Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for providing solder-rejecting
protection to the conductor paths on the solder side of circuit boards.
In the production of printed circuits the circuit boards are equip-
ped with the associated components and the electrical connections produced
by soldering All the solder joints are arranged in one plane on the so-
called solder side of the circuit boards so that they can be produced sim-
ultaneously in one solder path> for example, by surge soldering or trail
soldering
In soldering, one must distinguish between full soldering and sel-
ective soldering. Full soldering is generally used in connection with cir-
cuit boards of large construction, In a compact construction the use of the
full soldering process would considerably increase the danger of short cir- -
cuits through the formation of solder bridges. Therefore, selective solder-
ing is popularly used for circuit boards of compact construction, in partic-
ular those with miniaturised conductor patterns. In the selective soldering
process the solder side of the circuit boards is covered with a solder-
rejocting solder-stop lacquer. ~nly the required solder seams are not cover- --
ed with solder stop lacquer and thus accept the solder. The solder stop
lacquer is usually applied by the silk screen printing method in which, how- -
ever, the correct application of a printed pattern to the finished conductor
pattern presents considerable difficulties. In addition, the required print-
ing prototypes and printing tools are extremely expensive so that when there ~ -
sro only a small number of boards and a large number of types the application
of the solder stop lacquer by the silk screen printing method becomes un-
econo~ical.
Fro~ tho German Offenlegungschrift 1,640,269 it is known to pro- ~- -
toct tho conductor paths on the circuit boards during the soldering process
by a photo printing procoss, wherein the entire board is covered with a
photo lacquor with the oxception of those paths which are not to be soldered
~0 and which aro caused to reject solder by subsèquent passivation. Due to the - ~-
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good resolution of the photo lacquer the pattern produced by the photo print-
ing can be correctly applied to the conductor pattern. This known process,
however, is extremely uneconomical as it can be executed only with a sub-
stantial outlay and since photo masks which are dependent upon the patterns
are required.
It is an object of this invention to provide a process whereby
solder-rejecting protection may bo applied to the conductor paths of a cir-
cuit board while at the same time leaving certain portions of the circuit
board free to accept solder applied thereto.
It is a further object of this invention to provide a process as
referred to in the preceeding object which is easy to perform and economical -
to carry out.
It is a further object to provide a method for applying solder- -
rejecting protection to selected conductor paths of a circuit board which
does not require the use of pattorn depondent printing tools or masks.
~ . . ,,.,,;.,
Other objects and advantages of the invention herein will become
uore apparent when considering the following description and accompanying -~
drawings.
Figure 1 is an edge view showing a portion of a circuit boart with
conductor paths applied to both sides and with apertures for the reception
of components;
Figure 2 is an edge view of the circuit board of Figure 1 showing
the application of tho auxiliary lacquer to the top of the circuit board;
Figure 3 is an edgo view of the circuit board following the
application of a solder stop lacquer to the solder side;
Pigure 4 is an edge view of the circuit board following the re- -~ :
~oral of the auxiliary lacquer; and
Pigure S is an edge viow of the circuit board following the solder- - -
ing proces~.
The ai~ of the invention heroin is to provide a process for achiev-
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ing solder-rejecting protection of the conductor paths on a circuit board
which process can be carried out economically and without a special outlay,
and in which no pa~tern dependent printing tools or masks are required.
In accordance with the invention, there is provided a process for
the production of a solder rejecting protection for the conductor paths on the
solder side of circuit boards with apertures which are surrounded by solder
edges including the steps of: applying a viscous auxiliary lacquer to the
top surface of a circuit board; pressing the auxiliary lacquer into the
apertures in the circuit board in such manner that menisci of the auxiliary
lacquer cover the solder edges formed on the solder side; providing the
conductor paths on the solder side which are not to be soldered with solder-
rejecting properties; and removing the auxiliary lacquer.
