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Backbonding an Integrated Circuit to a Heat Sink

IP.com Disclosure Number: IPCOM000087698D
Original Publication Date: 1977-Mar-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Rivenburgh, DL: AUTHOR [+2]

Abstract

In packaging a large-scale integrated circuit, it is sometimes necessary to bond a silicon wafer to a heat sink. One way of accomplishing this is to solder the wafer directly to the wafer carrier. One problem with this technique is that thermal cycling can cause cracks in the solder bond. Eventually, these cracks propagate through the entire solder backbond and seriously impact the reliability of the wafer assembly.

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Backbonding an Integrated Circuit to a Heat Sink

In packaging a large-scale integrated circuit, it is sometimes necessary to bond a silicon wafer to a heat sink. One way of accomplishing this is to solder the wafer directly to the wafer carrier. One problem with this technique is that thermal cycling can cause cracks in the solder bond. Eventually, these cracks propagate through the entire solder backbond and seriously impact the reliability of the wafer assembly.

This cracking can be eliminated by connecting the wafer to the heat sink using a reinforced solder preform.

The preform can be fabricated as follows: 1. A copper mesh screen (pretinned for optimum soldering conditions) is laid on top of a Pb/In solder foil, which is 0.010 inch thick. 2. The components are then pressed together at a pressure of 0.5 tons/in . 3. The Pb/In/Cu mesh foil is then removed from the press and cut into the desired size and shape. The wafer is joined to the wafer carrier or heat sink as follows: 1. The Pb/In/Cu mesh preform is fluxed on both sides and laid on the wafer carrier. 2. The wafer is then positioned over the preform and the assembly is joined using any standard solder reflow technique.

The above solder preforms can be used to join silicon wafers to heat sinks such as KOVAR* or steel.

It is noted that if a microcrack should develop in the solder backbond during the thermal cycling, the copper wire isolates the crack and prevents it from propagating any further. * Trademar...