Well I had to put my two pence in to the debate. Blow holes are not caused by
component terminations or poor wetting of the plated through hole. If any one
thinks this they are misunderstanding what the term pin hole or blow hole or
their customer is. Read the following and test your own boards it make the whole
issue clear cut.

My training video on the suject has been available in the UK for five years.

The subject of outgassing, as it relates to printed circuit assemblies during
the flowsoldering operation, has been an area of debate for many years. Until
recently the soldering equipment always received the blame for pin holes and
blow holes; in recent years the source of the problem has now been established.
It is true to say that any soldering system would have to be operated well
outside effective soldering limits with poor quality control for outgassing to
occur.

Over the years, a number of articles have been published which have put forward
many suggestions as to the cause of the outgassing phenomenon. It was not until
the involvement of the National Physical Laboratory (NPL), as part of the
Soldering Science and Technology Club (SSTC), that any detailed scientific work
could be co-ordinated at a national level. The club worked closely with major
companies and with trade groups like the Joint Telecommunications and Components
Working Group.  The work of the joint activity highlighted the following points
which have been shown to affect the degree of outgassing.

Blowholes and pin holes are caused by the escape of gas through the plated
copper hole during the soldering operation. The gas is water vapour which is
super heated during the soldering operation and escapes through the solder
during its molten state. Voids are formed when the gas continues to escape
during the solidification of the solder joint. On examination outgassing may
result in pin holes, blowholes or sunken joints.

Outgassing can be eliminated if the copper plate is uniform in its thickness and
is impervious to the gas escaping. This depends on many factors like the quality
of drilling, the effective deposition of the electroless copper and the correct
thickness of electroplated copper to resist the flow of escaping moisture.

The drilling may cause a problem due to the irregular surface that the copper
will be required to cover and the need to bridge glass stands protruding into
the through plating. This may provide voids which will allow continued gassing
during the soldering operation.

Outgassing can be reduced during the soldering operation if the printed boards
are baked prior to the soldering process. The baking operation must be conducted
just prior to the assembly and soldering operation if it is to be effective in
removing the moisture from the board. The boards must be baked at the correct
temperature for the correct time duration to be effective. It should be noted
that all glass epoxy is hydroscopic and will take on moisture during storage.
This means that boards which have been baked need to be used if the baking
operation is to be effective.

The incidence of outgassing can be eliminated by correct specification of the
printed circuit board. Most national specifications quote a minimum thickness of
copper plating in the through hole connection. The specification quotes a
minimum of 20um but allows voids to also be present in the plating; this is, of
course, a minimum standard which circuit companies in the main far exceed in
terms of quality.

Many electronics companies can be accused of double standards. In the case of
printed circuits they request boards to be manufactured to the national
specification or, in some cases, have a company document which is slightly
adapted from those specifications. In this situation they are implying that, in
the worst case, they will accept boards which will outgass during soldering.

On the other hand the soldering standards operated in production, or imposed by
a customer for second party manufacture, do not allow pin holes or voids so they
are required to rework the solder joint to improve the cosmetic appearance. In
no case has it been shown that touch up of solder joints improves the quality of
the solder joint but in many cases the reverse has been shown; damage can be
caused through repeated rework.

A simple non-destructive method of evaluating printed boards which may be
conducted at goods receipt or during production is available on VHS video tape.
Generally if outgassing is seen it will continue for a considerable time, in
most cases until the heat source is removed; in cases during video filming the
gassing has continued for over 1 min. The complete oil test and the results
which may be obtained are shown on the video tape, the tape shows how the test
is performed and the outgassing taking place.
 
Where testing is conducted on a large sample size of boards a range of results
will be seen. It is difficult in most cases to establish the size of the problem
in the actual board but in situations where considerable gassing is seen it is
easy to judge the effect it would have on manufacture. The method of test is
very useful for demonstrating the phenomenon of water vapour escaping from both
thin and voided copper plating. It can also demonstrate the likelihood of solder
voids being seen in production. The test could be used as a criteria for the
need for pre baking of the board prior to soldering but at best it can only be a
guide.

The test has been successfully used in many companies to examine the effect of
other manufacturing processes; its greatest benefit is its simplicity and its
non destructive nature. Suggestions for its adoption as a standard test are
unfortunately limited as who will specify the number of bubbles formed in a
given time, the size of each bubble and the rate of escape?


NON DESTRUCTIVE EVALUATION OF PLATED THROUGH 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 METHOD

A sample board or part of a board is selected for examination. Each of the holes
for examination are filled with oil; provided the oil is optically clear both
visual inspection and photography may be undertaken.

The oil is injected into each hole using a hypodermic syringe which makes it
easy to perform and limits excess oil from being applied to the board. For
effective examination it is necessary for the oil to form a concave meniscus on
the surface of the hole to be examined. 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 trapment 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.

The next stage of the test method is to reflow the solder in the plated through
holes which also locally heats the surrounding board area. The easiest method 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 500oF 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 to 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.

The test and examples are all included in an EPS video at L39.95 if you want to
check your own boards.

In the case of delamination if boards are made to the highest standards they do
not delaminate. If boards are of a lower quality and use the current standards
for thin plating, voids in plating PTH or voids in the internal layer as a
reference for manufacture or cause moisture will form and delaminate.

Here end the lesson