Performance Assessment on Board-level Drop Reliability for Chip Scale Packages (Fine-Pitch BGA)

Abstract

Board level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, drop test of printed circuit boards (PCB) with four-screw support condition was conducted for a 15mmx15mm fine-pitch Ball Grid Array (FBGA) package assembly with solder ball compositions of 36Pb-62Sn-2Ag and Sn-4Ag-0.5Cu on PCB surface finishes of organic solderability preservative, electroless nickel immersion gold and immersion tin. The results revealed a strong influence of different intermetallic compound formation on soldered assemblies drop durability. The lead-based solder supersedes the lead-free composition regardless of the types of surface finish. Joints on organic solderability preservative were found to be strongest for each solder type. Other factors affecting drop reliability such as component location on the board and thermal aging effects are reported. IntroductionWith the decrease in size of consumer products such as cellular phones, PDAs, and camcorders, the frequency of accidental drops increases and will cause solder joint cracks in the board assembled Integrated Circuits (IC) packages that eventually leads to malfunction of the product. Research and studies have been conducted for the investigation of the reliability performance of IC packages during drop impact test [1][2][3][4][5][6]. Work on developing modeling methodologies for failure prediction of solder joints upon drop test are being investigated [7][8][9]. With the implementation of lead-free solders for the purpose of environmental friendliness, experimentation and analysis focusing on lead-free solder compositions have also been performed and documented [10][11][12][13]. Through finite element modeling and analysis, Wong et al [14] identified three main drivers for board level drop impact failure of interconnections; i) elongation and bending of interconnection due to differential flexing of PCB and package, ii) large inertia force from IC packages, and iii) stress waves generated from impact. There is however a lack of documentation in the use of lead-based (Pb-based) and lead-free (Pb-free) solder compositions on different printed circuit board (PCB) pad surface finishes, and their influence on drop impact reliability. The recent drop test study on a 35x35mm PBGA (plastic ball grid array) package by Chong et al [15] showed that different solder alloy of Pb-based and Pb-free when mounted onto different PCB surface finishes resulted in different intermetallics formation and contributed to different failure sites and mode of failure.

Effect of Zn on the intermetallics formation and reliability of Sn-3.5Ag solder on a Cu pad

“…This is qualitatively consistent with previous results indicating the brittleness of Cu 6 Sn 5 . 31,32 During isothermal aging at 150°C, thickness of the total IMC (␦ total ) at the solder joint increased, as shown in Fig. 6.…”

An eco-friendly Cu-Zn wetting layer for highly reliable solder joints

“…The solder joint interface was weakened as the IMC thickness increased [4,24,25]. The brittle Cu 6 Sn 5 layer could become damaged very quickly under impact loading, induced by high speed stress thus generating a worse drop performance [25].…”

An alternative JEDEC test board design and analysis

“…in test board design and drop test procedure, the drop test results can be compared, even if they are obtained by different suppliers. The standard has been widely adopted in the microelectronics industry and there are a large number of papers on this topic (Chong et al, 2006;Farris et al, 2008;Lai et al, 2006Lai et al, , 2007Lai et al, , 2008Mattila and Kivilahti, 2005;Ng et al, 2005;Syed et al, 2007;Varghese and Dasgupta, 2007). However, the standard test has its drawbacks and there has been continuing development of new test methods, despite the standardization by JEDEC (Wong et al, 2008).…”