Issue link: https://iconnect007.uberflip.com/i/881969
70 SMT Magazine • October 2017 PBGAs and CSPs are now widely used for many commercial electronics applications, including portable and telecommunication products. BGAs with 0.8−1.27 mm pitches are implemented for high-reliability applications and generally demand more stringent ther- mal and mechanical cycling requirements. The plastic BGAs introduced in the late 1980s and implemented with great caution in the early 1990s, further evolved in the mid-1990s to the CSP (also known as a fine-pitch BGA) having a much finer pitch from 0.4 mm down to 0.3 mm. Recently, fan-out and fan-in wafer level packages have gained significant interest. LGA packages cover a range of counterpart packages from fine pitch CSP to large pitch and high I/O BGAs. Similar to ball array versions, LGAs are surface-mountable. There are no solder balls in LGAs, only land pattern terminations like QFNs. Interconnec- tions are formed using solder paste and reflow during surface mount assembly—reducing the assembly height. This allows for a thinner assem- bly needed for mobile and computing products, especially the RF application that requires lower parasitic noise. LGAs are the preferred packages for applications that require an ideal combina- tion of low device sizes and profiles, and supe- rior thermal and electrical performance. LGAs have negligible internal stray parasitic elements associated with their external solder pads and closeness to PCBs which enable an extremely low thermal resistance to the device. This en- ables maximum heat transfer from the die to the package pads. However, thermal cycle reli- ability has an inverse exponential relationship with solder joint height. Therefore, there is a possible significant reduction in solder joint re- liability. FEA modeling projects lower cycles-to- failure trends for LGAs compare to BGAs, which are also verified by testing 10 . HASL Finish Limitation for WLP1600, 0.3 mm Pitch A test vehicle with various BGAs, sizes, and pitches was designed to determine assembly challenges for mix pitch and sizes as well as their reliability. Two PCB surface finishes were considered—one standard tin-lead HASL and the other with ENEPIG. The most challenging packages for PCB surface finishes and assem- blies were the following two sizes and pitches. • Two FPBGAs with 0.4 mm pitch and 13 mm 2 body size. One of the CVBGA432 components had SnPb solder balls whereas the other had Pb-free SAC305 solder balls. • Two daisy-chain WLPs with 0.3 mm pitch and 12 mm 2 body size. The WLP1600 had Pb-free SAC305 solder balls. Figure 2 shows a section of the PCB that compares the images of daisy-chained pad pat- terns for ENEPIG and HASL. The baseline for the pitch of 1.00 mm is also included. The en- larged sections of WLP1600 are shown for both surface finish conditions illustrated irregular- ity in HASL, and regularity in ENEPIG surface finishes. The HASL shows solder shorts cover- ing four pads. Even the solder dome formation is non-uniform. The ENEPIG finish, however, shows excellent consistency for 0.3 mm pitch and higher. So, the ENEPIG is a clear winner. For 0.4 mm pitch, the HASL finish is more consis- tent even though solder dome formation is still a common feature. ENEPIG: LGA1156 Assembly and Inspection To determine the effects of ENEPIG surface finish under a severe thermal stress condition, RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING Figure 2: The images of HASL (left) and ENEPIG (right) PCB finishes. The HASL finish is unaccept- able for WLP1600 with 0.3 mm pitch, whereas ENEPIG is acceptable.