Here is some more on Intrusive or PIHR Pin In Hole Reflow.

Surface Mount Technology (SMT) is now part of mainstream electronic assembly
with virtually all market sectors benefiting from the use of SMT. One problem
that has always proved a problem to design and process engineers is existing
through hole parts where no direct equivalent SMT part is available.  

It is possible hand solder conventional through hole components after surface
mount assembly and soldering operations are complete. This however is time
consuming and may leave more flux residues on the surface of the joints causing
a problem to in circuit test. Many through hole parts are used for direct
contact during test and residues can quickly clog test pins in a no clean
process.

A range of selective soldering equipment is available to either semi or fully
automatically solder through hole leads. This does of course require capital
expenditure on equipment. One method of soldering all surface mount and through
components in a single operation is Pin In Hole Reflow (PIHR) or Intrusive
Reflow soldering.

Possible Assembly Stages

There are three alternatives to adding solder paste for through hole components
prior to inserting parts:

Stencil Printing
Double Stencil Print
Paste Dispensing

Stencil Printing

Solder paste is printed into through holes and over the pad surface during
normal surface mount paste printing. The size of the aperture in the stencil is
adjusted to allow paste to fill the hole, cover the pad and fill the resist
aperture on the board. Component insertion is then conducted either manually or
automatically by robot prior to the whole assembly passing through the soldering
process.

Double Stencil Print

As it may be difficult to obtain the volume of solder for a through hole
components if lead to hole ratio is excessive a double print operation is
possible. In this case an initial printing operation is conducted to force paste
into the hole only. A second printing operation is then conducted to add
additional paste to the hole, pad and resist window. This increase the solder
volume in the joint. This operation does require two printers and two stencils.

Paste Dispensing

An alternative to printing is dispensing the paste in to the through hole which
can overcome all the problems of metal volume. Today due to the increase speed
of dispensing the through put would not be a problem for an in line
applications.

Figure 1. Example of solder paste displacement which can occur during component
insertion. It can also effect the solder volume if the paste does not reflow
back to the plated through hole. 

In each case the goal to through hole reflow is to achieve the required solder
fill and guarantee the reliability of conventional components. The desired
solder joint is a fillet visible on each side of the board after reflow. This
would be the same criteria applied to wave or hand soldering operations to
either IPC, IEC or British Telecom standards. However, it may not be possible to
obtain a positive fillet above the surface of the board. Often the solder fillet
remains flush with both sides of the circuit board but it is as strong and
reliable as a full connection.

The following process parameters effect the solder joint formation, each need to
be know to calculate the volume of solder required for each joint. In some cases
design modification can be made to improve manufacture and the final joint
quality.
Diameter of component contact
Diameter of plated through hole
Thickness of circuit board
Diameter of pads 

Component compatibility need to be confirmed with the suppliers. Today may
connectors, sockets and some active parts may be assembled by reflow. The main
problem that may occur is distortion of the parts causing them not to sit flat
to the board surface. Stress cracking may also be seen on the corner of parts
due to stresses build up during their manufacture.

Figure 2. This is an example of a section through a joint after Intrusive reflow
soldering

Care need to be taken over the reflow process to eliminate voiding in the
joints. As some much paste is confined in a hole rather than on the surface of
the board the evaporation of volatiles from the paste can case a problem. This
is particularly true when connectors are mounted on the top side of the board
during reflow. In this case volatile gasses find it difficult to escape from the
through hole. The profiles of most reflow systems may need to be re-examined as
most of the through hole parts like connectors, pin grid arrays and sockets can
have a market effect on local board temperatures. 

The SMART Group is looking to organise some production trials to define design
rules, inspection criteria and component specifications. Any company who may
like to contribute to the project can contact your local SMART Group office in
Ireland, Scotland or England.

Here are some Frequently Asked Questions (FAQ's) and Answers on the subject.

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
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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