Extrudable, non-flowing and non-aqueous solvent soluble hold down compound for printed wiring board assembly

Components can be held down on a printed circuit board without lead clinching by applying an extrudable, non-flowing, heat resistant completely diluent-solvent soluble composition over and around the components to keep them in place during soldering operations. After soldering, the hold down compound is removed by a solvent rinse. The hold down compound comprises a carboxylic acid, a cellulose compound, a solid hydrocarbon resin, a thickening agent, and a non-aqueous diluent-solvent.

This invention relates to a method of assembling components on printed 
circuit boards. More particularly, this invention relates to an 
extrudable, diluent-solvent soluble composition and method of using said 
composition to hold components on printed circuit boards in place during 
soldering operations. 
BACKGROUND OF THE INVENTION 
In a general method of assembling components on printed circuit boards, the 
component leads are mounted through holes punched in the board for that 
purpose and the leads are then clinched, or bent, to hold the components 
in place during subsequent fluxing, lead cutting and soldering operations. 
The leads are soldered to the boards and the excess leads cut off. In many 
cases, the cut lead ends must be resoldered to cover them. These 
operations require hand labor and many steps, which is of course 
expensive. Also, solder is lost on the discharged lead ends. Further, when 
a double soldering step is required, the danger of board warping, due to 
the high temperatures involved in soldering, is increased. 
With the increased complexity of electronic devices, more components are 
mounted on the boards, which creates more problems. The clinched leads on 
the boards interfere with each other, necessitating a stepwise component 
mounting whereby part of the components are assembled, soldered and their 
leads cut off before mounting additional components. 
Further, it is more difficult to remove faulty components which must be 
replaced or repaired when the leads have been clinched, without damaging 
the board or other adjacent components. Still further, access to both 
sides of the board is required. 
Thus, other ways of assembling printed circuit boards components without 
clinching leads have been sought. U.S. Pat. Nos. 2,973,499 and 3,230,612 
describe adaptors or griplets which are fitted into the board holes and 
which receive the lead ends and pin or grip them to hold them in place. 
However, these methods require additional steps and parts which also add 
to the expense of manufacture. 
U.S. Pat. No. 3,540,718 describes a component clamp and jig assembly 
whereby a sliced compressable pad, as of plastic foam, is pressed against 
the components to keep them in place during the soldering and lead cutting 
operation. This method is not entirely satisfactory either because it is 
difficult to maintain a uniform pressure against all components, which may 
differ greatly is size and weight. Also, the pad tends to become 
impregnated with flux and solder after several operations and requires 
extensive cleaning to retain its resiliency. 
Other means of holding the components down by means of pressure have also 
been tried. When all the components are of nearly uniform size and weight, 
a bag loaded with small particles, e.g., beans or plastic shot, placed 
over the components will help keep them in place. However, this method is 
also unsatisfactory when both large and small components must be assembled 
in close proximity to each other, because the smaller components will not 
be held down properly and they may become loosened or even dislodged 
during soldering. 
According to another method that has been tried, a molten wax coating is 
applied to the bottom of the board after the component leads have been 
inserted into the holes. After the wax cools and hardens, it holds the 
components in place while the leads are cut and soldered. This method has 
several advantages; it is particularly adapted to automatic soldering 
operations and the leads can be cut prior to soldering. The solder then 
covers all the lead ends in one operation. In addition, the wax acts as a 
flux for the soldering operation. The problems with the existing wax 
material are that the wax smokes during soldering, which is annoying to 
the operator, and the wax must be removed, either with organic solvents or 
with hot water, which requires a temperature over 150.degree. F. 
(65.56.degree. C.) which is expensive. In some instances also the 
soldering efficiency of the wax is inadequate for automatic soldering 
operations, necessitating the application of a second flux over the wax 
coating. 
Applications Ser. Nos. 771,680, filed Feb. 24, 1977, and 771,679, filed 
Feb. 24, 1977, assigned to the same assignee, solve the above problems 
through the use of an extrudable, non-flowable, completely water soluble 
composition that can be readily applied over the components assembled on 
the printed circuit board to hold the components in place on the wiring 
board without lead clinching during the soldering operation. The 
compositions are suitable for the generally more acidic water soluble 
fluxes and water rinses but, due to insolubility in the non-aqueous 
diluent-solvents, are unsuitable if milder alcohol-soluble rosin fluxes 
and solvent rinses are necessary to prevent component corrosion. The terms 
printed circuit board and wiring board are interchangeable. 
