Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


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Fiducials | 1 July, 2010

We build thousands of flex boards in array format. The fiducial quality for both global and local fids give us much trouble and it seems to be inherent of flex PCB mfg. I am trying to write an internal ducument for our PCB fab house to adhere to. I found a document on fids called SMEMA Standard 3.1. I would like to reference this particularly for fid. flatness guidelines. Is this a good reference? Is there a better one?

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Fiducials | 2 July, 2010

That depends what SMT machines you have. We use the same fiducial mark like in this standard and works fine every time.

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Fiducials | 5 July, 2010

I have found that lighting for the cam is very important for fiducial recognition. My machine illuminates with a light ring, which isn't so good. However, if I burnish the fiducial with a small fiberglass brush then it works every time.

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Fiducials | 6 July, 2010

Thanks - we have tried everything you can imagine with machine settings as well as burnishing and flatening fids with irons. I don't want to deal with it in house. The volumes are huge and we don't want to do any touch-ups. I am really looking for a recommendation on a standard to reference to get our PCB supplier to adhere to as the fid quality is not consistent.

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Fiducials | 6 July, 2010

What type of finish are the PCB's? HASL, tin immersion, silver immersion,........we've seen all the above problems with HASL finished boards. Usually either reheating w/ a soldering iron or "burnishing" with a scotch brite pad has worked for us.

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Fiducials | 6 July, 2010

I agree, setting a standard for the pcb fab shop is good. I use a fiducial a little smaller than the spec you mentioned, I use 30 instead of 40 mil, which is the minimum recommended for my machine. The rest of the spec (SMEMA Standard 3.1) appears the same that I use.

I still want to emphasize that if the true problem is the lighting technique on your machine, then you may need to enhance the lighting to achieve reliable recognition. Because I am only doing small runs, it is easier for me to burnish than change the machine.

Here is a link for more info on lighting:

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Fiducials | 7 July, 2010

Interesting info - thanks for the input.

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Fiducials | 8 July, 2010

I have this same problems on the flex i use, most of which has an ENIG or immersion silver finish. About the only thing I know to do is have your supplier package it in a vaccume seeled bag to keep it from oxidizing. Otherwise you end up having to burnish the pads.

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Fiducials | 9 July, 2010

We don't use flex pcbs but have the same issue here. Most of our pcbs are HASL finish. I have looked at silver and gold immersion and both look really good under my color camera on my AOI. However, gold is more pricey. There is no additional cost with silver, but there is a question with its performance if used with our RF products, which are the boards that provide most of the headache with fid recognition; however, silver may be a good fit for you for about the same cost. It has a shelf-life of about 3 months.

The questions I have for you are: 1) How are your boards packaged upon receiving and are they inspected when they are received? 2) Has your pcb design allowed for any reliefs around your fiducials?

Question 1 has to do with the shelf-life of the HASL finish. The metal begins to tarnish after some time and recognition under a camera becomes more difficult. I would have your fab house ship your boards in a sealed vacuum bag. If you do any receive inspection, reseal the bag with vacuum or have your pcb mfg send one additional board packaged seperately for pcb inspection. That way the boards will not be exposed to air until they are used.

Regarding question 2: I have found that a fiducial surrounded by either soldermask (no copper) or better yet, just substrate, registers more consistantly than one that has a few mils of relief surrounded by copper/solder mask. You need to have your pcb designer provide a square relief area around your fiducial, again substrate works better. This gives the best contrast ratio. A square ething that is 15-20 mil from the edges of the fiducial should be fine. A square will probably work better than a circle depending on you inspection algorithm. Most systems use a square inspection area and it will work well for future equipment you may procure.

I hope this helps.

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Fiducials | 12 July, 2010

Great info rdoss and reese. Thanks so much for the response. We'll look into all these things.

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Fiducials | 18 July, 2010

Thanks you for the post.


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