Pin Hole Reflow or Intrusive Reflow is a technique used to solder through hole
components into plated through holes at the same time as surface mount parts. It
may eliminate wave soldering or significantly reduce manual soldering
operations. 
The following are (FAQ's) Frequently Asked Questions on Pin Hole Reflow or
Intrusive Reflow. I hope that this may be of interest to some engineers please
feel free to add to the text your own points discussion is what makes the world
go "round and round" as the Average White Band used to sing. 

Why reflow solder through hole components?
In many electronic assemblies there are large multi leaded component still being
used in combination with surface mount. If wave soldering is to be eliminated
then either hand soldering, single point automatic soldering or reflow must be
used. The major driving force is manual cost reduction and a simplified process.

How is through hole reflow conducted?
Solder paste is applied by stencil printing to the through holes and to the
surface of the pads. This is conducted at the same time as the surface mount
printing process. The through hole components are then carefully inserted just
prior to reflow or before surface mount assembly to avoid the possibility of
jarring parts if any snap fixings are included on connectors.

Do I need two stencils for though hole printing one for through hole and one for
traditional SMT parts? Depends if you are feeling charitable to your stencil
supplier. Some people have used the technique to increase paste volume.

(Bob's Note) Make sure you tell your stencil supplier that the round apertures
are required in your stencil for through hole printing. They are so often
removing them for customers who have not supplied a solder paste file for the
stencil, those wonderful people may get carried away. I forgot to tell my
stencil manufacturer last week, do as I say not as I do!!!!!!!

How many components can be soldered in this way?
I don't know the answer to the question. Each component needs to be considered
for this process and needs to be assessed and discussed with the component
manufacture, just like immersion cleaning, wave soldering etc. Generally
speaking it is the high pin count devices like connectors, pin grid arrays, post
headers, sockets and dual in line parts that have been specifically produced for
reflow applications.

What is the best use of this technique? 
Back plane or junction boards where you have loads of connectors and
functionality with lots of surface mount components. It can also replace some
press fit designs. 
Its a god send to manual assembly lines.

What will the solder joint reliability be like for reflow joints?
There should be no difference in the solder joint there may be a difference in
the solder volume due to the limitations of the printing process. Just try
ripping a through hole lead out of an existing soldered plated through hole if
you are strong enough. The microsection I have done look good.

Are any voids left in the joints?
Yes you can find voiding due to the reduced escape of gas and other non metallic
materials during paste reflow. Care on setting process parameters will keep this
to a minimum. Voids have been seen to improve reliability not reduce it!!! Think
of honeycomb structures.

When the component is inserted does the paste get forced out of the holes?
Yes some paste is forced on to the tips of the pins. The amount is dependent on
the care during component loading.

What happens to the paste on the pin tips during reflow?
As reflow takes place the solder does remain on the pin evening out the
thickness on the pin. However there will always be some slight build-up of
solder on the pin tip. There is also a difference which way up the pin is during
reflow.

What about flux residues on the pin tips will it cause problems during in
circuit test?
If you use a high solids paste or you don't tell your test engineers your
preferred process the answer will be yes. Conventional joints that are to be
hand soldered or reflowed should not be used for test access. With a little
planning at the start of a project you eliminate the problems before they hit
the shop floor.

Bob Willis
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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