Composition and method for removing conformal coatings

A composition and method for removing conforming coatings such as polyurethane or epoxy from printed circuit boards is disclosed. The composition comprises separate base and activator components which are mixed to form the coating remover. The base component comprises methylene chloride or toluene, or a mixture thereof, preferably together with a thickening agent. In general, the activator component comprises (a) from about 75 to about 85 parts methylene chloride or toluene, or a mixture thereof, (b) about 7 to about 25 parts lower alkyl alcohol, and (c) about 2 to about 8 parts acetone or methylethylketone. For epoxy conformal coatings, the activator component preferably comprises 85 parts of (a): about 7 to about 15 parts of (b); and about 2 to about 6 parts of (c). The activator component for removing polyurethane conformal coatings preferably comprises 75 parts of (a), about 15 to about 25 parts of (b); about 2 to about 8 parts of (c). The base component is usually mixed with the activator component in a ratio of from 1:1 to 10:1, depending on the thickness of the coating to be removed. The composition is used by combining the base component with the activator component, treating a conformal coating on a printed circuit board by applying the coating remover thereto, and removing the treated conformal coating. Ultraviolet light accelerates the reaction between the conformal coating and the composition of the present invention.

BACKGROUND OF THE INVENTION 
This invention relates to removal of conformal coatings from printed 
circuit boards, and, in particular, to a composition and method for 
removing polyurethane and epoxy coatings from printed circuit boards to 
permit and facilitate repair thereof. 
A conformal coating, as that term is used herein, is a coating applied to a 
printed circuit board to protect the components and circuitry from 
moisture and contaminants, and, to some degree, from mechanical shock, 
vibration, shorting, and tampering. Although a large number of coating 
compositions have been used as conformal coatings, polyurethane and epoxy 
coatings are the most common. When properly applied, the conformal coating 
effectively surrounds the entire printed circuit board and the components 
thereon. 
Typical printed circuit boards are formed of woven glass fibers impregnated 
with an epoxy resin and having etched or printed metal cladding on one or 
both sides defining electrical circuits. Holes are drilled in the circuit 
board to receive components, which are soldered into place and into 
electrical contact with the conductive metal circuits on the board. 
Conformal coatings are becoming increasingly popular in consumer and 
business electronics products. However, their principal use is in 
applications in which the printed circuit board is likely to be subjected 
to moisture, shock, and abuse. 
In order to effectively fulfill their function, conformal coatings must be 
impervious to water and most common solvents and not susceptible to ready 
removal. These properties, although desirable from the standpoint of 
protection of the circuit board and components, create significant 
problems in effecting repair of circuit boards containing defective 
components. This problem is discussed at length in U.S. Pat. No. 4,451,523 
to Native, et al. Epoxy resins have been virtually impossible to remove 
chemically because any solvent that would remove the epoxy coating would, 
as a general rule, also damage the epoxy-resin impregnated circuit board 
and the potted electronic components on the circuit board. 
The thickness of conformal coatings may vary from application to 
application. A conformal coating remover that can satisfactorily remove a 
10 mil conformal coating may damage the board when used to remove a 2 mil 
conformal coating. The prior art has provided no effective solution to 
this problem. 
As a result, repair of boards with epoxy conformal coatings has been a 
time-consuming and labor-intensive process. The only satisfactory method, 
to date, for repairing such coated boards is to physically abrade or 
scrape the coating off of the soldered junctions to permit desoldering and 
removal of defective components. The scraping or abrading processes for 
mechanically removing the conformal coating carry with them the 
possibility of damaging the components or the circuits on the board. If a 
circuit is cut, the board must be discarded. 
Polyurethane coatings are also insoluble in most common solvents. Corrosive 
strippers which can remove the cured polyurethane coating will, as a 
general rule, also corrode metallic surfaces on the board. Thus, 
polyurethane coatings, like epoxy coatings, are typically removed by 
mechanical processes such as scraping and abrading to permit rework of the 
board. 
There have been great advances in recent years in automated in-circuit 
testing equipment for printed circuit boards. Such automated testing 
machines can analyze and test in the neighborhood of 250 boards per hour. 
However, because of the dielectric properties of conformal coatings, these 
machines cannot be used on coated boards without first removing the 
conformal coating. Ironically, this step is still performed by hand, one 
board at a time. 
