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Method for a pin heat insulator

IP.com Disclosure Number: IPCOM000009496D
Publication Date: 2002-Aug-28
Document File: 7 page(s) / 344K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a pin heat insulator. Benefits include improved reliability.

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Method for a pin heat insulator

Disclosed is a method for a pin heat insulator. Benefits include improved reliability.

Background

        � � � � � Solder joints are conventionally comprised of several layers (see Figure 1):

1.        � � Wave solder heatsink (WSHS) pin

2.        � � Copper (Cu) plating
3.        � � Silicon (Sn) or silicon-lead (Sn-Pb) plating

4.        � � Diffusion zone between solder from wave and pin plating

5.        � � Solver from wave
6.        � � Copper-silicon (Cu-Sn) intermetallic layer
7.        � � Plated through hole (PTH) barrel plating
8.        � � FR4 board laminate

        � � � � � A WSHS is an integrated design for flip-chip ball-grid array (FCBGA1) packages with an electroless nickel immersion gold (ENIG) surface finish. This solution incorporates both a chipset thermal solution and the mechanical strength of the BGA solder joint at the package corner (with four anchoring points). As the WSHS undergoes wave soldering, the heatsink transfers heat from the pins (which are hot from contact with wave solder) to the fins (which are cooler). This heat transfer has led to a lower pin temperature (see Figure 2). The temperature is less than 70oC, very much lower than the solder melting temperature, 183 oC. As a result, solder only wets with IMC formation on a hotter surface, such as PTH Cu plating. The pin-plating layer is still bonded mechanically, without intermediate metallic conduit (IMC), as the material is still below its melting temperature. The layers formed in this solder are brittle, and two weak interfaces exist.

        � � � � � During the qualification, parts fail, showing solder-joint cracking on WSHS pins that result in loose heatsinks (see Figure 3). Dewetting symptoms occur on the loosed WSHS pins (see Figure 4). The dewetting symptom and cracking is observed on assembled joints. Conventionally, no solution to this problem exists.

        � � � � � Mechanical bonding is a joint without intermetallic formation. It is weak and easily broken by mechanical or thermo-mechanical force.

        � � � � � Pure (100%) Sn has a high melting point, 231°C. Reaching this temperature is not achievable in the conventional process. As a result, a lower temperature solder (such as eutectic) is required.

                                � � � � � � � � � � � � � � � � � � � � � � � � �

General description

        � � � � � The disclosed method is a pin heat insulator that is a solution to WSHS solder-joint dewetting and cracking. The method ensures sufficient heat to achieve the melting point of WSHS pin coating material. In addition, the method ensures the solde...