Hi John - 

Try using these simple rules (they have worked for me anyway):

#1 Is there enough solder volume available for the gold to dissolve into?

#2 Is the soldering process of sufficient time ABOVE the melting point of the 
solder alloy (usually 183C) you are using?

The answer to #1 is nearly always yes but it is #2 that gets most of us into 
trouble! The soldering process has to be long enough, hot enough to allow any 
gold rich solder joint regions to disperse. Although this is possible to deal 
with in production by closely regimenting your process recipes and profiles I 
think that it is impractical. I have previously suggested that working with your
component vendors is a more practical solution to avoid problems. A good paper 
on this is:

"Manufacturing Concerns When Soldering With Gold Plated Component Leads or 
Circuit Board Pads" by Mark Ferguson, IBM, IPC technical paper IPC-TP-1103.

Dave Hillman
Rockwell Collins
[log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: goldfinish on PC boards
Author:  [log in to unmask] at ccmgw1
Date:    11/5/96 10:44 AM


     =
     
     Address,
     =
     
     What you and everyone else are saying makes a lot of sense.  However=
, =
     
     how is amount of gold diluted into the SnPb controlled.  Reflow and =
     
     solder rework dwell time (heat) is obviously a large contributor?  T=
he =
     
     amount of gold diluted is also dependent on what types of gold are =
     
     applied (Immersion Au tends to have a higher porosity than =
     
     Electroplated Au, in-turn more diluted gold in the SnPb).  =
     
     =
     
     Is the gold being soluable a concern at the Reflow operation or at =
     
     rework or both.  Could someone please comment on how the amount of =
     
     diluted gold can be controlled in PROCESS, without an SEM or other =
     
     Quality Control Device.
     =
     
     Regards,
     =
     
     John Gulley
     =
     
     =
     
     =
     
     =
     
     =
     
     =
     
     =
     
______________________________ Reply Separator __________________________= 
_______
Subject: goldfinish on PC boards
Author:  [log in to unmask] at Internet =
     
Date:    11/4/96 8:52 PM
     =
     
     =
     
     =
     
De:   Coderre - JCCODERR BRMVM1
*************************************************************** =
     
CLASSIFICATION    NON-CLASSIFIE ( )       CONFIDENTIEL ( ) =
     
Objet:   goldfinish on PC boards
     =
     
Gold is highly soluble in solder and should not affect the properties =
     
of the solder joint if the concentration is below 3% (wt). This is
true if the gold is completely  dissolved in the solder and one does not =
     
find a rich gold concentration at the interface. THis rich zone may occur=
 =
     
if the temperature cycle is such that the dissolution is not complete. =
     
This should be verified by SEM cross-sectioning 
Literature references are available upon request
At higher gold concentrations (either local or bulk), embrittlement =
     
will occur
     =
     
Merci/Regards
Directeur Ingenierie MLC / MLC ENG. MGR
Dept 881/23 BRMT (tel. 514-534-6591, US tieline 552) =
     
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     =
     
Would appreciate any source for info on effects on solderjoint =
     
reliability when assembling (solderpaste / IR Reflow ) a full gold =
     
(approx 30 microinches thick) finished board. What are the effects of =
     
the gold alloying with the tinlead in the joint ? / Thank you.
     =
     
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