This is an area which should be looked at in detail. At first site it would seem
perfectly straight forward to have  via holes filled with solder to prevent all
the possible problems that via holes can cause with the surface mount process
like printing, reflow, testing etc.

Via hole filling can work but not when you also have outgassing from the board
due to thin plating in the via holes. I had this problem recently with a
customer and even made a video showing it happen. The major issue is on double
sided SMT boards if the outgassing occurs the solder in the vias expand leaving
balls on the opposite side of the board making screen printing of paste
impossible. If the plating in the hole is like a string vest, lots of holes it
may explode as suggested.  Some of the new direct plate processes do suffer from
outgassing even with the minimum copper thickness of 25um. A test procedure for
outgassing is included below for any people to have a go.

My customer had to ask his production staff to remove all the balls on the vias
so that screen printing could take place on the second side prior to placement
and reflow. It also caused test problems where the via locations were used as
test points. Reworking all the vias a horrible job for any one.

This new process problem will be included on the second addition of my
interactive "Bob Willis Process Defect CD ROM" with the photographs and video
clips as its a nice new industry problem. I will also cover it at Nepcon West in
my workshops.

Just have the via holes covered with solder resist depending on the size of the
vias 90% will fill or cap preventing the need for solder fill. 

Outgassing may be a old problem that is still out there today ready to get us
again.

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.

TEST EQUIPMENT

1.Sample Printed Circuit Boards For Evaluation

2.Microscope with top and underside lighting. Alternatively a suitable
magnification aid of between 5 to 25x magnification and a light box.

3.Canada Bolson oil or suitable alternative which is optically clear for visual
inspection and can be easily removed after test. 
(The oil is the material used for oil immersion lenses on microscopes available
from any supplier)

4.Suitable hypodermic syringe for application of oil in each hole and blotting
paper for removing excess oil.

5.Soldering iron with suitable temperature control.

The test method is all shown on a video tape from a well known supplier.

Bob Willis
Process Engineering Consultant
Electronic Presentation Services
2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England.
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Home Page: http://ourworld.compuserve.com/homepages/Bwillis
Email: [log in to unmask]

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