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Plated Through Hole - Procoat Surface Geometry to Reduce Copper Dissolution During Rework

IP.com Disclosure Number: IPCOM000120772D
Original Publication Date: 1991-Jun-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 57K

Publishing Venue

IBM

Related People

MacGregor, A: AUTHOR

Abstract

Component rework is an essential process in an electronic manufacturing environment. The process utilizes a high, thermally conductive, liquid medium, such as solder, which is pumped through a chimney to the component rework site of a card or board. During the direct contact of the fountain solder with the plated through hole (PTH) solder, extensive dissolution of copper present in the PTH occurs. Presently, the need to ensure rework capability drives the initial plated copper thickness. However, increasing the copper thickness is limited by throughput and line width tolerance requirements.

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Plated Through Hole - Procoat Surface Geometry to Reduce Copper Dissolution
During Rework

      Component rework is an essential process in an electronic
manufacturing environment. The process utilizes a high, thermally
conductive, liquid medium, such as solder, which is pumped through a
chimney to the component rework site of a card or board.  During the
direct contact of the fountain solder with the plated through hole
(PTH) solder, extensive dissolution of copper present in the PTH
occurs.  Presently, the need to ensure rework capability drives the
initial plated copper thickness.  However, increasing the copper
thickness is limited by throughput and line width tolerance
requirements.

      The disclosed process presents a new method utilizing
PTH-procoat surface geometry to address the problem of copper
dissolution.  A modification to the geometry of the bottom surface of
the card, by either recessing the land relative to the board surface
or by increasing the thickness of the procoat, can significantly
decrease the rate of copper dissolution during rework.  By changing
the procoat surface geometry at the copper land (Fig. 1), the solder
flow dynamics are modified with a reduction in the Cu-Sn reaction
rate.  A 3-mil layer of vaccryl resulted in copper dissolution being
held to 0.2 mils after eight reworks.  Fig. 2 is a statistical
summary of copper dissolution vs. rework and demonstrates the
uniformity of the benefit of surface geometry modification. Re...