-----Mark, I tend to agree with the PTH board moisture  and poor copper 
theory. we pre-bake our bare pwb's @ 220 deg f for 4 hours to remove any 
moisture prior to build and solder. Check with your board vendor to have him 
check his platting process.

As to the reflow of SMT components on the component side I have found that 
as the assy crosses the wave, the top side temp will reach 181 deg c for 
approx. 2-3 seconds this can be enough to send the solder into a partial 
reflow.
we use fiberglass bags filled with steel shotgun shot. These are put on the 
assy  to act as a heat sink and also helps to hold hand installed components 
from floating during the wave solder process..

 - Ray Volk
Tel 612-957-4075
e-mail: [log in to unmask]
 ----------
From: [log in to unmask]
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: wavesoldering mixed tech boards
Date: Monday, November 11, 1996 4:07PM

MR MARK A WARYCKA wrote:
> > Good Day,
> >          This is a request for some assistance on a wavesoldering
> problem that we are currently experiencing on some mixed technology
> boards that we process. The boards have surface mount top and bottom
> and when the process is set to wave the bottom side with no defects
> we are experiencing a problem with some top side surface mount IC's
> de-soldering! I have tried using soldering pallets set at
> approximately  a 30 degree angle with some success but it slows down
> the processing time and the top side components still come loose.
> Initial profiling indicates that the top side of the board is showing
> temperatures below the eutectic point of the solder paste being used.
> When the boards are processed without the pallets the defect rate
> increases dramatically but the incidence of topside component des-
> soldering decreases! The wavesolder machine is a T/D Nu-Era with
> dancer wave and the flux being used is Kester 2331-ZX. Topside
> preheat temperatures just prior to wave entry are around 185 degrees
> F.
> >       Another question is in regard to voids and blowholes occurring
> on a PTH board that is auto-inserted. Just can't seem to get rid of
> them. The boards are inspected at incoming for voids in the plated
> thru holes. Profiling indicates that all process parameters are
> correct. Any ideas?
>               From: Mark A. Warycka
>                        Process Engineering Manager
>                        Electronic Systems, Inc.
>                        Sioux Falls, SD
> > 
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Mark, blowholes and voids in PTH boards are due to two main reasons:

     1. PCB laminate moisture
     2. Poor quality copper plating in barrel            ( rough drilling, 
thin copper, pin holes, plating voids etc)
Two factors when present together can affect blowholing and solder voids in 
PTH joint. Examine your as received boards for plating defects in barrel and 
try drying your PCBs before soldering.

As regards your reflow of SMT component on top surface during wave solder, I 
have a question. Are there vias in the SMT pads? Many times the wave will 
reflow the solder in the pad via and suck the sodler out of SMT joint on 
top.
 -- Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]

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