Accordingly, there is a significant need for compositions which can 
satisfactorily remove epoxy and polyurethane conformal coatings without 
damaging the printed circuit board or the components thereon. One attempt 
to provide a conformal coating solvent mixture is described in U.S. Pat. 
No. 4,383,867 to Elwell. This patent discloses a mixture of 70% 
dichloromethane, 20% dimethyl formamide, and 10% methanol that is said to 
be useful in removing polyurethane conformal coatings from printed circuit 
boards. This single-part composition merely softens the polyurethane 
coating, which must still be scraped off. Unfortunately, it also softens 
the underlying resin-impregnated board. In addition, this composition is 
not suitable for use on epoxy-coated boards. 
Another single-part composition for removing conformal coatings is 
disclosed in U.S. Pat. No. 3,625,763, to Melillo. This composition is said 
to loosen the resin in a few minutes, with complete removal occurring over 
a two-hour period. Two hours, of course, is much too long for practical 
utility. A practical conformal coating remover will remove conformal 
coating in 10 minutes or less, preferably in five minutes or less. 
Accordingly, it is an object of the present invention to provide a 
composition and method for effectively removing cured epoxy and 
polyurethane conformal coatings from printed circuit boards. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the invention, there is provided a material for removing 
conformal coating from a printed circuit board, comprising a base 
component and an activator component in separate containers, said base 
component and said activator component forming an active conformal coating 
remover when mixed, wherein: 
said base component comprises methylene chloride or toluene or a mixture 
thereof; and 
said activator component comprises (a) about 75 to 85 parts by volume 
methylene chloride or toluene, or a mixture thereof, (b) about 7 to about 
25 parts by volume lower alkyl alcohol, and (c) about 2 to about 8 parts 
by volume acetone or methylethylketone. 
In a preferred embodiment of the present invention, there is provided a 
composition for removing a conformal coating from a printed circuit board, 
preferably an epoxy conformal coating, comprising a base component of 
methylene chloride or toluene or a mixture thereof, and an activator 
component comprising (a) 85 parts by volume methylene chloride or toluene 
or a mixture thereof, (b) 7 to 15 by parts by volume lower alkyl alcohol, 
and (c) 2 to 6 parts by volume acetone or methylethylketone. The preferred 
composition for epoxy conformal coating removal has a base component 
comprising methylene chloride and an activator component comprising (a) 85 
parts by volume methylene chloride, (b) about 9 to about 13 parts by 
volume, and preferably about 11 parts by volume lower alkyl alcohol, and 
(c) 3 to 5 parts by volume, and preferably about 4 parts by volume 
acetone. 
In accordance with another preferred embodiment of the present invention, 
there is disclosed a composition for removing a conformal coating from a 
printed circuit board, preferably a polyurethane conformal coating, having 
a base component comprising methylene chloride or toluene or a mixture 
thereof, and an activator component comprising (a) 75 parts by volume 
methylene chloride or toluene or a mixture thereof, (b) 15 to 25 parts by 
volume lower alkyl alcohol, and (c) 2 to 8 parts by volume acetone or 
methylethylketone. In a preferred embodiment, the base component comprises 
methylene chloride and the activator component comprises (a) 75 parts by 
volume methylene chloride, (b) about 18 to about 22 parts by volume, and 
preferably about 20 parts by volume lower alkyl alcohol, and (c) about 4 
to about 6 parts by volume, and preferably about 5 parts by volume 
acetone. 
Preferably the base component in the compositions of this invention include 
a thickener. The preferred thickeners are pectin, cellulose thickeners, 
and silica thickeners. 
These two-part systems for removing conformal coating from printed circuit 
boards provide significant advantages over the one-part systems heretofore 
used. These advantages include the ability to tailor-mix the base and 
activator components to remove any particular desired thickness of 
conformal coating, as well as an apparent increase in stability under 
normal working conditions when the base and activator components remain 
separated. Accordingly, the present invention contemplates a conformal 
coating removal system, comprising a base component and an activator 
component in separate containers for mixing in a desired ratio. 
