Howdy Vic.,

Here are some insights I have on this subject.

Q: I just wonder if it is possible to forget all the preheat, ramp up 
periods and go straight  into the spike zone where solder paste will melt at 
that temperature?

A:  I don't think it's wise to do that. Typically, the  thermal profile is 
defined by the  flux chemistry. Each stage of the profile has a specific 
purpose. For example:
Preheat   - to fully preheat the SMT assembly to temperature 100 to 150C
Soak      - to ensure flux activation and achieve uniform heating
Reflow    - accomplish solder reflow (for 63/37=> 183 C)
Cooling   - to allow fine grain form within joint

If you go directly into the  "spike zone",  you will experience one or more 
 of the following problems. (Depend on your time)
 - Cold Solder
 - Solder Balls
 - Solder Splash
 - Board Warp
 - Component Crack

The key to have a perfect profile is to have uniform heat transfer 
throughout the board. If you are really interested in removing preheat and 
ramp up periods, you should try Vapour Phase soldering where time is the 
main parameter of control.

Hope this helps !!

Michael Yuen

 ----------
From: TechNet-request
To: TechNet
Subject: Changing of reflow profile.
Date: Monday, July 01, 1996 10:38PM

Hi,
I am a student doing research at the temperature issue during the IR
reflow process.  As I know that there is a well-defined temperature
profile for the PCB when passing through the oven.   I just wonder if it
is possible to forget all the preheat, ramp up periods and go straight
into the spike zone where solder paste will melt at that temperature?

What are the potential problems I have to face in this context, excluding
the thermal mass issue?

Any input is appreciated.  Thanks in advance.

Vic.
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