CA1227908A - Solder resist paste and method of soldering - Google Patents

Abstract

SOLDER RESIST PASTE AND METHOD OF SOLDERING
Abstract of the Disclosure A solder resist paste having the feature that it is relatively easily removed is disclosed. The paste is comprised of a liquid polyglycol (e.g. Pluronic* 25R2 by BASF) and either Kaolin or talc powder. In operation, the paste is selectively applied to holes in the PCB which it is desired to keep open during a soldering process. The paste is then washed out of the holes, additional components mounted via those holes, and soldered into place.

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Description

1~7~8 This invention relates generally to solder resists and more particularly to a solder resist that can be readily removed.
Background ox the Invention In the manufacturing of electronic devices it is well known to mount electrical or electronic components to a PUB (printed circuit board) and to wave (or flow) solder the components to the board.
Some of the components to be mounted to the PUB are more heat sensitive than others and they cannot withstand the high temperatures involved with the wave soldering process. As a consequence, these components are not initially placed on the PUB; rather, the other components are placed on the PUB and the PUB is wave ordered. Next, the heat sensitive components are placed on the PUB and are hand soldered into position.
This procedure has at least one drawback. During the initial wave soldering stage the holes that are to accept the heat sensitive components become blocked with solder. As a result, it is difficult if not impossible, to insert the leads of the heat sensitive components.
To overcome this problem, a temporary solder resist is applied to the PUB, at those locations that components will not be soldered during the initial wave soldering step. After the wave soldering step, and before inserting the heat sensitive components, the temporary solder resist is removed. The heat sensitive components are then inserted and hand soldered Summary of the Invention The present invention is directed to producing a temporary solder resist paste that is easy to remove. In one embodiment the :~Z;~79~8 solder resist is composed of 3.5 liters of 25R2 (a Pluronic* R
polyol by BASS) and 4.0Kg of Kaolin. More details of the Pluronic* R
polyols can be found in US. patent No. 3,036,118.
In other terms, the present invention is a solder resist paste comprising approximately 45% to 50% by weight of a polyglycol and the remainder being either Kaolin or talc.
Stated in yet other terms, the present invention is a method of soldering components to a PUB (printed circuit board), the method characterized by: selectively applying a temporary solder resist comprised of a polyglycol and either Kaolin or talc powder to at least one hole in the PUB; selectively situating first components on -the PUB
either on the surface of the PUB or into holes other than the holes containing the solder resist; soldering the first components to the PCBj washing the PUB with a solvent in which the solder resist is soluble and which is not harmful to either the PUB or the first components;
selectively situating second components on the PUB employing the holes that had previously held the solder resist; and soldering the second components to the PUB.

The present invention is a material composed of a Pluronic* R polyol in combination with either Kaolin or talc (or some combination thereof). The preferred form has the proportions of 3.5 liters of 25R2 (by BASS) to 4.0Kg of Kaolin. An alternative form is 3.5 liters of 25R2 and 4.0Kg of talc. This produces a material having a specific gravity of approximately 1.45. The constituent components by percentage weight are approximately 48% 25R2 (since it has a specific *Trademark ',~

~2~9~3 gravity of approximately 1.039) and 52% Kaolin.
The material is formed by combining the 25R2 in liquid form with either powdered talc or powdered Kaolin. Mechanical mixing at room temperature is all that is required. The end result is a paste that can be applied to the PUB.
The resulting solder resist material is a thixotropic paste, having a consistency, in its preferred embodiments, not unlike that of toothpaste It can be removed by washing with freon TAMS or by washing with hot water (with or without a detergent).
lo The paste can be applied to a printed circuit board either by a screening method or by dispensing from a caulking cartridge using a pneumatic system. In its preferred form it has a specific gravity of approximately 1.45 and a viscosity of approximately 400,000 to 500,000 cups (Brook field viscometer, spindle = TO, speed = 10 RPM).
The range of the constituent materials is not critical and can vary between approximately 45% and 50% polyglycol depending upon the desired viscosity of the resultant paste.
In operation, the paste made according to the present invention is inserted into selected holes on the PUB. The paste does not normally cover the pad (or land) area surrounding the hole; consequently, during the initial wave soldering operation the pad becomes coated with solder, but the solder does not block the hole.
After the initial wave soldering step, the PUB is washed with a fluorocarbon solvent (e.g. freon TAMS) to remove the temporary solder resist material. The hole in the PUB is now free to receive the lead of a component for mounting purposes.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A solder resist paste comprisisng:
approximately 45% to 50% by weight of a non-ionic surfactant polyol and the remainder being either Kaoline or talc.

2. The solder resist paste of claim 1 wherein said non-ionic surfactant polyol is a liquid Pluronic* R polyol or chemical equivalent.

3. The solder resist paste of claim 1 wherein said non-ionic surfactant polyol is Pluronic* 25R2 or chemical equivalent.

4. A solder resist paste comprising a mixture of Pluronic 25R2 and Kaoline in the proportions of 3.5 litres to 4.0 kilograms, respectively.

5. A solder resist paste comprising a mixture of Pluronic 25R2 and talc powder in the proportions of 3.5 litres to 4.0 kilograms, respectively.

6. A method of soldering components to a PCB (printed circuit board), said method characterized by:
selectively applying a temporary solder resist comprised of a non-ionic surfactant polyol and either Kaoline or talc power to at least one hole in said PCB;

selectively situating first components on said PCB
either on the surface of said PCB or into holes other than the holes containing said solder resist;
soldering said first components to said PCB;
washing said PCB with a solvent in which said solder resist is soluble and which is not harmful to either said PCB or said first components;
selectively situating second components on said PCB
employing holes that had previously held said solder resist; and soldering said second components to said PCB.

7. The method of claim 6 wherein said non-ionic surfactant ployol is a liquid Pluronic* R polyol or chemical equivalent.

8. The method of claim 6 wherein said non-ionic surfactant polyol is Pluronic* 25R2 or chemical equivalent.