We have encountered problems with pre-baking on a flex with bare-copper pads. It had caused oxidation of the pads leading to excessive "exposed copper" problems after reflow. Thus, we had a case where the solution to a problem led to another equally serious problem.

-Luke Mendoza-
Electronic Assemblies, Inc.
Manila, Philippines
http://www.globe.com.ph/~eai

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From: 	[log in to unmask][SMTP:[log in to unmask]]
Sent: 	Tuesday, June 25, 1996 12:40 PM
To: 	[log in to unmask]
Cc: 	[log in to unmask]; [log in to unmask]
Subject: 	Re[2]: ASSY: Baking before wave soldering

     Just a caution.  Many solderpaste actually recommend or even require a 
     prebake prior to reflow.  Note that an N2 atmosphere prevents 
     oxidation of the PWB, the pastes (and fluxes) are formulated to 
     tolerate (and even benefit from) some pre-baking (to activate the flux 
     and drive off solvents) and prebaking also acts to reduce thermal 
     shock during reflow.
     
     Jim Maguire
     Boeing
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: ASSY: Baking before wave soldering
Author:  [log in to unmask] at esdigate
Date:    6/19/96 5:59 AM


Teik,
 I'm not sure about the baking of inserted boards before wave 
soldering, but I do know that baking a board that has solder paste on 
it prior to reflow is not a good practice to start.  If you are using 
an extremely good paste and conditions are ideal, you might get away 
with it, but even then you would be lucky.
     
 When you bake solder paste, you are oxidizing the board, component 
leads and the solder balls in the paste.  Now flux is active to some 
degree at room temperature, and that activity increases with 
temperature.  How much the activity increases depends on the flux 
formulation.  So here you have you solder surfaces oxidizing and your 
flux becoming more active (trying to remove all of this oxidation) 
and before long your flux puts up a white flag because there is more 
oxidation than it can remove.
     
 Now you have an oxidized board with an exhausted flux going into 
your reflow oven.  Your board will have less that good results.
     
 If you feel you really need to do this, I would get some test boards 
and dummy components.  Use these to develop a bake cycle that works 
with the flux and board conditions you have to deal with.
     
 Best of luck.
     
>      A customer of ours is asking what is the best practice baking 
>      procedure (or pre-conditioning) of loaded board prior to IR reflow or 
>      wave soldering.
>      
>      All of our specs in house deal with bare boards. I know there must be 
>      an IPC assy spec that specifies boards pre-conditioning at reflow. Can 
>      someone tell me the spec#, temperature and duration?
>      
>      Thanks.
>      
>      Teik M. Ng
>      [log in to unmask]
>      PC World
>      Toronto, Canada
> 
> 
     
Sincerely,
John Guy
     
EMPF
tel:  (317) 226-5630
fax:  (317) 226-5615
E-Mail: [log in to unmask]