SOLDER RESIST PASTE AND METHOD OF SOLDERING

PATE RESISTANT A LA SOUDURE ET MODE DE SOUDAGE

English 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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Claims

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.

Description

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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

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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



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*Trademark

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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.