結論 •錫鉛、錫銀銅比較這兩種銲料，得到錫銀銅 銲料有較低的蠕變應變率，即有較高可靠度。 •在CSP和倒裝芯片封裝，在極端的負荷條件下， 以非彈性耗散密度，作為損傷參數，錫銀銅 銲料表現較差，其主因是低蠕變應變率，當 受到的應力較高時，將導致產生更高磁滯曲 線與多耗能。 •模擬結果，錫銀銅對於QFN封裝較好，幾乎無 蠕變引起。 2011/3/22 14 版權所有 翻印必究 •在熱循環測試中，發現錫鉛不同於錫銀銅銲 料的裂紋擴展，有關應變與能量密度之損傷 參數，當有夠多數據時可證明可靠性。 2011/3/22 15 版權所有 翻印必究 Thank you for your attention 2011/3/22 16