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Wave Soldering Defects - Pin Holes
& Blow Holes
Pin holes or blow holes are the same thing and caused
by the printed board outgassing during soldering. Pin and blow
hole formation during wave soldering is normally always associated
with thickness of copper plating. Moisture in the board escapes
through either thin copper plating or voids in the plating. The
plating in the through hole should be a minimum of 25um to stop
the moisture in the board turning to water vapor and gassing through
the copper wall during wave soldering.
The term pin or blow hole are normally used to indicate the size of
the hole, pin being small. The size is solely dependent on the volume
of water vapour escaping and the point the solder solidifies.
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| Figure 1: Blow hole |
The only way to eliminate the problem
is improve the board quality with a minimum of 25um of copper plating
in the through hole. Baking is often used to eliminate the gassing
problems by drying out the board. Baking the board takes the water out of
the
board,
but it does not solve the root cause of the problem.
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| Figure 2: Pin hole |
A simple test to prove if the PCB is causing the problem is outlined here:
Nondestructive Evaluation of PCB Holes
The test is used to evaluate printed circuit boards with plated through
holes for outgassing. It indicates the incidence of thin plating
or voids present in through hole connections. It may be used at goods
receipt, during
production or on final assemblies to determine the cause of voids
in solder fillets. Provided that care is taken during testing the boards
may be used
in production after test without any detriment to the visual appearance
or the reliability of the final product.
Test Equipment
- Sample printed circuit boards for evaluation;
- Canada Bolson oil or a suitable alternative that is optically
clear for visual inspection and can be easily removed after test;
- Hypodermic syringe for application of oil
in each hole;
- Blotting paper for removing excess oil;
- Microscope with top and underside lighting. Alternatively,
a suitable magnification aid of between 5 to 25x magnification
and a light box;
- Soldering iron with temperature control.
Test Method
- A sample board or part of a board
is selected for examination. Using a hypodermic syringe, fill
each of the holes for examination with optically clear oil. For effective
examination, it is necessary for the oil to form a concave
meniscus
on the surface of the hole. The concave form
allows an optical view of the complete plated through hole. The easy
method of forming
a
concave meniscus on the surface and removing excess oil is
to use
blotting paper. In the case of any air entrapment being present
in the hole, further
oil is applied until a clear view of the complete internal
surface is obtained.
- The sample board is mounted over a light source; this
allows illumination
of the plating through the hole. A simple light box or illuminated
bottom stage on a microscope may provide suitable lighting.
A suitable optical
viewing aid will be required to examine the hole during test.
For general examination, 5X magnification will allow viewing of bubble
formation; for
a more detailed examination of the through hole, 25X magnification
should be used.
- Next, reflow
the solder in the
plated through holes. This also locally heats the surrounding
board area. The easiest way to do this is to apply a fine-tipped
soldering iron to the pad
area on the board or to a track connecting to the pad area.
The tip temperature can be varied, but 500°F is normally satisfactory.
The hole should be examined
simultaneously during application of the soldering iron.
- Seconds
after the complete reflow of the tin lead plating in the
through hole, bubbles will be seen emanating from any thin or porous
area in the through
plating.
Outgassing is seen as a constant stream of bubbles, which
indicates pin holes, cracks, voids or thin plating. Generally if outgassing
is seen, it
will continue for a considerable time; in most cases it
will
continue until the heat source is removed. This may continue
for 1-2 minutes; in these
cases the heat may cause discoloration of the board material.
Generally,
assessment can be made within 30 seconds of application
of heat to the circuit.
- After testing, the board may be cleaned in a suitable
solvent to
remove the oil used during the test procedure. The test
allows fast and effective examination of the surface of the copper or
tin/lead
plating. The test may be used on through holes with non
tin/lead surfaces; in the
cases of other organic coatings, any bubbling due to
the coatings will cease within a few seconds. The test also provides the
opportunity to record
the results both on video or film for future discussion.
Wave Soldering Defects:
Pin/Blow Hole | | | | |
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