Plated through-hole via cracking in laminated substrate of heat slug PBGA packages

Multilayer laminated substrate is widely used for molded Plastic Ball Grid Array (PBGA) packages for higher density packages. In a substrate, plated throughhole via functions as the electrical connection between each metal layer. Because of material property mismatch at substrate level (between plated copper, via fill material and core, etc) and that at package level (between substrate, mold compound, and die, etc), via cracking could be found when PBGAs gone through thermal loadings. Now, a new version of PBGA package emerges in electronics packaging industry, in which a copper-alloy heat slug (H/S) is embedded into the molded body to increase package thermal performance. However, introduction of such material may also bring some new reliability issues because new mismatch factor is being introduced into the package. We encountered intermittent open failures in the later stage of Temperature Cycling (TC) test (-55°C to 125°C; air to air) for 35 mm heat slug PBGA. Cross-section pictures showed barrel cracks in the laminated BT substrate, at the center portion and the knee of the through hole. Interestingly, all the failed vias were located at the rim of heat slug. We ran TC test (-55°C to 125°C; liquid to liquid) 1000 cycles for bare substrate with new via plug material β and thicker copper. It was confirmed that no failure was found in open short testing and there was a reduction of number of micro-cracks as compared to via filled material (α. Package level testing on two heat slug PBGA parts passed 1000X TC. To understand the via plug effect, FEA studies (using 2-D non-linear axisymmetric model) were done on substrate level. The emphasis was placed on the via plugged material. This paper shows that substrate configurations are significant in package reliability. These factors are especially important for heat slug PBGA since the embedded H/S is proven to have a negative effect on fatigue life particularly for small diameter vias that are located under the H/S rim. This should be taken note of during substrate design stage.