Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Stencil Printing

RossC

#6376

Stencil Printing | 20 July, 2000

Bob We are just starting to look in to this process and have been told that it can't be done using a stencil to print the solder paste onto the PCB (of course it was by an equipment manufacturer trying to sell a paste dispnecer). Is this a far comment. What problems are we likely to come across if we try to stencil print the PCB

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Bob Willis

#6377

Re: Stencil Printing | 20 July, 2000

If you are talking about Pin In Hole Reflow that is not true, I hope it was not my friends at Speedline that told you that.

Stencil printing of paste for through hole reflow can be done. If you get all the parameters right it can give you joints like wave solder. If you do my SDRSS process you can also have bulbous joints.

PIHR Frequently Asked Questions

The following are a list of typically asked questions on PIHR and the answers often provided. These may be beneficial as a quick guide to the process and any questions you may have. They have all appeared in the AMT, EM&T and Asian Electronics Engineer magazines.

Why reflow solder through hole components?

In many electronic assemblies there are large multi-leaded components 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 manufacturer, 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 godsend 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 looks 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.

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