The process of the invention presents a number of advantages in
comparison to the known processes. For example, all matching problems dis-
appear as, irrespective of the position of the apertures in the solder edges,
only the solder edges are ever wetted by the auxiliary lacquer and those paths
which are not to be soldered can be caused to reject solder, completely
irrespective of the pattern. In addition, no expensive pattern-dependent
printing prototypes or masks are required. Thus the economics of the process
of the invention are not dependent upon the number of boards so that it is
particularly suitable for the now usual large number of types, coupled with
small lot sizes. Furthermore, the process of the invention can easily be
automated as it can be carried out with an extremely low outlay in apparatus.
With particular advantage those paths on the solder side which are
not to be soldered are caused to reject solder by passivation. The passivation
may be carried out by oxidizing the surface of the paths or by the applicaSion :
of a solder-rejecting substance.
Preferably, after the application of the solder lacquer, a solder
stop lacquer is applied to the whole of the solder side and is dried and then -
the auxiliary lacquer is removed by a solvent having a selective action, and
the solder stop lacquer is removed fro~ the solder edges. As a result of
the removal of the auxiliary lacquer the solder stop lacquer loses its
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848
adhesion to the solder borders so that in these regions it can easily be
removed. The removal of the auxiliary lacquer and thus of the solder stop
lacquer from the solder borders can, depending upon the nature of the lacquer
systems employed and the layer thicknesses, be made easier by a spray treat-
ment, a light brushing, blowing with compressed air or by the use of ultra-
sonics during or following the treatment with the selective solvent.
When an auxiliary lacquer having a flux additive is used, the
danger of poor solderability of the solder borders in the case of non-optimum
removal, can be reduced.
Figure 1 shows a circuit board which consists of an insulating
carrier 1 to both sides of which conductor paths 2 have been applied. Aper- -~
tures 3, with solder edges 4, are provided for the reception of components. -~
Figure 2 shows the application of a viscous auxiliary lacquer 5
to the top 6 of the circuit board. The application of the auxiliary lacquer
S may be carried out with the aid of a curved doctor 7. The auxiliary lac-
quer S is pressed into the apertures 3 in such manner that initially spher- -
ical drops 9 are formed on the solder side 8 of the circuit board. These
drops 9 then withdraw back into the apertures 3 due to surface tension, to
such an extont that only menisci 10 remain which wet and completely cover
the solder edges 4. To enable this meniscus formation to occur, the aux-
iliary lacquer 5 must have a specific ~iscosity. The required viscosity may
bo detormined expori~entally and depents in part upon the diameter of the
aperturos. After the application, tho auxiliary lacquer 5 which may consist,
for exa~ple, of a water soluble mixture of polyvinylalcohol, gelatine,
glycerine and dye, is dried.
Pigure 3 shows the circuit board following the application of a
solder stop lacquer 11 which covers the whole of the solder site 8. Commer-
cially available solder stop lacquers, for example, a two compo~ent lacquer
havin8 an epoxido base can bo used which are applied by spraying, roller-
coating or similar ~ethods.
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After the drying of the solder stop lacquer 11, the auxiliary
lacquer 5 is completely removed by means of a selective action solvent.
When the auxiliary lacquer 5 has been removed from the solder edges 4, the
solder stop lacquer 11 loses its adhesion in these regions.
Figure 4 shows the circuit board following the removal of the solder
stop lacquer 11 from the solder edges 4. The solder stop lacquer 11 can,
for example, be removed by light brushing. ~hen merely the solder edges 4
are exposed on the solder side 8 and all the other regions have been covered
with the solder stop lacquer 11, components may be applied to the circuit
board and soldered in place.
Figure S shows the circuit board after the soldering. Through
capillary action the solder 12 fills up the entire aperture 3 and thus pro-
duces a durable solder connection between the terminal wires 13 of the com-
ponents and the solder sdges 4.
It will be apparent that I have advantageously provided a process
whereby solder-rejecting protection may be applied to the conductor paths on ~ -
a circuit board which is simple and economical and requires very little in
the way of expensive apparatus. -~
While a preferred manner of carrying out the invention has been ~-
disclosed, it will be appreciated that this has been shown by way of example -
only, and ths invention is not to be limitet thereto as other variations may
be apparent to those skilled in the art and the invention is to be given its
fullest possible interpretation within the terms of the following claims. -~
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