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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


PCB Warp Issues

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#62267

PCB Warp Issues | 16 July, 2010

We're expereincing issues in our pick and place machine due to what I consider a server warping of the PCB. The PCB is received this way from our PCB manufacturer.

Before I push to much, I would like to know options for a cause. The warp is so bad, that transport between gantry's in our QSP-2 is affected. Baking the PCB's post manufacture has been suggested to decrease warp. Any opinons on this?

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#62268

PCB Warp Issues | 16 July, 2010

Consider: * Bow & Twist IPC-TM-650; Method 2.4.22 * Previous threads on SMTnet, like: http://www.smtnet.com/Forums/Index.cfm?CFApp=1&Message_ID=54498

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#62272

PCB Warp Issues | 16 July, 2010

It is a PCB design issue where the copper patterns are not balanced among the layers. Baking the boards in order to correct the warpage will not improve it.

We've had a problem with one bare board in particular warping the same way from two different vendors. We have a QSV-1 (similar to yours, but one gantry) but we have not had problems on the pick and place. We had problems after running through the reflow oven; they warp even worse. So we made an aluminum plate (with the least amount of mass as possible while keeping rigidity) that secures the four corners with screws as it goes through the oven. After cooling, we remove the screws and the warpage is good enough to pass IPC as Dave referenced.

You may have to use a similar technique of making a carrier plate and screwing down the board during pick and place as well. Sometimes we have to make aluminum carrier plates for very small individual boards, not in arrays for the P&P.

One advantage of this oven trick is that the joints are reflowed and cooled in a flat position. Otherwise, trying to flatten a warped assembly will put constant stress on the joints. Just keep in mind that the aluminum will suck up a lot of heat, so we run it without a board through the reflow so it gets hot and then quickly mount the board to be soldered. Hope this helps!

Dean

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#62282

PCB Warp Issues | 20 July, 2010

I've seen this warp issue across several different PCB's we assemble. One of the biggest head ache's is getting the boards to transfer between machines, converyors, and inside our pick and place. Due to the warpage, transport between conveyors is difficult. Because of the warp issue, only one edge of the board contacts the next conveyor in line, and often, the board will jamb.

The PCB manufacturer is telling me they will bake the PCB's to ensure flattness. I have yet to get an answer about what actually causes them to warp.

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#62285

PCB Warp Issues | 20 July, 2010

You may want to look at IPC 7721 Method 3.2 Bow and Twist Repair. In extreme cases you may need clamp or weight the pcb with bow in the other direction to yield a "flat" pcb in the unrestrained state. You will have to take the PCB up to or slightly above its Tg.

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rem

#62290

PCB Warp Issues | 21 July, 2010

I have some 13" long boards that warp considerably during first pass through reflow (no center support), what I've done to improve this type of warpage is stiffeners along the front and back edge. This may help with twisted boards as well.

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#62292

PCB Warp Issues | 21 July, 2010

To eliminate warping during processing, we have provided customers with composite fixtures to support the board on all 4 edges, with retractable pull down claps that can be added to the top of the PCB after the screen printing. Simply by pushing the pallet down on a flat surface and turning the claps over the edge of the PCB, this will eliminate warping. The camp height is .2” to clear during the pick and place process.

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#62301

PCB Warp Issues | 23 July, 2010

I think that it can be solved by placing ths into a baking oven with the set point above the PCB Tg. For this process to be succesful, weights need to loaded onto the PCB stack during the heating cycles. Once the cycle is complete, the PCB stack will cool and straighten because of these weights. The drawback however is that the possibility of pad oxidation increases by time and may results in wetting problms. I would suggest having a supplier agreement level where they acknowledge any non-conformance as a cost on their side.

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#62332

PCB Warp Issues | 28 July, 2010

I believe we are seeing some wetting issues as we have tried to bake these assemblies ourselves. Our PCB supplier is coming in next week to address the issue. Is there a rule of thumb or formula for determining correct parameters.

Thanks

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