Many conditions can cause dewetting, but if you are seeing the condition
as received than the issue is the PCB supplier.  If you further
determine by solderability testing that the part are non-compliant than
the issue can be directed to the fab house.  

One major cause of dewetting and subsequent poor reflow soldering is
thin HASL coating on the pads.  The SMT pads should measure in central,
nominal looking areas from 200 microinches minimum.  Thin areas that
measure below 100 microinches can be an issue, certainly after storage
or exposure to poor environments, thermal cycles, etc.   

Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED 
           76 Technology Drive - POB 1890 
              Brattleboro, Vermont 05302
                Voice - 802.257.4571 ext 21
                    Fax - 802.257.0011
                       <http://www.vtcircuits.com/> 
                           
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of sumxp
Sent: Wednesday, July 26, 2006 10:38 AM
To: [log in to unmask]
Subject: [TN] What causing dewetting on HASL pcb

We have a HASL(Sn/Pb) board(FR4) with gold finger edge that having
dewetting issue across 30% of the lands.We have done below evaluation to
find out the root cause;
1)use short,medium,long profile,increase and reduce peak temp.
2)change to different brand of solder paste.
3)clean the board with IPA.
4)Apply a layer of flux on pcb and reflow it without paste,clean the
board
& follow by normal smt solder paste reflow process.
All above countermeasures couldn't solve dewetting issue.We brought in
the
pcb vendor & they couldn't give any answer except coming back to us to
say
that the board is nothing wrong.
Could anyone explain to me what went wrong with the board.It is a Sn/Pb
process.We are seeing dewetting even though the baord is not populated.

Thanks in advance..

rgds
sumxp

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