Solder paste test print
Bob Willis
“Testing first-off paste prints without contaminating the PCB, its simple and does not leave any
mess!”
Solder paste test print
What are paste test prints? Well, before you
print a board with solder paste, you should
print a sample to establish the quality of
printing. I have often heard the statement
in SMT production lines world wide, “Our
second print is always better,” so why print
the first board when it’s not necessary? If
you print a sample board and the quality of
the print is not satisfactory, or outside your
criteria, technicians will take the board and
clean it prior to reprinting. These boards
are referred to as wash offs and should
always be marked on their edge for future
reference in production and tractability.
Where ever possible, cleaning
should be avoided, as it’s difficult to do
successfully and is very likely to leave solder
balls on the surface of the board. You will
find solder balls in through holes, vias
and also lodged in solder mask windows.
This will be obvious with manual cleaning
and may also be found during semi
automatic cleaning of assemblies. It can be
a nightmare after first side build, so why
Figure 1. Example of a poorly printed board after manual wash off!
do it?
There are simple and practical ways
of printing a first off without the paste
provided on a reel and can be spliced to Turning the board over allows
touching the board surface. Using a low
specific widths by the supplier. A section of confirmation of successful coverage
tack clear plastic film on the PCB surface
the roll is placed on the surface of a table without any lifting or crinkles in the film
of the printed board allows successful
or flat surface. A sample board is placed that may impede successful printing due to
printing to be confirmed prior to starting
on the tacky surface of the film. A knife is the uneven surface lifting the stencil. The
production. In the UK, the film is
used to remove the film overhang. board can then be loaded into the printer
Figure 2. Examples of test prints on different component packages using the film overlay.
4 – Global SMT & Packaging – August 2009 www.globalsmt.net
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