[OpenPnP] BGA fiducial use ? BGA place in OpenPnP good practice?

[manu f]

Hello,

Due to semiconductor shortage I will need to place small BGA (28 pins).

I saw that my BGA footprint have a fiducial.

Is OpenPNP could use it ? is it necessary?

If somebody could tell me good practices for BGA placement before I route my PCB ?

Regards.

Manu

[manu f]

Nobody to explain good practices in bga PnP ? 

[ma...@makr.zone]

AFAIK, there is nothing special or difficult about BGA placement with OpenPnP.

The pitch is relevant, i.e. your machine must be precise enough for the pitch. I do not think there is a difference between pins and balls. BGA is probably even simpler, because it is a more compact package, given a certain number of contacts and pitch.

Other questions are solder paste application and reflow, and related quality control. Unfortunately, I know nothing about these. But I'm very interested, if you can report about your experience later.

Routing is also a problem if you have a fine pitch and only regular PCB tech.

_Mark

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[Marek T.]

BGAs are very well-positioned parts even if they are placed not perfectly. We put about 10pcs of different BGA parts with maybe 2 badly soldered - that means not soldered, short circuits do not happen.

Like Mark said, "other question are solder paste application...". The stencil for the parts like that should be very thin - 0.1mm is perfect. It creates the problem with the paste printing. Read about the relation between the size of the hole in the stencil, its thickness and size of the solder balls in the soldering paste. If this is choosen wrong, the paste detaches from the pcb when lifting the stencil. Solution is: thicker stencil (not possible, too much paste), paste with smaller balls (not always perfect solution) or bigger area of holes in the stencil. I have found official recommendation of some parts' manufacturer (don't remember which one) to use square holes instead of oval. See the picture attached - it is not the mistake that squares are bigger than the oval pads and the paste is partially printed out of the pad - this is not the problem. While the soldering the paste goes to the pads and there are not any short circuits. This way the size of the hole is bigger and paste is printed perfectly.

Second advice is to add the frame or stripes around the part, in Cu layer not in overlay!!! It should be like i.e. 0.5mm distanced from the part's body. The same distance at every sides.

Why it's helpful? Some BGA parts have the balls over all surface of the case and they are visible well when you look from the side - to check if balls sit nice on the pads. But, many parts (usually memories) have the case like 12x12mm but the balls are cumulated on the central 8x8mm only - so you can't see them when you look from the side. The stipes are helpful to verify if the part sits perfectly as should do. The same stripe's distance makes verification very fast and easy at the first glance.

I agree, part's fiducials could be helpful - but it doesn't work in Openpnp (in 1.0 for sure, probably not in 2.0 either - but I can be wrong).

From other side and from my point of view, this is time loosing. Placement without fiducials is not the problem if general fiducials are processed well and machine has no oversteps or missed steps. And even if the part's fiducial worked, I would never trust them, but I would like to verify the placement with my own eyes (instead of potential play with soldering, reballing, etc.).

[Dave Park]

Using paste on BGAs is not best practice. Just using flux and trusting the balls to do their job seems best.

Or is that just me?

Dave

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[Marek T.]

Maybe, if you make reballing and have a bit of old paste on the cleaned pads the flux is ok - I use it then.

But I don't think it's true for the new boards. I think that thin layer of paste is very good. At least works perfect for me (statistics).

And tell me how to print the paste for 500 parts and flux for 3 BGA on the same pcb side. Do you think that anybody has time when need assemble the serie of 1000 boards?

Anyway, recommendation that I said told nothing about flux printing but regular paste. And 2 or 3 other - the same.

Maybe just me but I rather think - just you :-)

[Harjit Singh]

Marek, can you elaborate of the second point?

You said: Second advice is to add the frame or stripes around the part, in Cu layer not in overlay!!

Any chance you have a picture?

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[Marek Twarowski]

I've added there the part of pcb project. I can add real picture of the mounted board tomorrow or after.

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[manu f]

Thanks Marek for all this experience return. This is very interresting !

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[Marek Twarowski]

You're welcome, I hope it helps.

Small mistake in my long post. These 2pcs failed BGAs were between 10Kpcs not 10pcs :-).

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[Harjit Singh]

Marek, I look forward to the pictures.

I have also read about using square solder paste openings. The reason given was that because it has a sharp corner, the paste releases from the stencil and attaches to the PCB. With round shapes in the stencil, the surface tension of the paste is such that the solder paste remains attached to the stencil opening.

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