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Method for reducing brittle solder joints using protruded BGA pads

IP.com Disclosure Number: IPCOM000028045D
Publication Date: 2004-Apr-21
Document File: 3 page(s) / 347K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for reducing brittle solder joints using protruded ball-grid array (BGA) pads. Benefits include improved functionality and improved design flexibility.

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Method for reducing brittle solder joints using protruded BGA pads

Disclosed is a method for reducing brittle solder joints using protruded ball-grid array (BGA) pads. Benefits include improved functionality and improved design flexibility.

Background

BGA packages conventionally uses flat pad that with electroless nickel immersion gold (ENIG) surface finish (see Figure 1).

These packages are very susceptible to solder joint reliability failure. The reliability issue that affects these packages is brittle solder joint failure, which caused by horizontal separation and cracking at interface between intermetallic compound (IMC) and nickel-phosphorus (NiP) layer. (see Figure 2).

Brittle solder joint failure typically occurs when the solder joints experience flexing due to board bending during processing/handling. The failure is more severe when a defective pad surface finish exists, such as mud cracking, blowholes, and solders resist undercut (see Figure 3 and 4).

Defective pad surface finishes and solder-resist undercutting act as stress raisers and stress concentration points that accelerate failure when the BGA joint is stressed. Brittle type of fracture is an abrupt crack, which propagates horizontally across the joint, resulting in a sudden open joint.

Conventional solutions to prevent brittle solder joint failure include the following:

•         Implement adequate board support fixtures during the board process, such as through-hole heatsink insertion, stiffening feature installation on the board, and test guidelines to minimize board bending and flexing. These containment measures create design constraints, inconvenience in manufacturing, and added cost.

•         Optimize and tighten the process control of ENIG process to obtain defect-free surface finishes. This solution is not foolproof as ENIG is a complicated wet process and not all process parameters can be effectively controlled.

•         Change the solder resist sidewall profile by optimizing the solder resist curing and etching process from undercut to taper cut. It reduces the opportunity of failure but does not eliminate it.

•         Control stress distribution by optimizing the sizes of the BGA solder resist pad opening, the BGA solder ball, and the motherboard metal pad opening.

General description

         The disclosed method includes a protruded section on the BGA pad to arrest or redirect horizontal crack propagation to vertical crack propagation. By stopping or diverting the ho...