In accordance with yet another aspect of the present invention, a method 
for removing conformal coatings, preferably epoxy or polyurethane, from a 
printed circuit board is disclosed, comprising the steps of forming an 
active coating remover composition by combining the base component and the 
activator component of one of the coating remover compositions disclosed 
above, treating a conformal coating on a printed circuit board by applying 
the active coating remover thereto, and removing the treated conformal 
coating. 
Thus, one embodiment of this invention provides a method for removing a 
polyurethane or epoxy conformal coating from a printed circuit board, 
comprising the steps of: 
obtaining the base component and the activator component of the conformal 
coating remover system described above; 
selecting a ratio of base component to activator component in the range of 
about 1:1 to about 10:1 by volume, wherein the selection of the ratio is 
based on the thickness of the conformal coating to be removed; 
forming an active conformal coating remover composition by combining the 
base component and said activator component in the selected ratio; 
treating a conformal coating on a printed circuit board by applying the 
active coating remover thereto; and 
removing the treated conformal coating. 
The action of the active coating remover may be accelerated and improved by 
exposing the treated conformal coating to an ultraviolet light source. 
Between about 1 and 15 minutes after treatment, and preferably after about 
5 minutes, the treated conformal coating can be loosened by scrubbing and 
removed by rinsing with water. The ultraviolet light preferably has a 
wavelength of 180-235 nm. An intensity of 110 microwatts per square 
centimeter of board is particularly preferred. When the light is spaced 
one meter from the work, the conformal coating may be removed in as little 
as one minute. 
The present invention also includes a method for removing conformal 
coatings of various thickness, by varying the ratio of base component to 
activator component. Thus, in most instances, a 6 mil coating may be 
removed by applying a mixture of four parts base component with one part 
activator component. This ratio is increased for thinner coatings and 
decreased for thicker coatings. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Unlike prior art compositions, the composition of the present invention is 
a system, preferably two-part, that rapidly removes epoxy and/or 
polyurethane conformal coating without damaging the printed circuit board 
itself. The composition can be rinsed off of the board with water, 
together with the treated conformal coating. 
The first component, or base component, of the present composition 
comprises methylene chloride, toluene, or mixtures thereof. The base 
component also preferably includes a thickening agent. The purpose of the 
thickening agent is to increase the viscosity of the composition, so that 
localized application of the conformal coating remover composition is 
possible. Localized application is not only more economical, it also 
prevents contact between the remover and electronic components that could 
be damaged thereby. The particular thickening agent used is not critical, 
so long as the thickener is compatible with the other ingredients of the 
composition. A large number of conventional thickeners are known. Such 
thickeners include starches, gums, casein, gelatin, phycocolloids, 
semi-synthetic cellulose derivatives (such as carboxymethyl cellulose, 
cellulose acetate butyrate, cellulose acetate propionate, cellulosic acid, 
and modified cellulose), pectin, carrageenan, polyvinyl alcohol, 
carboxy-vinylates, bentonite, silicates, and colloidal silica. Of course, 
the properties of the various thickeners and their relative advantages and 
disadvantages are well known to those of ordinary skill in the art. Those 
thickeners that also have emulsifying and suspending properties are 
particularly valuable because they facilitate water removal of the 
composition. Preferred thickening agents are pectin, cellulose acetate 
butyrate, colloidal silica such as that sold by Cabot Corporation under 
the trademark "Cab-O-Sil", and vinyl polymers such as those sold by B. F. 
Goodrich Company under the trademark "Carbopol". 
The amount of thickener in the base component is not critical, and 
obviously depends to a large extent on the particular thickener used. The 
optimum amount of any particular thickener can be readily determined by a 
person of ordinary skill in the art. Of course, the amount of thickener is 
also dependent on the manner in which the coating remover is to be applied 
to the boards; e.g., by squirting, brushing, or dipping, and is also 
dependent on the orientation of the board (vertical or horizontal). For 
most applications, the thickener will comprise between 2 and 16 percent 
(by volume) of the base composition, and usually about 8 percent by 
volume. The preferred consistency is a soft gel that will not run when 
applied to the board in modest quantities. 
There are three essential ingredients in the activator component of the 
present composition. If desired, the activator component may consist 
exclusively of these ingredients. The first ingredient is methylene 
chloride or toluene. Methylene chloride is preferred. 