SUMMARY OF THE INVENTION 
Extrudable, non-flowing, completely non-aqueous diluent-solvent soluble, 
low cost compositions, compatible with alcohol rosin fluxes, have been 
found that can be readily applied over components assembled on printed 
circuit boards that will maintain the components in place on the printed 
circuit board without clinching leads during soldering operations. The 
compositions are comprised of a carboxylic acid, a cellulose material, a 
hydrocarbon resin, a thickening agent, and a non-aqueous diluent-solvent. 
These ingredients can be mixed together with the diluent-solvent so as to 
obtain a non-flowing viscosity which can be extruded at room temperature.

DETAILED DESCRIPTION OF THE INVENTION 
The carboxylic acids suitable for use in the invention must be compatible 
with the alcohol rosin flux and the other components in the composition. 
Compatible as employed herein means miscible and able to form a single 
phase system. In addition, the carboxylic acid must be soluble in the 
cleaning diluent-solvent used after the soldering operation. The 
carboxylic acid is employed for its acid constituent which is used to etch 
and clean the contacts which are to be soldered. 
The concentration of the carboxylic acid is variable depending upon the 
consistency of the final composition desired. However, one of ordinary 
skill in the art would recognize that too high a concentration of the 
carboxylic acid would render the composition too fluid and therefore 
unacceptable while too little carboxylic acid would produce a dry 
composition which is not easily extrudable and has insufficient etching 
properties. Generally, the carboxylic acid concentration can vary between 
about 15 and 30 parts by weight and preferably about 20-24 parts by 
weight, based on 100 parts by weight of the total composition. Exemplary 
of suitable carboxylic acids are palmitic acid, levulinic acid, linoleic 
acid, and, preferably, oleic acid. 
A cellulose material is added to provide sufficient film strength to the 
composition. Film strength is defined as the lack of shearing or cracking 
of the extruded composition after removal or evaporation of the 
solvent-diluent. The cellulose material must be soluble in the diluent and 
the cleaning solvent used after the soldering operation. The cellulose 
material tends to increase the resistance of the composition to removal by 
the solvent rinse, i.e., the greater the amount of cellulose material the 
longer the composition will have to be subjected to solvent rinse to 
remove the composition. 
The cellulose material must be compatible with the carboxylic acid. Any 
cellulose material which meets the above criteria is considered to be 
within the scope of this invention. By way of example, if an oleic 
acid-trichloroethane diluent-solvent system were selected, then cellulose 
propionate, cellulose acetate butyrate and, preferably, an ethyl cellulose 
such as Ethocel #7 by the Eastman Kodak Co., would be compatible. In this 
system, methyl cellulose would not be compatible because it is not soluble 
in trichloroethane. This does not preclude the use of methyl cellulose in 
a different carboxylic acid-diluent system. The concentration of the 
cellulose may be varied from about 5-14 parts by weight and preferably 
about 7-11 parts by weight based on 100 parts by weight of the total 
composition. 
The ratio of carboxylic acid to cellulose material gives an indication of 
ease of removal of the composition during the solvent rinsing step. Under 
conditions generally encountered with solder wave machines known in the 
prior art, a ratio of carboxylic acid to cellulose, such as oleic acid to 
ethyl cellulose, of less than about 2.25, while within the scope of the 
invention, will require longer rinse times or higher spraying pressures to 
remove the composition after soldering. 
The hydrocarbon resin is used to control the viscosity of the composition, 
i.e., give body to the composition. Any solid hydrocarbon resin which is 
soluble in the solvent and is compatible with the other components in this 
composition is within the scope of the invention. The hydrocarbon resin 
concentration can vary from about 12-30 and preferably about 16-25 parts 
by weight based on 100 parts by weight of the total composition. The solid 
hydrocarbon resins employed within this invention generally may have a 
minimum viscosity of at least about 25 centistokes as determined by the 
Gardner Holt method of a solution of 75% hydrocarbon resin and 25% 
toluene. 
The solid hydrocarbon resin utilized must permit the final composition to 
be readily extrudable and remain thick, i.e., not fluid, at the soldering 
temperature. An example of a suitable hydrocarbon resin is XL-30 of the 
Velsicol Chemical Corp. The hydrocarbon resin is low in olefinic 
unsaturation and has an average molecular weight of about 3400 and 
viscosity of 36 centistokes as determined by the Gardner Holt method of a 
solution 75% hydrocarbon resin and 25% toluene. 
