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Tented Via's

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#53744

Tented Via's | 20 February, 2008

Yea, I'm still here.....sigh.

Got bridging problems.

These assemblies are SMT topside, then go thru wave for thru-hole components on topside. These assemblies have many, many, many vias VERY close together. We have a problem with solder from the wave pushing up thru via's to the topside, if the vias are under an I.C on the topside, then we have had, and the potential for, a solder bridge between via's under the I.C. Can't SEE the bridge with the naked eye, if you know what I mean. Real pain in the ass when trying to troubleshoot the problem. Also some via bridges on the wave side of the assembly that seem to "sneak" there way thru inspection and falls into the lap of the troubleshooter........ ME.

We have used that water wash off "Mask" stuff before in known problem areas. But, we are trying to get away from that step.

Anywho..

Thinking about getting all the via's, topside and bottomside "tented" with soldermask during the PCB manufacturing stage, to midigate the above problems.

Do you folks see any reason NOT to "tent" the via's?

Anyone run into any issues post manufacturing with "tented" via's?

Thanks for your input.

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#53746

Tented Via's | 20 February, 2008

Tented vias have solved problems for me in the past, but never created any.

I have heard seemingly pointless discussions about whether or not the EE that designed the board designed it with filled vias in mind (a consideration of current carrying capacity I suppose, but why provide increased capacity in single points on a route?) but never on a product I was associated with. Not that I'm much help, but that's my experience.

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#53757

Tented Via's | 21 February, 2008

I asume you're not ICT-ing this board and using the vias for test points?

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#53758

Tented Via's | 21 February, 2008

That is a correct assumption Doug.

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#53759

Tented Via's | 21 February, 2008

Just have your board house take them out of the mask layer.

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#53760

Tented Via's | 21 February, 2008

KRIKIES! tenting WILL inhibit solder flow through the top-side me lad.

In the meantime, me chap, have you tried to adjust wave parameters? i've read threads here that a vibrating wave will tend to push solder up through holes.

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#53762

Tented Via's | 21 February, 2008

If the vias are large and don't always tent, you can have your pcb supplier double mask the pcb. I do this for COB assembly sometimes because the globtop wicks through the vias and also causes air bubbles in the globtop.

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#53763

Tented Via's | 21 February, 2008

if you are not using them for test points or want them filled with solder for current carrying capability then tent them puppies. i was able to move our company away from exposed vias many years ago and this eliminated lots of hairline shorts between pads and solder balls hidden under components.

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#53779

Tented Via's | 22 February, 2008

Are you using nocleans, especially when you get to SMT?

Years ago, I had OA wave flux get into vias that were plugged from the top under parts. It is really difficult to clean small, high aspect ratio vias. Cross sections showed that the copper in the middle of the barrel was actually eaten away enough that it had fractured opening up the via. We talked the customer into going from 8 mil holes to 12 (saved them some fab money in the process), and got enough cleaning margin to make it work.

You have to decide which side will get exposed to what, and work out the plug (there is no tenting with LPI, that only worked for dry film mask) from there. e.g. are you trying to keep stuff from the top from going down into the holes (encroached but still open vias under BGA's are prefereable to trying to plug them) or are you trying to keep wave solder fluxes from going up from the bottom?

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#53780

Tented Via's | 22 February, 2008

Steve,

Trying to keep wave solder from coming up the via' from the bottom and adhering to the via's and via's pads during wave. We use WS flux thru-out the whole process.

I have been concerned about possible flux intrapment in the via's. That's a good point.

We really don't want/need to "Plug" the via's. My understanding of "Tented" via's is just a layer of solder mask over the via's not allowing solder to adhere. Sometimes it plugs the via and sometimes it does not, but it ALWAYS coats the vias and the via pads. The molten solder can come up the via, and with nothing to adhere to, will flow back down.

Thanks for your suggestions.

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#53798

Tented Via's | 25 February, 2008

Rather than having the artwork cover the via as with tenting dry film, and letting the mask do what it will, the preferred way is to do a second op silk screen with something like SR1000 just in the vias themselves to plug them. This doesn't always fill the entire via, but usually plugs most of it. This will minimize the entrapment except for vias with aspect ratios above 10:1

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