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Using Folding Adhesives with Polymer Filler Spacers and Conductive Polymers to Maintain Uniform Bond Line Thickness and Enhance Package Reliability in Folded Stack Chip Scale Packages

IP.com Disclosure Number: IPCOM000101589D
Publication Date: 2005-Mar-16
Document File: 4 page(s) / 249K

Publishing Venue

The IP.com Prior Art Database

Abstract

Described in the document below are techniques for reliabile assembly of folded packages. These include a) use of polymer spheres within the folding adhesive of a folded stack CSP package for creation of uniform BLT and b) method to achieve package to package interconnection via the use of conducting polymer based material.

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Using Folding Adhesives with Polymer Filler Spacers and Conductive Polymers to Maintain Uniform Bond Line Thickness and Enhance Package Reliability in Folded Stack Chip Scale Packages

Described in the document below are techniques for reliabile assembly of folded packages.  These include a) use of polymer spheres within the folding adhesive of a folded stack CSP package for creation of uniform BLT and b) method to achieve package to package interconnection via the use of conducting polymer based material.

Background

FSCSP packages use a folding adhesive for folding and adhering the polyimide based substrate to the mold compound cap (see Figure 1). The package-folding concept enables a second package to be stacked onto the first, using solder joint interconnects. Current FSCSP packages use a paste-based adhesive that is dispensed onto the mold surface at room temperature. The folded substrate flap is then folded and adhered, with heat and pressure applied to cure the folding adhesive. The BLT of the folding adhesive is then determined by the volume of the adhesive dispensed, the curing rate of the folding adhesive, and the pressure and temperature applied.

In the FSCSP packaging technology, warpage on the bottom package is significantly high
(i.e. ~ 50 -100 um).  In addition, there is significant unit-to-unit variation, leading to varying warpage levels on the bottom package after singulation. As a result, the volume of the folding adhesive that is dispensed on the mold surface must be adjusted for every unit, depending on the warpage of the bottom package. However, in a high volume manufacturing environment, it is impractical to change the dispense weight of the adhesive for every unit. As a result, the folding adhesive BLT varies unit-to-unit, and often within a unit, leading to high stress points in the package.  The reliability of the package is compromised, leading to high rates of delamination in the pre-conditioning and temperature cycling.

Also, polyimide-based substrates absorb moisture very rapidly and act as a sink for moisture.  During reliability testing (i.e. pre-con + reflow) the moisture expands, leading to package blistering. One failure mode in FSCSP packages is that the bottom package blisters, which leads to package-to-package interconnect shorting. During pre-con, moisture is absorbed in the package. At reflow conditions, moisture diffuses out rapidly leading to bottom package bulging (i.e. blistering). At reflow conditions, the package-to-package interconnect solder is above temperature, and therefore even a slight deformation of the bottom package folded flap leads to solder bridging of the molten solder (see Figure 2).

General Description

In the disclosed method, uniform BLT is achieved by adding polymer spacers in the folding adhesive (see Figure 3).  These polymer spacers are incorporated in the folding adhesive material itself.  The polymer filler spacer does not compress under pressure during the fold and gel...