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Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab

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#58683

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 29 April, 2009

I have a Product with 2 Different PCB Fiducial allocation in the Design. It has 4 Fine Pitch components. The first PCB DO NOT have Fiducial Mark on the board itself BUT has Fiducial on the Break-OFF Tabs. The second PCB has no Break-Off Tab but has fiducial mark on the PCB itself. I believe that the first board was panelized due to the addition of Fiducial Mark which the designer forgot to add.

The PCB are 100 percent identical except that one has break-off tab and the other doesn't have.

The excitement begins when comparing the placement of the fine pitch component on the PCB on the 2 different boards. The board with fiducials on the break-off tabs exhibits random placement misalignment while the board with no breakoff tab having the fiducial on the board itself is always SPOT ON (Centered).

What's causing my misalignment on the board with the fiducial marks on the break-off tabs?

regards

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#58684

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 29 April, 2009

One more thing:

I have a Panelized PCB with 8 boards on the X directions.

As I move to the next PCB towards the right my misalignment moves to the right by 0.02mm. By the time I reach the 8th PCB I'm Off by between 0.12mm to 0.14mm.

My fine pitch component becomes misaligned...what's causing this issue? Is the panelization the main culprit?

regards

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kpm

#58690

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 30 April, 2009

In regards to your panelized PCB it sounds like your step and repeat in you pnp program is slightly off, by 0.02mm from the sound of it. Are all of the components moving in the same direction as the fine pitched IC?

I don't know what type of machine you are using or if you have access to this but I would say you need to subtract .02mm from your step and repeat in the X direction.

The other possibility is that perhaps your step and repeat on the PCB is not consistent. I would measure the PCB step just to make sure.

I would also recommend using individual PCB fids if you have them or local fids for the fine pitch IC since it is giving you trouble. This should help minimize these problems as your pnp machine should use the step dimension to locate the fids and then the fid information to actually place the parts and .02mm should be within the search area to find the fids.

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#58692

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 30 April, 2009

Could be board support. Breakaway boards are a little more flimzy than boards without.

A bowed board will cause miss placements - again can be overcome with proper supports (depending on the machine)

I would bet that the boards with breakaways has the fids in the wrong spot as compared to your pick and place data. Even slightly off will cause problems like this - depending on the software you use.

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#58700

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 30 April, 2009

Fids ON the PCB will have a reference to the PCB 0,0 point in CAD data generally or can be measured with good accuracy in gerber if CAD is not available.

Panel fiducials are not always referenced from the PCB 0,0 point but many times are referenced from the outer edge of the panel. Trying to reconcile this offset can be a serious chore without gerber artwork or CAD data and your results sound very familiar.

The best thing you can do is to try and find out the true position of the panel fids in relation to the PCB 0,0 point. I'm not talking calipers here but real data. If you don't have access to the PCB artwork and you're lucky you might be able to get in touch with the fab house's CAM dept and see if they can give you this info. If your customer has the data, that's obviously a quicker route.

To piggyback onto KPM's post - While you're at it, confirm the PCB panel pitch (distance from point A to point A on two adjacent PCBs). Although if you're stencil print doesn't have the same offset and your P&P program is using the same pitch, this wouldn't be your problem.

Good Luck.

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#58725

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 4 May, 2009

I'm with hoss67. Trust the local pcb fids. Have you run the camera down one edge of the board to see if it runs off due to a poorly scored breakaway? Some fab houses do not hold very tight tolerances for breakaways.

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#58753

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 8 May, 2009

Hi,

I have had similar issues. First as the above replies suggest pcb fids are better than panel scrap fids, even if you have to use fids under QFP's just palce those parts last.

If your machine has board skip. Place 1-2 QFP's on pcb # 1 and check it. If they are off on one board your fids are wrong in respect to the parts and may have been added by the PCB shop on the scrap at a later date.

If the placement looks good, place 1-2 QFP's on the board furthest away in X,Y and check them. This would tell you if your step and repeat is wrong.

Clear double sticky tape helps, but may not be esd approved. Wait until no one is looking.

I have had to use round test points or fids under parts instead of incorrect panel fids that did not correspond to the part X,Y. Not the best solution, but if they come from the same X,Y gerber coordinates they will work.

rob

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#58808

Misalignment Brainstorm - Fiducials On Board VS On Breakaway Tab | 14 May, 2009

I've gone round and round on this very subject with our manufacturing engineer. He prefers to use panel fiducials and I prefer board fids. I've seen the result in placement from bowed PCB's in the machine. Skewing the board a fraction will throw off placement. Improper tooling or a fiducial that's slightly off will also do this.

From the sounds of it, one of the fiducials your using is slightly off in the X direction. I wouldn't say the step and repeat is necessarily off. You could try subtracting between .10mm and .14mm in the X to the second fiducial (that's if the second fiducial is on the far right towards the offset)

XY machines have a hard time trying to triangulate placement when the fiducial locations don't match the signal layer data.

One way you can tell if the panel offsets are off is to drive to the same placement location on each panel (not using the fiducials for alignment) and see if there is a general offset between first and last.

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