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Flexi boards and uBGA

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dougs

#42416

Flexi boards and uBGA | 26 June, 2006

We have a new product in where a 0.7mm pitch uBGA is to be placed on a 2 layer flexi board. Also a 20thou pitch connector. Has anyone done anything like this before, what should i look out for. Not done much flexi work before so any tips will be gratefully received.

regards

Dougs

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Rob

#42427

Flexi boards and uBGA | 26 June, 2006

Yep,

Book a holiday and let someone else sort it out.

Otherwise e-mail me and I'll talk you through pallets, rigidisers, printer tooling, spray mount, Kapton tape, local fiducials (panels can strech & shrink) etc.

Oh, yeah - for the connectors you need a really flat PCB finish - most flex suppliers claim they're good - most of them aren't.

Best of British,

Rob.

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#42581

Flexi boards and uBGA | 6 July, 2006

Rob,

do you have any tip on reliable supplier of "really flat" flexible pcb? We tried several but all with issues.

Thank you.

Pavel

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SWAG

#42586

Flexi boards and uBGA | 6 July, 2006

We've had much trouble with flex circuits. As previously mentioned, you must have local fids for the board or the BGA itself (if panelized). Other troubles we experience are drilled holes for pinned connectors. Unless the PCB manufactur has some way of locally finding/drilling these on an array using new 0,0's for each PCB, then they will vary so much that you might need to break alignment pins off connectors to get them to fit. Also related to this is the fact that holes will not drill very clean at all. As for BGA's on a thin board, be very careful with all handling processes after the reflow oven (especially test if bed of nails!). Make a carrier for the oven that keeps the PCB flat during reflow. Too resrictive of a carrier can actually cause the BGA to pop off the board during reflow as the PCB will get convex when it expands. 0.005" gap around perimeter (0.010" total x and y) is usually O.K. for most carrier materials. As stated in previous posts, flatness and board finish will be major consideration.

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dougs

#42595

Flexi boards and uBGA | 7 July, 2006

Hi folks, thanks for all the tips. We have run our first batch of these flexi boards now. I had the PCB suppliers make me some 1.6mm FR4 carriers to lay the boards on to carry them through the process. (1 flat & 1 with cutouts for the second side) they included some locating holes to help us align the panels onto the carriers. We taped them down with kapton tape around the 4 edges of the board making sure they lay as flat as possible. The panels are 120mmm x 100mm 10up's, we didn't see any issues with panel shrink/stretch, don't know if that was just luck or not. We managed to get the boards through the line and tested with 1 fail, turns out this was due to a damamged track which looked like it happened at the PCB manufacturers. Looks like our board manufacturer has done an ok job otherwise though as they seemed to run through like a standard PCB once we had taped them down. I will look though at some of the suggestions here, like local fids, although not sure at this stage if there is enough room on the board.

again, thanks for the help.

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Rob

#42596

Flexi boards and uBGA | 7 July, 2006

Hi Pavel,

We tried 6 or 7 but they all had issues with one part or another of their process. The only consistantly good ones we experienced were the Japanese manufacturers Nitto & Fujikura - these were more expensive than the other options, but probably not if you looked at the total cost.

Cheers,

Rob.

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SWAG

#42652

Flexi boards and uBGA | 11 July, 2006

Over time, you might see problems with delam., blistering and warpage if using FR4 carriers. Depending on volumes, you might consider using durostone or some robust material like that for the production runs. Keep in mind that switching materials will likely affect your profile which is important when reflowing BGA's.

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dougs

#42654

Flexi boards and uBGA | 11 July, 2006

Thats a good point SWAG, i'll look into that for the next run of these.

cheers

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