[log in to unmask] wrote:
> 
> I am sending this question again, soon after I sent the question, I lost
> contact with the forum and have just now signed back on.  Still looking for
> an answer:
> 
> Question: Our company processes circuit boards in panel form, (eg. six
> three inch square boards forming a panel that will be six inches wide by
> nine inches long.)  The individual circuit boards are joined to each other
> with small webs, 0.25"long by 0.060" wide, separated by two to three inch
> spaces.  A shear is used to separate the individual boards from the panel
> after the wave solder operation.  Most of our products have been Type III
> assemblies, CEM1 board material, but we have also been doing Type II
> assemblies, FR4 board material for the past year.  Our problem is that the
> larger, multi board FR4 panels(0.060" thick) are bowing during the reflow
> process across the width of the panel by as much as 1/4".  The bow creates
> problems in the next operations of thru-hole insertion or SMT placement.
> The bowing appears to be compounded by the panels while they are in a
> magazine.  I feel that this is due primarily to the practice of using the
> webs to connect the circuit boards within the panel.  However, if the
> amount of web material is increased, we will have to switch from shearing
> to a routing process, scored panels or punch/return style boards(I hear
> that these warp also!).  Another option is a pallet, but this will affect
> the flow-thru process, non-value added labor, and increases tooling cost
> (we are a high volume shop).  Is there another method to eliminate the warp
> problem, or will FR4 material always warp in the reflow process, if so by
> how much?  Is there a panel width, with or without webs that the material
> will not warp?  I am presently planning to purchase a new reflow oven, and
> a center board support is another option.  I am using a standard reflow
> profile with standard soak zone temperatures, liquidous temperatures, and
> peak temperatures.
> 
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Have you tried using clip on 'Board Stiffener' during wave solder 
process? First you may try leading edge stiffener, add a trailing edge 
and then on sides to see what helps most.
-- 
Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]

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