Other suitable hydrocarbon resins are XL-37, Polyvel G100 and Polyvel G110 
of the Velsicol Chemical Corp; Hercules 6100 of the Hercules Chemical 
Corp.; and Nevchem 100 of the Neville Chemical Company. 
Thickening agents must be added to give the final composition the proper 
yield structure and to control flow. Yield structure is defined as the 
ability to maintain an extrusion shape, i.e., round, square, rectangular, 
at the soldering temperature. If too little thickening agent is employed, 
the composition is too fluid to be extrudable. However, lack of 
extrudability does not preclude the possible use of the fluid composition 
to act as an adhesive after drying to hold down components inserted into 
the circuit board after application of the composition. If too much 
thickening agent is employed, the composition is too stiff to be easily 
extruded at room temperature. Any thickening agent which is compatible 
with the other components of the composition and soluble in the cleaning 
solvent is within the scope of this invention. The concentration of the 
thickening agent may be from about 5-12 parts by weight and preferably 
about 6.5-10.5 parts by weight based on 100 parts by weight of the total 
composition. A suitable thickening agent is glyceryl tris 1,2-hydroxy 
stearate (Thixcin R of the National Lead Co.). Neither the hydrocarbon 
resin nor the thickening agent alone are sufficient to provide the proper 
viscosity and yield structure to impart proper extrusion properties to the 
composition. 
The diluent-solvent can be any commercially available solvent compatible 
with the other components of the composition. Diluent-solvent as used 
herein means any non-aqueous liquid compound(s) which is compatible with 
the composition of the invention and can be used in the rinsing operation 
of the soldering machine to remove the extrudable composition. The 
diluent-solvent chosen should have no solvent or limited solvent power for 
the components on the printed circuit board. In addition, it is desirable 
to choose a diluent-solvent that is sufficiently volatile so as to permit 
the composition to set up relatively quickly. 
The diluent concentration can vary but generally should be between about 
30-50 parts by weight and preferably about 35-45 parts by weight based on 
100 parts by weight of the total composition. One skilled in the art will 
realize that the greater the amount of diluent-solvent employed or the 
less volatile the diluent, the longer the composition will take to set up. 
Set up is defined as the time after extrusion that the composition takes 
to develop adequate holding power. 
Examples of appropriate diluent-solvents are chloroethane, i.e., 
(1,1,1-trichloroethane), tetrachloroethane, and perchloroethane. 
The diluent-solvent is preferably the same as the cleaning or rinse solvent 
for the printed wiring boards. As used herein, the terms cleaning solvent 
and rinse solvent and diluent-solvent are synonyms. 
Within the parameters of compatibility of the components and 
diluent-solvent solubility, the exact proportion of the several 
ingredients are not critical. As used herein, compatibility of the 
components means the composition is able to form a stable single phase 
system. These mixtures can be applied by extruding, as from a hand gun, 
over and around the mounted components. The mixtures do not flow into the 
lead holes, even after standing for several days, or when passed through a 
soldering operation which produces board surface temperatures of about 
150.degree. F. (65.56.degree. C.) or higher. If, upon application, some of 
the composition should flow into the wiring board holes, the composition 
does not impair the solderability of the components to the wiring board. 
These mixtures are completely solvent soluble and readily removed after the 
soldering operation by a rinse of cleaning solvent, without staining or 
corroding the printed circuit boards' surface or the components mounted 
thereon. The mixture is storable for long periods of time and is 
extrudable for at least 3 months. If the composition is stored past the 
useful extrusion life then the composition may be reconstituted by the 
addition of more of the original diluent-solvent or a different but 
equivalent diluent-solvent. 
The compositions are compatible with rosins such as gum rosin, Tenex, Helix 
Rosin and Panbro, (trademarked products of Tenneco Chemical, Newport 
Division), and Pentalyn and Staybelite (trademarked products of Hercules, 
Inc.), etc. 
In the method described of soldering components to printed circuit boards, 
whereby the foregoing mixtures are applied to the mounted component, the 
component soldered and the mixture washed away, an added advantage is that 
the component's leads may be pretrimmed to a uniform length prior to the 
mounting on the boards, thus obviating the need for cutting leads after 
the components are mounted. 
