“Can I Order a Double Stack?”
SINGAPORE - In a bid for the mobile handset market, STATS ChipPAC developed a method of stacking 3-D packages, creating a package-on-package (PoP) with up to seven stacked die. The technology allows for up to two stacked die - most likely logic die or a processor - in the bottom package and up to five - typically various types of memory - stacked in the top package.
“Since memory dice in mobile applications tend to generate little heat, thermal management between die in the top package is not a major concern,” said Flynn Carson, advanced packaging manager at STATS ChipPAC. “And then you have a natural air gap and insulating properties of the package molds between the packages, which buffers the top stack from heat generated by the logic or microprocessor in the bottom package.” This makes the PoP concept more attractive than a 3-D package encompassing seven stacked die. Additional thermal support comes from the heat-dissipating properties of chosen substrates, underfill, or other materials. Die in both packages are wire bonded.
Carson sees the mobile handset market as lucrative and tailored to the possible benefits of PoP technology. “We target primarily the cell-phone market,” said Carson, “this sector is going to grow a lot in the next two years, and the top five makers are headed toward PoP integration for high-end products.”
The company, which already produced the bottom package, introduced and ramped production on the top package at the end of 2006. Since both packages are fully tested prior to assembly, they present no more concerns in terms of function than a single-die package. STATS ChipPAC also controls package design, bill of materials (BOM), and other parameters to ensure compatibility between the packages to be stacked. This service can simplify the global supply chain operated by most, if not all, handset manufacturers, added Flynn, though customers can mix-and-match with other packages from other providers.
The handset market presents two major technical challenges for the seven-die PoP: minimal package height, and reliability for the countless times an end user will say “Oops I dropped my phone!” in the product’s lifecycle. However, the PoP performed well in drop tests, according to Carson. He explained that reaching the 1.4-mm mounted height of the PoP (in certain die combinations) required thinning the die, substrate, and package mold on the top package, while compressing the cap and substrate of the bottom package is less aggressive. “Mold-cap thickness on the bottom helps determine the package-to-package air gap that provides thermal control,” he said, “the substrate that contacts the PCB is maintained at a thickness that prevents warpage.” Controlling warpage increases reliability, as does attention to die attach and other materials.