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>>> Pin Hole Reflow <<<
 
 
 
  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.  
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  Do I need two stencils for through 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) Ensure 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!!!  
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  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 point count devices like connectors, pin grid arrays, post headers, sockets and dual in line parts that have been specifically produced for reflow applications.  
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  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.  
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  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 microsections I have done look good.  
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  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 depending on which the up the pin is during reflow.  
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  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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Bob Willis, Process Engineering Consultant Electronic Presentation Services
2 Fourth Avenue Chelmsford Essex CM1 4HA England Tel: (44) 01245 351502 Fax: (44) 01245 496123
Email: bob@bobwillis.co.uk