Abstract
For portable and wearable devices, having good functionality and being lightweight are a must. To improve the functionality of the device, System-in-Package (SiP), Package-on-Package (PoP), Multi-Chip-Module (MCM), or Chip-on-Wafer-on-Substrate (CoWoS) are good packaging options. These devices and their printed circuit boards (PCBs) are designed to be thin and lighter for many applications and the severe warpage is one of the serious reliability concerns. The warpage has caused reliability issues such as soldering defects, bridging, stretching, head-in pillow, and non-wet opens. The warpage is caused by the CTE mismatches of the materials under temperature excursion during the manufacturing process of the package. Therefore, a good selection of materials or operating temperatures may help mitigate the deformation. Unfortunately, this type of warpage control is very difficult and complex for SiP, PoP, or McM packages because there are many components to be soldered together using different processing temperatures.
To address the serious issue of warpage, we have developed a reflowable jig holder which can hold an array of MCM packages. Our jig holder is different from traditional ones that use end-clamping or only have clamp force in the inactive regions outside the functional package areas. Our reflowable jig holder implements magnets to apply forces on both inactive and active regions. With a Sm-Co magnet inside the reflowable jig at the bottom and a ferromagnetic cover on the top, we were able to better control the warpage. Experimentally, we have designed, fabricated, and tested four different magnetic jigs. Two different magnets having different diameters and weights, and two jigs with different thicknesses and materials were used. The magnetic flux densities and magnetic forces as a function of components’ structure or material were analyzed to optimize the performances.
After the manufacturing of the packages with our innovative reflowable magnetic jig, the amount of warpage and shape of the package were inspected by Thermal Shadow Moire (TSM). The integrity of solder joints was inspected by X-ray and cross-section analyses. We have compared the warpage data of the packages using our innovative reflowable jig with those using a traditional reflowable jig. The experimental results show that the number of magnets and their configurations impact not only the local warpage but also the overall warpage. A reflowable jig with stronger magnetic force results in smaller warpage of the package. In conclusion, our innovative magnetic reflowable jig is a good method to control the warpage of these complex packages during the reflow process.
