Effects of Joint Structures and Materials on the Reliability of LTCC/PWB/BGA Assemblies

摘要

The reliability of LTCC/PWB/BGA structures under thermal cycling conditions is known to be affected by several factors the main being the global mismatch of coefficients of thermal expansion between LTCC ceramic and organic PWB substrate. In addition, the stand-off height has great influence on the assembly reliability as well as the stiffness of LTCC and PWB boards. In this study we have examined several other parameters that might influence on the resistance of the joints to thermal cracking. These included the LTCC metallization materials AgPd and AgPt, the printed circuit board material (FR-4 or Hitachi Chemical MCL-E-679F), the use of modified structures in the LTCC joints which are the cavity structure and the large LTCC metallization lands with solder-mask (organic or glass). The LTCC modules (9x9 bumbs) soldered onto PCBs were stressed in thermal cycling tests in the temperature range of -40°C - 125°C. The test boards were examined by resistance measurements of Daisy-chains in LTCC modules, by Scanning Acoustic Microscope (SAM) and representative cross-sections of joints were subjected to a metallurgical examination. The results showed that providing adequate adhesion strength between metallization and ceramic the AgPt metallization with solder mask gives improved reliability of BGA modules against thermal stresses in the harsh test conditions compared to the AgPd metallized modules. Effects of other factors on the failure mechanism, like joint geometry and elasticity of non-collapse ball material of BGA joints, are also discussed in this paper.