The second ingredient of the activator component is a lower alkyl alcohol. 
As used herein, the term "lower alkyl alcohol" means alcohols containing 
from 1 to about 5 carbon atoms, such as methanol, ethanol, the propanols, 
the butanols, and the pentanols, and also diacetone alcohol. Methanol, 
ethanol, propanol, and butanol are preferred. 
The third ingredient of the activator component is acetone or 
methylethylketone. Acetone is preferred. 
In general, the activator component comprises (a) from about 75 to about 85 
parts by volume methylene chloride or toluene, or a mixture thereof, (b) 
about 7 to about 25 parts by volume lower alkyl alcohol, and (c) about 2 
to about 8 parts by volume acetone or methylethylketone. 
The composition of the activator component is varied depending on whether 
an epoxy or a polyurethane coating is to be removed, as described below. 
The activator component for removing epoxy conformal coatings comprises 85 
parts by volume of methylene chloride or toluene (or a mixture thereof), 
about 7 to 15 parts by volume of a lower alkyl alcohol, and about 2 to 
about 6 parts by volume acetone or methylethylketone (or a mixture 
thereof). The preferred composition contains from about 9 to about 13 
parts, and preferably about 11 parts by volume lower alkyl alcohol, and 
from about 3 to about 5 parts acetone or methylethylketone, preferably 
about 4 parts by volume. 
The activator component for use on polyurethane coatings contains 75 parts 
by volume of methylene chloride or toluene, 15 to 25 parts by volume of a 
lower alkyl alcohol, and 2 to 8 parts by volume of acetone or 
methylethylketone. The preferred composition contains from about 18 to 
about 22 parts, and preferably about 20 parts lower alkyl alcohol, and 
from about 4 to about 6, and preferably about 5 parts acetone or 
methylethylketone. 
Compositions within these ranges, and particularly within the preferred 
ranges, exhibit markedly superior properties in comparison to compositions 
outside those ranges. Although the effects of the various components are 
interrelated, the amount of alcohol appears to be particularly important. 
The composition is used to remove conformal coatings having roughly an 8 
mil thickness by combining 75 parts base component with from about 18 to 
about 32 parts activator component, preferably in a ratio of about 3 to 1. 
This forms an active coating remover composition. The activity of the 
coating remover is time-dependent, and the most satisfactory results are 
achieved by using the active coating remover within about 2 or 3 days 
after formulation. 
The present invention may be used to remove conformal coatings of varying 
thicknesses by varying the ratio of base component to activator component. 
By varying this ratio, the activity of the conformal coating remover can 
be altered. In this way, the user may formulate an active conformal 
coating remover composition that will effectively remove the conformal 
coating without harming the board. In other words, the removal of a 
specified thickness of conformal coating exhausts the properly-compounded 
active conformal coating composition so that no damage to the board 
occurs. 
The ratio of base component to activator component may vary from about 1:1 
to 10:1, and preferably from about 2:1 to 6:1. Thus, for a 10 mil coating, 
an effective active conformal coating remover composition is made by 
combining the base and activator components in the ratio of 2:1. For an 8 
mil conformal coating, the ratio is preferably 3:1; for a 6 mil coating, 
the ratio is 4:1; for a 4 mil coating, the ratio is 5:1, and for a 2 mil 
coating, the ratio that is preferred is about 6:1. 
The ideal ratio of base component to activator component for any particular 
coating can be readily determined by testing various ratios of base 
component to activator component on an innocuous part of the coated board. 
The ratio of 3:1 will ordinarily be used as a starting point. 
The active coating remover is applied to the printed circuit board by any 
suitable method, such as by dipping one or both sides of the circuit board 
into the composition, squirting the composition onto the circuit board, or 
painting the composition on the desired area. For spot removal of the 
conformal coating, the best method is to squirt a small amount of the 
composition onto the coating to be removed. A conventional squeeze bottle 
has proved satisfactory for this purpose. Many electronic components on 
printed circuit boards are encapsulated in epoxy, polyurethane, or other 
polymer material, so, in order to avoid damage to these components, it is 
best to avoid applying the remover directly to the components. 
When automatic board testing equipment is to be used, it is generally 
preferable to remove the conformal coating from the entire back side of 
the board. Manual or automated techniques for rolling, brushing, or 
spraying the composition onto the board may be used. 