The invention will be further illustrated by the following Examples but it 
is to be understood that the invention is not limited to the details 
described therein. In the Examples, all parts are based on 100 parts by 
weight of the total composition and percentages are by weight. 
EXAMPLE 1 
Part A 
A mixture was made of 21.9 parts of oleic acid, 8.6 parts of ethyl 
cellulose (Ethocel #7 of the Eastman Kodak Co.), 38.1 parts of 
1,1,1-trichloroethane as the solvent and diluent, 21.2 parts of a 
hydrocarbon resin XL-30 of the Velsicol Chemical Corp. and 10.2 parts of a 
thickening agent Thixcin R of the National Lead Corp. The respective 
components were mixed together and allowed to stand for about 1/2 hour to 
permit sufficient diluent to evaporate to allow the composition to set up. 
The composition remains extrudable for at least three months. 
Part B 
The mixture of Example 1 formed a non-flowing, room temperature extrudable 
mixture which is applied to hold down variously sized components with 
pre-cut leads on printed circuit boards during fluxing with Kester #1571 
flux, preheating and wave soldering operations at 260.degree. C. with a 
60% Sn-40% Pb solder. The mixture can be completely removed from soldered 
circuit boards which have been standing for several days by a warm 
solvent, 1,1,1-trichloroethane (Chlorothane) rinsing spray. 
EXAMPLE 2 
A mixture was made of 23.5 parts oleic acid, 9.2 parts ethyl cellulose, 
32.6 parts of 1,1,1-trichloroethane as the diluent and solvent, 26.8 parts 
of a hydrocarbon resin XL-30 of the Velsicol Chemical Corp. and 7.8 parts 
of a thickening agent Thixcin R of the National Lead Corporation. The 
respective components were mixed together and allowed to stand for about 
1/2 hour to permit sufficient diluent to evaporate to allow the 
composition to set up. 
The mixture formed a solvent soluble, non-flowing, room temperature 
extrudable mixture which can be successfully used to hold components down 
on printed circuit boards during soldering operations. 
COMATIVE EXAMPLE 1 
A mixture was made of 18.5 parts oleic acid, 9.2 parts ethyl cellulose 
(Ethocel #7 of the Eastman Kodak Co.) 49.9 parts of 1,1,1-trichloroethane 
as the diluent and solvent, 22.2 parts of a hydrocarbon resin XL-30 of the 
Velsicol Chemical Corp., and no thickening agent. The respective 
components were mixed together and allowed to stand for about 1/2 hour to 
permit sufficient diluent-solvent to evaporate to allow the composition to 
set up. 
The mixture formed a one phase fluid, marginally extrudable mixture which 
required extended drying time to afix components to the circuit board 
prior to soldering, and extended rinsing time after soldering to remove 
the composition. The carboxylic acid to cellulose ratio was 2.01 to 1. 
COMATIVE EXAMPLE 2 
A mixture was made of 21.5 parts oleic acid, 8.6 parts ethyl cellulose 
(Ethocel #7 of the Eastman Kodak Co.), 35.4 parts of 1,1,1-trichloroethane 
as the diluent and solvent, 21.0 parts of a hydrocarbon resin XL-30 of the 
Velsicol Chemical Corp., and 13.5 parts of a thickening agent Thixcin R of 
the National Lead Corporation. The respective components were mixed 
together and allowed to stand for about 1/2 hour to permit sufficient 
diluent to evaporate and allow the composition to set up. 
This mixture contained an excess of Thixcin R. It was extrudable but too 
stiff, i.e., rested on top of the electronic components and would not flow 
around the composition on the printed circuit board. 
COMATIVE EXAMPLE 3 
A mixture was made of 16.9 parts oleic acid, 11.9 parts ethyl cellulose, 
(Ethocel #7 of the Eastman Kodak Co.), 50.0 parts of 1,1,1-trichloroethane 
as the diluent and solvent, 21.2 parts of a hydrocarbon resin XL-30 of the 
Velsicol Chemical Corp. and no thickening agent. The respective components 
were mixed together and allowed to stand for about 1/2 hour to permit 
sufficient diluent to evaporate to allow the composition to set up. 
Without a thickening agent the mixture was too fluid, i.e., not enough of 
the mixture remained on the components at the soldering temperature to 
hold them in place. Extensive solvent washing time was required to remove 
the composition after soldering.