Dear WM Cheng,
                In your post you said:

                Currently we have changed to use PCB with OSP (Organic
                Solderability Preservative) finished instead of HASL (Hot
Air
                Solder Leveling) finished, the PCB is a single side one,
CEM-1
                or CEM-4 material, mixed technology with through hole and
                surface mount component, which then go through wave
soldering
                process. The PCB itself has some big copper land/trace
design
                for high current flow which patterned with circular opening
by
                solder resist in order to increase overall trace thickness
but
                not create solder coalesce problem, the circular pads is 1mm
                in diameter and with 1.5mm centre to centre distance. After
                the wave soldering, all the through hole lead solder joint
and
                surface mount device seems no difference in quality when
                compare with the HASL finished board, but a random solder
                skipping happened on those circular pad opening, the skip
                solder area can be readily take up solder by applying solder
                wire with solder iron, the question is :

                1. What is the root cause of such problem? Flux (the flux
                using is a rma no clean type)? Machine setting? Pad design
or
                others?

                2. The way to eliminate this problem, but if it cannot,can
we
                just let the skip solder pad (with copper expose) remain
                unsolder, will this create reliability problem? What I heard
                from some of the OSP supplier that this should be OK.


Dear WM Cheng,
        As your "skipped solder" pads are easily touched up manually, it is
likely that solderability is not the problem.  More likely, the relatively
small aperture in your soldermask is the problem.  Due to surface tension of
the wave solder, It is difficult to get the wave solder to contact the
copper pad where there is no lead to create turbulence.  You are seeing the
same problem that HASL process has in coating small flat surfaces.  While
you may get some relief by switching fluxes, your better solution is to open
up the apertures, perhaps to 3mm.
        There are a number of studies that confirm the reliability of
assemblies with exposed copper.  I suggest you contact your board supplier
and have them forward this information, or contact Enthone at 203-934-8611.
If you are reassured by that information, then you could acknowledge the
skips but not view them as a defect.
        By the way, I don't understand the purpose of the apertures.  Why do
you want "to increase overall trace thickness"?  While I just build and
assemble boards, and am not a designer, it has always been my perception
that the solder has little effect on conductor current capacity.  It could
be that your best solution is to eliminate the apertures altogether.

Greg Anderson
Advanced Manufacturing Engineer
GE Fanuc Automation, N.A.
Charlottesville, VA
        e-mail:  [log in to unmask]

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