After the activated composition has been applied to the coating, it rapidly 
reacts with the coating. The particular compositions of the present 
invention exhibit very precise depth and duration properties. They have 
been formulated to rapidly remove the outer conformal coating layer, yet 
become inactive as the coating layer is penetrated and the composition 
contacts the resin of the printed circuit board. 
The speed with which the composition removes the coating can be 
significantly accelerated by exposing the treated board to light, 
preferably ultraviolet light. Although an ultraviolet light source of 100 
watts at 1 meter is particularly effective, good results are also seen 
with ordinary incandescent light. The intensity of the ultraviolet light 
and the wavelength of the ultraviolet light have significant effects. The 
preferred wavelength corresponds to short-wavelength ultraviolet, in the 
range of 180-235 nm. A 500 watt ultraviolet light is notably more 
effective than a 100 watt ultraviolet light, permitting removal of the 
conformal coating in one minute or less. 
After the composition has worked on the conformal coating for an 
appropriate period of time, generally between about 1 minute and about 15 
minutes, and preferably about 5 minutes, the treated coating is scrubbed 
to loosen it and is then rinsed off with ordinary tap water. 
The method of the present invention effectively and completely removes the 
conformal coating from the treated area of the board. Usually a thin layer 
of the outer "gel coat" of the board itself is also removed; however, this 
gel coat is only cosmetic, and the etching, as it were, does not extend 
into the fibers of the board or in any way weaken or damage the board. In 
contrast, ordinary solvents tend to simultaneously soften both the 
conformal coating and the resin comprising the board itself, so that when 
the coating is effectively removed, the board is softened and damaged.

EXAMPLE 1 
An epoxy conformal coating remover is formulated as follows: 
Base Component: 
92% by volume methylene chloride 
8% by volume cellulose acetate butyrate 
Activator Component: 
85% by volume methylene chloride 
11% by volume ethanol 
4% by volume acetone 
Three parts by volume of base component were combined with one part by 
volume activator component, and the composition was mixed by shaking. The 
resulting active coating remover composition was applied to an epoxy 
conformal coating on a printed circuit board with a squirt bottle and the 
treated area was exposed to sunlight for 4 minutes. The conformal coating 
was then loosened with a stiff brush, and the board was rinsed with water. 
The treated area was smooth and clean and showed no traces of conformal 
coating. Although the surface pattern of the fibers in the board was 
visible in the treated area, the fibers were not exposed and the resin 
impregnating the fibers was not softened or otherwise damaged. Electronic 
components were easily removed from the treated area by desoldering. 
EXAMPLE 2 
A polyurethane conformal coating remover was formulated as follows: 
Base Component: 
92% by volume methylene chloride 
8% by volume cellulose acetate butyrate 
Activator Component: 
75% by volume methylene chloride 
20% by volume methanol 
5% by volume acetone 
Three parts by volume of base component were combined with one part by 
volume activator component, and the composition was mixed by shaking. The 
resulting active coating remover composition was applied to a polyurethane 
conformal coating on a printed circuit board with a squirt bottle and the 
treated area was exposed to sunlight for 4 minutes. The conformal coating 
was then loosened with a stiff brush, and the board was rinsed with water. 
The treated area was smooth and clean and showed no traces of conformal 
coating. Although the surface pattern of the fibers in the board was 
visible in the treated area, the fibers were not exposed and the resin 
impregnating the fibers was not softened or otherwise damaged. Electronic 
components were easily removed from the treated area by desoldering. 
EXAMPLE 3 
The conformal coating remover composition of U.S. Pat. No. 4,383,867 was 
formulated by combining 70 volume percent methylene chloride, 20 volume 
percent dimethyl formamide, and 10 volume percent methanol. A printed 
circuit board was immersed in the composition as instructed in the patent. 
The treated conformal coating was checked at approximately five minute 
intervals to determine whether the polyurethane conformal coating had been 
sufficiently softened to permit removal thereof. After about 20 minutes, 
the conformal coating could be removed by scraping. The resin of the 
underlying board was softened and damaged by the composition. The paint 
had been removed from the resistors, and many of the potted electronic 
components had been damaged. The insulating varnish on the copper wire in 
inductors on the board had been removed. 
