Integrated solder preform array having a tin outer coating

An integrated solder preform array is composed of a metal or metal alloy suitable for use in a soldering or bonding process. The preform includes a series of discrete solder islands disposed in a desired matrix. Each island contains a hole for receiving a pin, and bridging strands for joining each of said islands in a matrix. The bridging strands are formed of the same solder material as the islands, with the solder preform containing a substantially uniform coating of tin over its entire outer surface for the purpose of improving solder wetting and therefore elimination of unwanted shorting of circuits and other electrical components being soldered.

BACKGROUND OF THE INVENTION 
The invention relates to solder preforms for use in joining one object to 
another, and more specifically to an integrated preform array (preform) 
used for joining electronic components to connectors, circuit board 
terminals and other elements. 
U.S. Pat. No. 5,242,097 teaches that a solder preform once applied to 
terminals or pin components to be soldered, will not become easily 
dislodged when the assembly is handled prior to the soldering operation. 
More specifically, U.S. Pat. No. 5,242,097 provides an integrated solder 
preform array in which individual bridging strands of solder cojoin solder 
islands positively, and reliably separate in the midregion of the strands 
so that the separated halves retract to adjacent solder islands in a 
positive and uniform fashion. 
The preforms of the type taught by U.S. Pat. No. 5,242,097 typically are 
made of an alloy which, when exposed for any significant period of time to 
the atmosphere, tend to form surface oxides. The formation of these 
surface oxides sometimes creates problems by impeding the proper wetting 
of the solder, in-turn preventing the strands connecting the integrated 
preform array islands from efficiently separating and retracting to the 
preform islands. When this occurs, an electrical short can be caused due 
to the unwanted electrical contact between adjacent individual pins. 
It can therefore be seen that there is a need for an improved integrated 
solder preform array which provides for ease of wetting during the 
soldering operation, and for a clean break of the strands connecting each 
solder preform island. Such a preform would eliminate the problem of 
unwanted shorting of circuit boards or other electrical components being 
soldered. It is, therefore, an object of the present invention to overcome 
the problems noted above. 
SUMMARY OF THE INVENTION 
The above and other objects of the present invention are accomplished by 
providing a thin tin coating or film over the solder preform after the 
final manufacturing step in making the preform. It has been found that a 
thin tin coating over the solder preform significantly reduces the 
formation of unwanted oxides. These oxides have been found to prevent or 
make wetting during the solder operation difficult or inconsistent from 
preform to preform. 
The formation of the tin coating on the solder preform may be carried out 
by any convenient method. The preferred method involves immersion coating 
the preform array in a bath having a suitable source of a tin component. 
Other methods of coating the surface with tin include electrolytic 
deposition, sputtering, etc.

DETAILED DESCRIPTION OF THE INVENTION 
Referring initially to FIG. 1, there is shown an integrated preform array 
10 which contains a plurality of washer shaped solder islands 12, which in 
one embodiment, would be equal in number to the number of pins in a pin 
array more fully described in U.S. Pat. No. 5,242,097 which is 
incorporated herein by reference. Certain solder islands 12 may be 
equipped with a tab 16 that extends radially into the hole area 18 of the 
island so that the tabs will frictionally engage pins that are passed into 
the selected holes. The washer shaped islands are joined together by 
bridging strands 14 which are optionally notched at 20 to provide a 
reduced solder area at the midregion of each strand 14. 
As will be more fully described herein, and as illustrated in FIG. 2, the 
entire preform 10 contains a thin tin coating or film 22 over its entire 
outer surface. 
A preferred method of applying the tin coating to the solder perform is by 
immersion coating in a tin containing bath formulation. A suitable bath 
formulation for use in the present invention is set forth below. In the 
table, the concentration of the five components are listed with their 
quantities required in a liter of deionized water. 
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IMMERSION TIN BATH 
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Stannous Chloride 20 g/l 
Thiourea 75 g/l 
Concentrated hydrochloric acid 
50 m/l 
Sodium Hyposphosite 16 g/l 
Wetting Agent Triton X-100* 
0.5 ml/l 
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*Commercially available from J. T. Baker Inc. of Phillipsburg, New Jersey 
Typically a tin outer coating of about 0.75 to 1.30 microns has been found 
to be satisfactory. A preferred coating of about 1.20 microns has been 
found to be particularly suitable. 
The bath is generally maintained in a temperature range of about 80 to 
150.degree. F. and the preform immersed in the bath for about 5 to 20 
minutes. The solder preforms, and method of manufacturing the preforms is 
specifically set forth in applicant's U.S. Pat. No. 5,242,097 which is 
incorporated herein by reference. 
The present invention is applicable to any metal or metal alloy suitable 
for use in soldering or bonding. Typical metals and alloys include 63% Sn, 
37% Pb; 62% Sn, 36% Pb, 2% Ag; 96.5% Sn, 3.5% Ag; and 60% In, 40% Pb; all 
in weight percent. 
While this invention has been explained with reference to the structure and 
method described herein, it is not confined to the details set forth in 
this application and is intended to cover any modification or changes that 
may come within the scope following the claims.