Browse Prior Art Database

Methods and Solder Joint Structure for Avoiding Secondary Reflow

IP.com Disclosure Number: IPCOM000106829D
Original Publication Date: 1993-Dec-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Hayden, TF: AUTHOR [+2]

Abstract

Disclosed is a means of preventing the unwanted secondary reflow of eutectic solder by locating a small amount of palladium, other metal which would raise the liquidus temperature of the solder, in contact with the solder before it is reflowed the first time (primary reflow). This technique is intended for use in direct chip attach applications on FR-4 epoxy glass laminate or in the case of chips attached to a multi-chip molule which is then attached to some other substrate. The palladium may be added to the structure by electroplating or immersion plating on to the substrate joining metallurgy.

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Methods and Solder Joint Structure for Avoiding Secondary Reflow

      Disclosed is a means of preventing the unwanted secondary
reflow of eutectic solder by locating a small amount of palladium,
other metal which would raise the liquidus temperature of the solder,
in contact with the solder before it is reflowed the first time
(primary reflow).  This technique is intended for use in direct chip
attach applications on FR-4 epoxy glass laminate or in the case of
chips attached to a multi-chip molule which is then attached to some
other substrate.  The palladium may be added to the structure by
electroplating or immersion plating on to the substrate joining
metallurgy.

      During the primary reflow, the solder joint is formed by the
solidification of the transient liquid phase (the molten Tin-Lead
solder) which first appears at about 183ºC.  Once the liquid
Tin-Lead appears, the palladium is rapidly dissolved by the molten
lin-lead, forming a ternary tin-lead-palladium alloy.  The melting
point of this alloy is higher than the melting point of the original
tin-lead eutectic solder.

      If such a scheme were used for attachment of the chip to the
chip carrier, the formation of the higher melting point alloy during
the primary reflow would prevent the solder joint from remelting
until temperatures above 240ºC were reached, well above the
temperatures normally encountered in the eutectic solder reflow which
is used to attach the chip carrier to the second...