EXAMPLE 4 
The composition of Example 3 was thickened with cellulose acetate butyrate 
until it was approximately the same consistency as the active compositions 
of Examples 1 and 2. A printed circuit board having the same polyurethane 
conformal coating as the board in Example 2 was treated by squirting the 
thickened composition onto a small area on the back of the board. The 
treated board was exposed to sunlight with no apparent effect. The treated 
area was periodically tested to ascertain whether the conformal coating 
could be removed. After approximately 30 minutes, the coating had been 
sufficiently softened to permit removal by scraping. The underlying board 
had also been softened and damaged. The scraping procedure removed not 
only conformal coating, but also part of the resin impregnating the glass 
fibers of the board. 
From the foregoing examples, it is apparent that the novel two-part 
composition of the present invention provides significantly superior 
results over the composition of U.S. Pat. No. 4,383,867, and permits ready 
removal of conformal coating without damage to the printed circuit board. 
EXAMPLE 5 
In order to demonstrate the effect of the two-part composition on the 
effectiveness of the active conformal coating remover, base component and 
activator component for epoxy conformal coating were prepared. The base 
component and activator component were maintained in separate closed metal 
containers. Some of the base component and activator component was used to 
formulate active conformal coating remover composition by combining the 
components in the volume ratio of 3:1. This active composition was also 
placed in a closed container. 
The three containers, respectively containing base component, activator 
component, and active (mixed base and activator) conformal coating 
remover, were stored for a period of several weeks in a storage building 
where the temperature ranged from about 55.degree. F. to about 90.degree. 
F. Additional active conformal coating remover composition was then 
prepared from the base and activator components, and this fresh conformal 
coating remover was compared to the previously-mixed conformal coating 
remover. The fresh conformal coating remover effectively removed an epoxy 
conformal coating from a printed circuit board in under five minutes. The 
old conformal coating composition, on the other hand, required more than 
15 minutes to act, and removal of the conformal coating was not complete. 
The explanation for this loss of activity is not known. Under controlled 
laboratory conditions, when the experiment was repeated in 
carefully-sealed laboratory containers maintained in a 
temperature-controlled environment, the loss of activity noted above in 
the previously-mixed composition did not occur. 
The ordinary commercial user of conformal coating removers, of course, is 
unable or unwilling to exercise the care necessary to store conformal 
coating compositions under laboratory conditions and under controlled 
temperatures. Under such circumstances, a two-part composition would 
appear to reduce the chances of accidental inactivation of the conformal 
coating remover. 
EXAMPLE 6 
The effect of the ratio of base component to activator component and the 
relationship of that ratio to coating thickness is studied by varying the 
ratio on a volume basis to ascertain the optimum ratio for a given coating 
thickness. 
Various conformal coating remover compositions having ratios of base 
component to activator component from 2:1 to 6:1 are applied to a printed 
circuit board having a 6 mil epoxy coating. The conformal coating remover 
composition having the best characteristics for removing this 6 mil 
coating has a ratio of base component to activator component of 4:1. 
Similar results are achieved with a polyurethane conformal coating. 
EXAMPLE 7 
The test of Example 6 is repeated on four other printed circuit boards 
having epoxy conformal coating thicknesses of 2 mils, 4 mils, 8 mils, and 
10 mils. It is determined that the best volume ratio for a 2 mil coating 
is 6:1; the best volume ratio for a 4 mil coating is 5:1; the best volume 
ratio for an 8 mil coating is 3:1; and the best volume ratio for a 10 mil 
coating is 2:1. 
EXAMPLE 8 
A conformal coating remover composition is prepared having a volume ratio 
of base component to activator component of 3:1. This composition is 
applied to a printed circuit board having an 8 mil epoxy coating. After 
approximately five minutes at 70.degree. C., the coating has been loosened 
and may be removed with water. The same composition is then applied to 
another part of the same board. The board is then exposed to a 500 watt 
short-wave ultraviolet light at a distance of one meter. The conformal 
coating may be removed after approximately 45 seconds. 
This demonstrates the acceleration of the chemical reactions involved in 
the conformal removing system of the present invention when exposed to 
ultraviolet light.