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Author Topic: How do they bond the Pads in a Multi layer SMD PCB.  (Read 4023 times)
KD8MJR
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Posts: 2157




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« on: September 15, 2013, 01:31:19 PM »

I know how the boards are put together and how the layers are made and bonded but I don't understand how they join the contact points through the multi layers.  Is it that they have holes cut out in the Bonding sheet and use a mask sheet to fill in some conductive substance in the holes?

On thru hole you have options but on SMD it's all sealed up below the pad so I am confused.
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M0HCN
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« Reply #1 on: September 15, 2013, 02:05:44 PM »

Unless you do special things the pads on an SMT part ONLY exist on whatever side of the board the part is mounted.....

If you want to connect a buried trace to an SMT pad you need to place a via to get to the outside of the board then connect the pad.

Now that via can be a microvia in pad (subject to some process restrictions on aspect ratio, but very useful with BGA packages, these are small enough thst the copper plating fully closes them so no solder wicking problems), a 'blind' via (that is a normal via that only extends through some of the layers) or a perfectly normal plated through via of the normal sort (BAD idea to put these in pad in general as unless you tent the back they can wick solder away from the joint during reflow).

Where a through hole part will have a pad stack that extends right through the board, a SMT pad exists only on the outside of the board (Appropriate copper, resist, screen and paste layers).

The usual process for generating a PTH hole (Be it a via or through hole pad) involves chemically plating a thin layer of copper then electroplating up to the required thickness.

HTH.

73 Dan.
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WX7G
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« Reply #2 on: September 15, 2013, 04:41:23 PM »

For microvias the vias are plated as with a standard PCB, the holes are then plugged with a non-conductive materia, and copper is plated over the filled hole. Microvia construction raises the price of a PCB and when possible they are avoided. To get by without microvia construction the pad has a short trace going to a standard via. This construction consumes more board space and does not work with small IC ball pitches.
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KD8MJR
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« Reply #3 on: September 16, 2013, 08:08:36 AM »

I still can't wrap my head around this, I appreciate the replies but I think I need to see a video of how this is done.   I am very familiar with the through hole process but the SMT pads just don't seem to be possible unless they are bonded before a new layer is thrown on top.
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M0HCN
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« Reply #4 on: September 16, 2013, 01:17:52 PM »

Query, what do you think an SMT PAD is?

I ask because the meaning of the phrase to me implies something that can only exist on the outermost copper layers, after all they are the only surfaces the completed board has. You cannot place an SMT pad on a buried layer, because that makes no sense.

SMT does not have a layer stack in the sense that a through hole component pad does.

73, Dan.
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AF6WL
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« Reply #5 on: September 16, 2013, 01:38:24 PM »

I still can't wrap my head around this, I appreciate the replies but I think I need to see a video of how this is done.   I am very familiar with the through hole process but the SMT pads just don't seem to be possible unless they are bonded before a new layer is thrown on top.

You might be referring to buried vias - where connections exist between inner layers of the PCB.
Here sequential lamination construction is used:



http://blogs.mentor.com/tom-hausherr/blog/2011/01/21/pcb-design-perfection-starts-in-the-cad-library-part-11/
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KD8MJR
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« Reply #6 on: September 16, 2013, 09:32:51 PM »

AF6WL that's exactly what I am talking about  Smiley
I was wondering how they bond the surface pad to the tracks running underneath without a via.   With through hole I know it's just done by electrolysis and plating but on SMD boards they have a solid adhesive layer between each board layer so my thought was that they actually punched holes in the adhesive layer at the needed points and use something like a liquid solder (applied like how they do it with solder mask) to then fill in the holes and after that put on the next layer. I figured the special solder type material flows and bonds all the connection points when they heat up the board or expose it to something.  Of course regular via's could also do it but I have noticed on boards with large FPGA chips that there are not enough via's on the board to make all those connections that I see on the schematic so I figured that some off the top layer pads must be multi layer bonded directly.

I will look at that link and see how its actually done, it's been puzzling me for some time now.

Thanks
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KD8MJR
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« Reply #7 on: September 16, 2013, 09:41:27 PM »

Ah Ha so my guess was sort of correct.
MICROFILLâ„¢ EVF Via Fill provides enhanced via filling.

http://www.dow.com/products/product/microfill-vf-n/

Seems to be like a 2 part conductive epoxy.
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AF6WL
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« Reply #8 on: September 16, 2013, 11:17:29 PM »

The HDI lamination shown in the mentor example is extreme.

A cheaper and more common sequential build would be something like 4+4+4.
i.e. like a sandwich of three individual four layer boards (each sub lam with it's own local 1...4 vias )
The next steps would be to drill all the way through the board, plate up then fill; giving vias that can connect any layer to another.
With a final VIPPO ( via in pad plated over ) process coating all you have to look extremely closely to see on the surface if a via connects to the pad.
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KD8MJR
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« Reply #9 on: September 17, 2013, 10:49:51 AM »

Ok I can see how that would be a lot easier and cheaper.

Thanks.
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