Browse Prior Art Database

Method for a roller apparatus to improve coplanarity of solder balls on BGAs

IP.com Disclosure Number: IPCOM000008967D
Publication Date: 2002-Jul-26
Document File: 4 page(s) / 249K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a roller apparatus to improve coplanarity of solder balls on ball grid arrays (BGAs). Benefits include improved reliability and improved yield.

This text was extracted from a Microsoft Word document.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 50% of the total text.

Method for a roller apparatus to improve coplanarity of solder balls on BGAs

Disclosed is a method for a roller apparatus to improve coplanarity of solder balls on ball grid arrays (BGAs). Benefits include improved reliability and improved yield.

Background

      Different size solder balls cannot be used on a BGA package because solder balls are mostly spherical and co-planarity between balls must be within 8 mils to be assembled in surface mount technology (SMT) manufacturing. Increased solder volume is required in certain areas of a ball grid array where high thermal coefficient of expansion (TCE) mismatch causes high solder joint stresses. Specifically between the silicon and the substrate package.

      Conventional BGA packages that do not meet coplanarity specifications are either scrapped or reworked during assembly. Conventional 1-mm pitch BGA packages specify ball heights between 28 and 19.4 mils. Solder balls can vary over 8 mils on a package. Yields on flip-chip ball grid array (FC-BGA) package manufacturing lines are about 98% with about 2% of parts either scrapped or reworked. The disclosed method removes the requirement to rework solder balls due to co-planarity because the cold rolling process makes all the balls within 1-mil coplanar of each other. Smaller solder ball pitch packages (with a distance of one ball to the next in a grid) require thinner solder paste to prevent solder paste bridging between balls and to enable solder-screen apertures not to plug. With the conventional method, new silicon products may not migrate from 1.27-mm pitch to the cost effective 1.0-mm pitch FC-BGA.

      Conventionally, smaller pitch packages require tighter co-planarity specifications than 1-mm pitch BGAs with tighter solder ball specifications from the supplier. Tighter solder ball specifications lead to and lower supplier yields resulting in higher production costs.

General description

      The disclosed method is a cold rolling process for BGA solder balls after BGA package assembly. The method enables different size diameter of solder balls (up to 25% larger) to be assembled on BGA packages and enables larger solder joints on the package and board-to-solder joint interface. The larger solder joints in high stress areas under the silicon increase the solder joint strength.

      The disclosed method reduces the scrapping and reworking of BGA components and increases permanent channel attachment (PCA) assembly yields. Rolling solder balls improve package ball coplanarity to within .001 inch. This better package coplanarity improves printed circuit assembly yields solder ball opens.

      The disclosed method also enables smaller-pit...