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Electrical Conductors with Chromate Solder Barrier and Method of Forming

IP.com Disclosure Number: IPCOM000078514D
Original Publication Date: 1973-Jan-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Ameen, TJ: AUTHOR [+2]

Abstract

Printed-circuit conductors are selectively coated with a chromate conversion film to confine molten solder to selected areas during reflow and thereby prevent wicking.

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Electrical Conductors with Chromate Solder Barrier and Method of Forming

Printed-circuit conductors are selectively coated with a chromate conversion film to confine molten solder to selected areas during reflow and thereby prevent wicking.

Referring to the figures, an insulating substrate 10 is formed with electrically conductive lines 11 thereon to which a circuit device 12, such as an integrated circuit chip, is to be attached.

At selected land areas 13, quantities of solder in the form of mounds 14 are deposited for subsequently attaching the circuit device which usually has corresponding solder mounds 15, although they not required.

The substrate, if it can withstand the temperature, and chip can be passed through a furnace which will raise the temperature of the previously fluxed, solder mounds to the molten state. If the substrate cannot withstand the heat, the device 12 can be held in place while hot gasses are applied to localized areas to melt the solder.

Generally, it is highly desirable to confine the molten solder to the particular land area, and this is done by constructing barrier coatings 17 adjacent the land areas 13.

The process, to be described is that used when the substrate has copper circuit lines thereon and solder mounds have been selectively plated in place at the desired land areas of the lines, such as with plating resists. In the following process all baths are at room temperature and all rinses are in deionized water.

In accordance with step 1 of the process, the substrate with circuit lines 11 and solder mounds 14, is first dipped in a bath of ammonium acetate to remove any coating of lead salts such as sulfates. Treatment may both be necessary if the sulfate has not formed.

Following the dip, the substrate is rinsed in deionized water.

Step 2 of the process is to insure that any copper oxide on the circuit lines is removed, and this is accomplished by approximately a one minute dip in a bath of hydrochloric acid at 5.3 +/- 0.2 Be., also at room temperature. Upon removal from the bath, again the substrate and circuits are rinsed.

Step 3 of the process is that of passivating the tin-lead solder mounds to deter the deposition of any chromate coating thereon. This is accomplished by immersing the substrate, with its circuits and solder mounds, in an ammonium persulfate bath for approximately one minute. The bath is constituted of water, 225 +/- 25 grams per liter of ammonium persulfate, and a 4% solution by volume of sulfuric acid. Immersion is continued for a one minute duration. Thereafter, the substrate is rinsed.

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Step 4 is the application of the chromate conversion coating to the circuit lines. A satisfactory bath is that mixed in accordance with the manufacturers recommendation for KENVERT* No. 31.

This coating produces a heavy chromate film on copper and copper alloys and is mixed in the ratios of 160 of the KENVERT No. 31 solution and 840 cc of water. In addition, 6.5 cc grams of sod...