| Okay here is the deal. I am fairly new to the company here, but one thing I have noticed is that we read way to many fiducials, in my opinon anyway. One for example is this. One assembly was reading 17 fids. Smallest pitch was .25mil I say you only need two globals. I changed it to that, they still placed fine, yet everyone else in my department dissagress with me. I heard someone say something about reading locals for ALL fine pitch (.25 and smaller) within 2.5' radius around the part. I have never heard of that before. If I am right, (which I think I am) When should you use locals? If ever.. Please help give me any feedback you can.. thanks.. | Larry,
Ask five people how many fiducials to read and you'll get five different answers. Not to try to overly validate my opinion or anything, but this worked fine on a board populated with 16 10-mil pitch devices on it:
Read three globals. As far out on the corners of the board as possible. The three will give your machine X,Y, and theta offset. If you think your boards have non-linear stretch, read a few globals in between, to. I believe the IP-3 has mapping capabilities. I know the universal GSM does. But start with three globals.
If I were placing 25-mil on a maintained, calibrated IP3, I wouldn't even bother with locals. I'd go there for 20-mil and below. If the cal and maintenance isn't stellar, then I'd add a local, but not until after I compared the placements with and without that local.
People are hesitant to stop reading fiducials. They seem to provide some type of abstract safety margin that doesn't exist. People should actually be hestitant to add fiducial reads.
Every time you read a fiducial, you're taking away production time and adding yet another adjustment to your placement. I think that someone should justify making an adjustment before making it, rather than having to justify not making an adjustment that he really doesn't know whether or not he needs.
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