Some questions for the Technet,
     
     One of our PCA Assembly subcontractors uses IR Reflow to reflow 
     through hole parts instead of the traditional Wave.  They typically 
     use this technique for high density double side SMT/Through hole 
     assemblies where there is no room for the keepouts necessary to mask 
     the SMT parts during the wave process.  
     
     The technique involves a double Paste screening operation to squeegee 
     paste into the through holes.  The board is run through IR to reflow 
     both the SMT and the Thru Hole parts.  The solder joints look 
     completely different than a typical wave reflowed thru-hole part.  In 
     fact I think they would be considered rejectable per IPC-A-610B.  The 
     solder joints look as follows (forgive the art, I hope this tranlates 
     well):
     
     
     
                     |      | 
     ___________     |      |     __________
                |    |      |    |
                |*   |      |   *|
                |**  |      |  **|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |****|      |****|
                |**  |      |  **|
                |*   |      |   *|
        ________|    |      |    |___________
                     |      |
                     |      |
                     |______|
     
          Where "*" is solder.
     ---------------------------------------
     
     I have several questions for the forum and would greatly appreciate 
     feedback:
     
     
        1.  Is there an industry term for this type of reflow.  I've heard 
     it referred to as "intrusive reflow".  Since I love catchy names, I'm 
     soliciting one.
     
        2.  Is the technique in widespread practice in the industry or is 
     it new?
     
        3.  Any comments on the reliability of such a solder joint.
     
        4.  Do standards (IPC or otherwise) exist that describe a "good" 
     joint and a "bad" joint.  I'm interested in IPC Class II product.
     
        5.  Are there any tricks of the trade in designing boards for this 
     type of assembly.  For example do I need to use modified (i.e. 
     smaller) thermals on my through holes to insure good reflow.
     
        6.  Are there special processing techniques in assembly that will 
     produce good joints.  Example - screening paste on both sides or using 
     special pastes.
     
        7.  How can these joints be non-destructively evaluated since they 
     can't be seen other than microsection.  Do periodic destructive tests 
     need to be done per an AQL.
     
        8.  I'm aware that any through hole components that use this 
     technique must be made of high temperature material to withstand IR, 
     but are there any other restrictions regarding what type of components 
     this assembly technique can be used for.
     
     I  would appreciate any feedback or comments you might have on this.
     
     Thanks,
     
     [log in to unmask]
     

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