Can local fiducials improve BGA placement?

Greg,

Need to give more to get more! What are the details?

What are the problems you are seeing exactly? What type of part are you trying to place? What type of machine are you using? What type of vision recognition? What type of PCB finish?

Phil

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Unless your placing micro-bga, board edge fiducials on alternating corners will provide more than enough placement correction ( Micro-bga's should also be fine depending on how your equipment maps the board ). The part data and vision recognition need to be looked at closer. Are you using 100% ball inspection? Does the size of the parts exceed your machine's limits? Are you rotating the part after vision inspection? When inspecting BGA sometimes the placement equipment confuses the reflection from the edge of the part as extra balls. You should be able to change the gray scale tolerance to correct. That's just my opinion, I could be wrong!!!

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Typically, yes.

Local fiducials may be used as a reassurance of proper alignment prior to a specified placement. This of course, is under the assumption that said local fiducials are taught accurately and with the proper light levels. We typically like to use local fiducials on fine pitch devices >.020" The logic behind this is that from the time of the initial fiducial check, to the time of the critical parts placement which are typically placed last, there may be changes. What I mean by changes is that depending on what type of assembly equipment you have, the mechanical fixation of a board may actually shift during the initial placements of a program. Hence, causing placement inaccuracies in the later part of a program. This mechanical inaccuracy is obviously not desired nor is it acceptable, but local fiducials are an insurance for a critical placements. This is especially true if you have various operators performing mechanical set-ups.

BGA's as we all know are more forgiving than QFP's, so I would recommend for you to work a little with your quality control department in order to determine whether placement inaccuracies are a problem. Otherwise, you may be targeting a non-existing problem.

Good luck.

Deon Nungaray GMI USA

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local fiducials and component recognition are two seperate issues.

Local fiducials may improve the placement accuracy in X and Y on larger boards (~ > 8" ). For calculation of the theta angle, the global fiducials should be placed over the diagonal.

Component recognition can be improved if your vision system has different light zones. The steep light should be turned off and the flat light turned to max. Also the vision system should be capable to recognize only the balls defined in the vision editor.

If you experience a large number of BGA rejects on your vision system:

Incorrect number for pocket spacing in the tray carrier. Component is picket offset and outside search area.

Incorrect number for ball grid spacing, or tolerance.

Incorrect number for component height; component out of focus.

Bad vacuum. Component is rejected because of vacuum error.

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Local fiducials will help component placement but will have no effect on part recognition. Local fiducials help you compensate for board skew or streatch by looking at a small area of the board vs. the board fiducials that cover placement over the entire brd.

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