Robert Willis says:

>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.

You are quite correct in saying this, IMO.

>The subject of outgassing, as it relates to printed circuit assemblies during
>the flowsoldering operation, has been an area of debate for many years.

This used to cause me periodic headaches. Fortunately technology has
overtaken the matter and now PTH connections are a small percentage of the
total in my Company's products.

>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.

In my experience it had a 1.0 correlation with rough drill and non-smooth
(nodular) PTH walls. I once read a convincing paper showing beautiful
microsections with strands of glass which were not fully wetted by resin in
the cloth plys, and demonstrated a statistical relation to blowhole
incidence in actual boards. Their conclusion was that it was a property of
the core material or rigid laminate used in PWB construction moreso than
drilling or plating weakness.

Anyway, it AIN'T GOING TO BE bad wetting in my forthcoming book (sometime
in the next 40 years after I retire, check your local bookstore).

(Long section describing interesting test method for measuring tendency to
outgas deleted.)

>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.

Even if you have an encyclopaedic specification with enough materials
science and measurement requirements to choke a horse (such as MIL-P-55110)
requiring periodic testing and such, another type of defect not covered, or
some characteristic which renders the product less than perfect will come
up.

It is my general theory that I probably will not know as much about the
board as a user as the producer does. There is a great difference in the
level of sophistication between PWB shops. This is not to say that some
simply equipped shops who maintain a craftsman's attitude do not perform
well, only that a document cannot provide you full protection against
getting inferior boards. My greatest confidence has always been reliance on
a good vendor.

As to your specific case regarding copper thickness Interphase drawings (in
a one-line note) call for a much greater minimum copper thickness than you
mention. We refer generally to IPC-RB-276 class II which seems to me to be
a reasonable specification.

(More stuff about the test method left out for brevity's sake)

>In the case of delamination if boards are made to the highest standards they do
>not delaminate.

We agree, although the term "highest standards" is ambiguous. Assume you
mean best conventional practice.

>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.

As we often reply here in Texas when experiencing a failure to infer any
cohesive ideas from a proffered logical statement: Huh?

Perhaps thin Cu plating would allow penetration of moisture into the hole
wall and further into the laminate?

Electroless copper baths are water based. When the drilled panels are
sumbersed, there must be some tendency for exposed glass fibers to adsorb
moisture, at least until the Cu has begun to coat. When the copper is one
monolayer thick and continuous over the surface, it would no longer be able
to contact the exposed fibers. If the holes walls were especially rough,
the copper may begin to form pocketed deposits with voids which might
entrap liquid from the bath.

The thickness of the copper seems to have little relation to the potential
problem. My thought is the entire structure after various laminating,
drilling, plating operations should be of solid epoxy resin, glass fiber,
and metal. No voids should be present, with all materials having cohesive
strength and adequate adhesion between dissimilar structures.

This is my hypothesis regarding baking and delamination:

With improved resin formulations, resin -> cloth impregnation in the
laminate construction, and use of vacuum laminating presses; conventionally
designed, competently built FR-4 boards of the 90's and later will
withstand much more stress than that imposed by conventional IR/convection
reflow or wave soldering. When such boards less than two years old have
been stored in a controlled environment, there is no need for bakeout prior
to typical assembly operations.

My advice to those who do not share this experience is to review the
possibility that the conditionals in the above paragraph do not exist. It
is based on experience in assembling computer and telecom grade 0.062"
thick digital PCB's, (4-14 layer) in the period 1978-1995. PCB vendors
whose products I have used in the last 10 years include AT&T, Fujitsu,
Hadco, Hallmark Circuits, Hitachi Chemical, Multek, Praegitzer, Tyco Group
(Atlantic Backplane), and Zycon. I have experienced no memorable
delamination problems with any of these makers products. In most cases, no
pre-assembly baking was performed, although Interphase currently does so.

Zycon once gave me a T-shirt, so I may be biased.

I will now join Mr. Einarson in net silence on this topic unless provoked.

cheers,

Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com