You will need to hold the PCB at the recommend cure temperature for that
material for a period of time dependent on the cure state of the
package.  Standard FR4 (140 Tg) is generally cured at 350-360 F for 45
minutes minimum.  The problem with this temperature on a completed PCB
is the finish.  Solder will begin to move at 320 F and other finishes
like IT and IS will be adversely affected for later wetting.  ENIG
should survive this cycle and may need to be cleaned prior to soldering
to remove any oxides.  

I would recommend a 300 F bake for 1 hour to improve the Tg to a nominal
performance range.  Any temperature above the epoxies Tg will cure the
material with time being the variable.  

What issues are your seeing that leads you to question the cure?
  
Jeffrey Bush
Director, Quality Assurance and Technical Support

VERMONT CIRCUITS INCORPORATED
  76 Technology Drive - POB 1890
    Brattleboro, Vermont 05302
      Voice: 802.257.4571.21 Fax: 802.257.0011
           http://www.vtcircuits.com
 

-----Original Message-----
From: Kaye Clayo [mailto:[log in to unmask]] 
Sent: Wednesday, April 27, 2005 3:10 PM
To: [log in to unmask]
Subject: [TN] How to Bake FR4 Boards

Dear Technet:

Can someone tell me what temperature and duration
we should bake some unpopulated FR4 (HASL) boards?

We are attempting an experiment to see if we can "bake" a
potential problem out of certain boards, but we don't know
what the baking temperature / time should be.  The potential
problem we are looking to bake out is under cured laminate
which could be causing a hook problem.

Please help.

Kaye


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