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Plated through via's in pads.

bpan

#23266

Plated through via's in pads. | 5 February, 2003

Hello Everyone, Is anyone out there building smt boards that have plated through vias directly located through the pads. We are seeing some of these designs and the soldering is very poor because the solder is flowing through the via. We think that we will have to use a thicker stencil (8 mil) to help with the soldering but it may cause shorts with the fine pitch. Are there any alternatives for this problem. (touching up the parts is not a fix)..haha..... The paste we use is Kester HM531 watersoluble..dont know if that will help

Thanks

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#23270

Plated through via's in pads. | 5 February, 2003

First off, get them designed out. I had this problem once and filled them with SMD adhesive and cured prior to printing. We were careful to be sure the solder lands were clean of any adhesive prior to curing. It worked out very well and we did this process until the customer changed the design and sent us new boards. Hope this helps.

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#23276

Plated through via's in pads. | 5 February, 2003

You're correct that a thicker solder deposit will make bridging on fine pitch devices a bigger risk. Another alternative could be to slightly over-print the pads (onto the soldermask) ala pin-in-paste process. Basically adding enough solder to fill the via during reflow but leave enough for a good joint.

This would require a new stencil and some calculation of the additional solder volume required to fill the via. Of course, you would need to have clearance to adjacent components/conductors. Solderballs would become the possible side effect but because you are using a water soluable paste, they would most would likely be removed by a good wash process. Some pastes are more prone to ball up than others. For fine pitch, over-printing on the toe side of the fillets would be wise. You may be able to pull this off without adding an extra process (glue dispense).

Let us know how you make out.

Joe.

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mk

#23285

Plated through via's in pads. | 5 February, 2003

Investigate "Solid Solder Deposition"

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iman

#23286

Plated through via's in pads. | 5 February, 2003

If by the Powers Ta Be (customer/designer), you can't change the PCB design, negotiate for the PCB fab house, to block the via holes with their (green?) solder mask

some customers/designers want to have via holes to be present in the PCB pad, due to some belief that it helps in heat dissipation.

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#23289

Plated through via's in pads. | 5 February, 2003

Iman

On your situation: As an earlier poster stated, your options are limited to opening you stencil aperture to compensate for the paste lost to the via.

On your designer's belief that "it helps in heat dissipation": Could be true, but: * Hole will fill with solder and that will decrease the reliability and reduce thermal conduction. * Less solder on the solder connection, due to the via taking on solder, will probably reduce the reliability of the product. * Less solder on the solder connection, due to the via taking on solder, reduces the current handling capability of that connection.

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genny

#23294

Plated through via's in pads. | 6 February, 2003

Actually, the most common reason I have seen vias in pads is for grounds in RF applications where the frequencies are high enough that you need a ground RIGHT THERE!... not .1" away. Vias and traces have RF properties of capacitance and inductance, so running a short trace to a ground via can have a completely different response than grounding right on the component connection.

If we can, we move the via just far enough that there is a few mil of soldermask dam between the component land and the via. If we can't, I have had good success with making the solderpaste stencil opening up to 20% larger than the land size, rather than increasing stencil thickness. Giving more paste a chance to stay on the pad.

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MA/NY DDave

#23299

Plated through via's in pads. | 6 February, 2003

Hi

"some customers/designers want to have via holes to be present in the PCB pad, due to some belief that it helps in heat dissipation. "

As I think about this I don't know exactly why they would be concerned. I can imagine in some exotic products with really hot traces due to amp loads that they would want to get to a heat sink or spreader as fast as possible. Yet a tiny hole doesn't help much and without doing the calculations I don't know since I can imagine all kinds of failure mechanisms.

And the ideas to cover the hole won't satisfy them since they will want solder in the hole for thermal reasons.

I can recall one product (an op amp) that the component manufacturer had a screwy pcb recommendation that couldn't be done.

Ask them What's Up on the Thermal Front??

YiE, DDave

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matherat

#23322

Plated through via's in pads. | 10 February, 2003

BPan,

I would like to discuss a possible solution to this that involves some of the suggestions found here but, with a twist that our process offers. This would be better discussed one on one somewhere other than this forum so if you still need suggestions please email me direct or provide contact information so I can contact you.

m.j.kehoe@att.net

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bg

#23323

Plated through via's in pads. | 10 February, 2003

Specify a .1 mm diameter FHS with tolerance of +.3 -.1 using a .15 mm stencil.

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bpan

#23329

Plated through via's in pads. | 10 February, 2003

Thanks for all the info guys. Dont think that the problem is a thermal one and I believe the guys designing the board just need to design them out. I have directed the customers we get the boards from to read this posted question so that they could see some of the responses. It should help with them understanding that it is "not normal" to design the via's in the pads.

Chow, bpan

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MA/NY DDave

#23333

Plated through via's in pads. | 10 February, 2003

Hi

You are probably right.

The only thing I will tell you is that one time as thee DFM engineer working between many electronic designers, component manufacturers and CEMs (contract electronic manufacturers) the component manufacturer would not budge and say they didn't know what they were doing.

They didn't even want to answer my phone calls.

I fought successfully to fix a design recommendation from the component manufacturer, yet all before me lost. I was on the Product Owner side so it made things easier.

So Good Luck, Yet you know what they say in Indiana

If the customer specifies it, and you do your best the customer buys it.

YiEng DDave

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Chad

#23338

Plated through via's in pads. | 11 February, 2003

I believe I've seen some board houses advertise the ability to plate vias completely shut, so that they can be placed within the outlines of a pad without causing the problems you noted. I've seen this scheme used on very densely populated digital boards to conserve real estate.

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Jim Mills

#23343

Plated through via's in pads. | 11 February, 2003

Use a PCB fab house that is capable of "Conductive Via Filling" is the way to do it right. A conductive epoxy is used to fill the drilled via PRIOR to final plating. After final plating, the surface of the "Filled Via" will appear to be the same as the other BGA pads. This will not require a manufacturing process change to use different stencil thickness. ... More Paste, ... More Waste!

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Paul T.

#25253

Plated through via's in pads. | 22 July, 2003

Have the PCB fabricator use a conductive paste to fill the vias and then finish by plateing over the via. Make sure it's coplanar to the side they're on, usually secondary side for discretes.

PT

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