High volume EMI-shielding process for LGA and BGA components

Abstract

Electro Magnetic and Radio Frequency Interference (EMI/RFI) occur in mobile electronics as high functioning devices with multiple operating frequencies are packaged in dense form factors. The most fundamental protection is through metal shielding to produce a "Faraday Cage" (e.g. a metal box) [1], a structure that absorbs or reflects EMI/RFI and boosts performance. A simple and direct means to shield packages is through physical vapor deposition (PVD). Temporary bonding adhesives are used to affix land grid array (LGA) and ball grid array (BGA) packages onto substrates and sent into the PVD line. The properties of the adhesive must be thermally stable, exhibit reduced sidewall creep, prevent under-side deposition (back spill), and be free of residue. Several adhesive solutions exist for EMI shielding processes [2], they exhibit low outgas to 300°C, and are proven to accept package bow and warp of greater than 30um [3]. LGA and BGA packages are affixed using a sufficient adhesion force that is tuned by mixing different resin molecular weights (MW) and activator amounts, all known to have a direct effect on peel force. Adhesion force has been shown to trend with MW of the resin and seemingly supports affixing, while simultaneous adjusting down activator until tearing begins to produce unwanted residue (Fig. 3). Through tuning, solutions are created that allow rapid sealing of small components with topographies exceeding 300um, metal deposition, and rapid satisfactory removal. These temporary bonding adhesives are formulated to achieve a desired modulus and elasticity to accept small packages, allow rapid processing and support high volume manufacturing. Similar work has been conducted to reduce the bow/warp of interposers during stacking with micro-bumped chips. The C4 bumps are encapsulated and protected during thermal exposure [4]. Simple tapes may support low profile LGAs whereas the high standoff in BGAs can create challenges in proper affixing, sealing, and processing. One solution in affixing packages with topographies is based upon the use of rigid etched or "pocket" carriers [5]. Once believed to be a solution for high-density placement of BGAs, there exist challenges in geometric design, glue placement, execution, and recycling (cleanup). Another possible solution is tape that is patterned using CAD-driven laser tooling, producing pockets within the flexible backing. However, the same flexible plastic backing that provides simplicity and ease creates challenges during high-density placement. Placement on flexible tapes can shift to lower density (increased separation), reduction in throughput, and increased cost of ownership (COO). New affixing introductions include laminates and composites that combine the design targets of rigid carriers with the simplicity of tapes [2]. While activity in affixing technologies remains high, PVD systems are also shifting towards small footprints and lower operating temperatures. While low temperatures may be more compatible with epoxy molding compound (EMC), it is uncertain if this will create a main effect and drive down PVD processing temperatures without affect on throughput. This paper will discuss irregularities in package type and shape, adhesive system, adhesion force, and the entire process execution, including PVD systems.