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Method for post assembly BGA coplanarity reduction

IP.com Disclosure Number: IPCOM000009296D
Publication Date: 2002-Aug-14
Document File: 5 page(s) / 177K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for post assembly ball-grid array (BGA) coplanarity reduction. Benefits include improved reliability.

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Method for post assembly BGA coplanarity reduction

Disclosed is a method for post assembly ball-grid array (BGA) coplanarity reduction. Benefits include improved reliability.

Background

      Conventional BGA solder balls are round (see Figure 1). Substrate warpage increases ball coplanarity and creates issues for substrate attachment to motherboards (see Figure 2). As ball coplanarity increases, the substrate does not have sufficient seating on the motherboard surface, resulting in open electrical connections.

      Conventional solutions include reducing package size and ball counts to limit sources of ball coplanarity variation. External stiffeners are added to offset warpage and drive reduced ball coplanarity.

              Thermal and mechanical stresses change the flatness of the substrate. A change in die/package size yields different substrate flatness values. Substrate flatness and ball coplanarity are closely coupled.

General description

      The disclosed method reduces substrate ball-grid array coplanarity post assembly processing through mechanical grinding or chemical polishing (flattening) of a BGA tips (see Figure 3). The method enables high coplanarity parts to meet industry standards for minimum coplanarity for substrate-to-motherboard assembly.

      The key elements of the method include:

•             Standard substrate assembly and test

•             Complete die attach, solder ball attach, and test

      Prior to packaging and shipment, the units are placed in required fixtures and processed for ball-grid array coplanarity reduction. Ball-grid array coplanarity reduction consists of a unit or group of units constrained at or near the substrate surfaces or edges. A mechanical grinding or chemical polishing surface is placed adjacent to substrate balls. Controlled pressure and depth provides a coplanarity of ball-grid array that meets minimum assembly requirements.

      The process may be utilized as a rework-only process.

Advantages

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