Re: http://www.automata.com/ipc/TechNet/0436.html
    
    Hi, Jim:
    
    I found the request for info that you posted some time back on the "average" 
    number of solder defects that occur on PTH assemblies.  I don't know whether 
    you got any responses, but I thought I'd throw my comments your way.  I have 
    talked to people who state that the solder defect rate in their factory is 
    in the hundreds per million (check with Jerry Rosser, Dave Hillman, Ralph 
    Woodgate), and years ago I visited a Hewlett Packard plant in Cupertino 
    which was running in the single digits per million, so I know that very low 
    rates are possible - not just on occasion but consistently.
    
    Every defect has a cause - it isn't just a random event. The manufacturing 
    engineer who is responsible for the process must identify every cause and 
    eliminate it.  The cause may not be the process - it may be a bad design or 
    acceptance of bad material.  This latter category would include not only 
    solderability, but also plating of the through holes.  There should be no 
    need to require solder fill to compensate for conditions like plating voids 
    and cracks in the knee - a board with such conditions should never get into 
    the factory (and the reliability of the "fix" is dubious).  Many 
    manufacturing engineers regard problems of design and material as outside 
    their domain and too difficult to deal with, so they just learn to live with 
    chronic pain as they attempt to "make do" with the processes that are under 
    their control.
    
    A defect may also be "caused" by the inspector or the acceptance 
    requirement, in that a condition that does not threaten the reliablity may 
    be identified as a defect because of a belief.  "Grainy solder" is an 
    example of what I'm talking about.  Solder that freezes slowly is not smooth 
    and shiny, but it's just as reliable, and it was just as reliable even back 
    when it was believed that solder had to be shiny to be acceptable.  Each 
    such belief impedes progress to the intended goal; eliminating each takes a 
    lot of time and discussion (and may not even be successful, since some 
    people seem to prefer problems they understand to solutions they don't).
    
    Some inspectors reject connections that they don't like the looks of, 
    regardless of whether a nonconformance to a requirement exists.  In fact, of 
    all the processes going on in the factory, the one most likely to be out of 
    control is the solder inspection process, as every study that has ever been 
    reported has shown.  Which connections get reworked is largely a matter of 
    who looked at them.
    
    The defect may even be caused by the standard - you probably remember when 
    the Weapons Spec set a limit on how hot the top of the board could get 
    before the board entered the wave, limited the temperature of the solder, 
    and limited the time in the wave, while also requiring that the solder flow 
    out onto the top land.  That combination of requirements artificially set a 
    limit on the heat capacity of an assembly that could be wave soldered, since 
    the top land has to get above the melting temperature of the solder in order 
    to wet.  We've come a long way since those days, and today soldering 
    standards do not impose arbitrary process limits.
    
    There is no reason why there should be any defects at all on a 
    properly-designed assembly if built only with parts that actually meet their 
    procurement specifications, and if only those conditions that reduce 
    reliability (which I term "risky defects") are counted, by people who have 
    been trained (and certified) to make good decisions.
    
    The biggest problem I have encountered in trying to reduce the occurrence of 
    solder defects has been the tightly-held (and widely-held) belief that 
    "we've always had solder defects and we always will" - a self-fulfilling 
    prophecy. This fatalistic attitude regards solder defects as inevitable, as 
    if the most that could be hoped for is keeping them under control.
    
    Gordon Davy
    Northrop Grumman ESSD
    410-993-7399

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