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Via in Pad - Good or Bad?

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c_oflynn

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Hello,

Are there any disadvantages to using vias in pads on PCBs? I have looked around but haven't found a huge amount of material on this subject, but I'm sure I'm missing something...
Regards,

-Colin
 

M!k

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Normally you wouldn't do that because the solder is sucked in the hole of the via (assumed it's a continous via from one to the other side of the PCB) and therefore is missing on the pad. There are some special techniques (e.g. for BGAs) which will allow that, but not in a standard production process.


Mik
 

    c_oflynn

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zlop

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Research again,I'm sure there is some material on the internet.
I used to compute the L&C of the via in pads using a exp fomula,but I have lost it!
 

Beepster

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The via can be filled solid (very expensive).

A micro via, less than .1mm in a fairly large pad won't suck much solder off the pad.
 

carvinguy

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It is not common practice to place a via in a pad. If it is an SMT pad, though, I would think when printing the solder paste the via would effectively be filled with paste as well as pasting the pad. When in reflow, the via is plugged with solder so it would not wick from the pad. On the other hand, if the via was just adjacent to the pad with no solder mask in between (no solder paste filling the via), the solder would wick over to the via in reflow causing a poor joint.

I'm sure pad geometry and other factors apply, so you'd have to experiment.
 

IanP

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Maybe you will not agree with me, but if I have to design a PCB for small series of devices (specially if I know PCBs will be assembled by hand) I don't care if vias is in the middle of pads - doesn't matter at all.
If however it is for mass production purposes that is a diffrent kettle of fish: not recommended at all.
 

TheLizard

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If you're sending this to a board house to have done for you, their computers will flag on the via in the pad. At the very least this would delay the boards...at worst they could refuse to cut them. I've had some board houses refuse to make boards that don't pass all of their computer checks.

Just my $0.02.
 

ME

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carvinguy said:
It is not common practice to place a via in a pad.
I don't know where you got this information from or what kind of experience you have in this field, but I strongly disagree with this opinion.

I used to be a electronics repairman for 5 years and mostly reapired brown goods.
I have seen many diiferent PCBs, and all the 2 or 4 sided PCBs I have seen had through hole plating in all the solder pad holes.
This can make it very annoying to desolder 40 pin DIP IC's etc, so that's one disadvantage. You need to have a good desoldering station with good suction to remove all the soler. Somtimes you have to put on new solder and repeat the process to get all the solder inside the holes removed.
I cannot recall I have ever seen a though hole plated PCB without plating in the solder pad holes, so I believe this is not a common pratice.
I have seen some double sided PCBs without through hole plating at all; but at all the through hole plated PCBs I have seen, there have also been plating in the solder pads.
Has anyone seen through hole plated PCBs without plating in the solder pad holes?
I think this is very rare, what do you say?

There's also an advangate in using through hole plating in the solder pads, because this will save you some extra additinal though hole platings outside the solder pads. This can save space and it can also make the PCB cheaper because the PCB manufacturer don't have to drill as many holes in the PCB for extra vias.

Try to check all the PCBs (for example development kit's etc.) you have laying around from other manufactures and check how many of the through hole plated PCBs has plating in the solder pads too. My guess is that it is all of them. Ofcourse it can sometimes be hard to verify when the boards are soldered, but just check if there is solder at both sides of the hole at some of the holes.

It seems such a common pratice to make though hole plating in solder pads, so I would just say go ahead and use this. There must be a reason why almost everyone uses this practice.
 

carvinguy

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I should clarify that I meant it is uncommon to have a via in the middle of a surface mount pad. A through-hole pad basically is a via. A large one maybe, but still is capable of passing a signal from top to bottom of board. You are right, they are usually always plated as far as what I've designed.
 

ME

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carvinguy said:
I should clarify that I meant it is uncommon to have a via in the middle of a surface mount pad. A through-hole pad basically is a via. A large one maybe, but still is capable of passing a signal from top to bottom of board. You are right, they are usually always plated as far as what I've designed.
Ok, I see. I misunderstood your previous post and thought you ment plating in through hole pads.

I agree, it's not very common to have vias inside SMT pads. Usally they a via is place right outside the pad instead. I'm not a professional PCB designer, but I have seen many different PCBs. You don't see vias inside SMT pads very often; but I think I have seen iyt a few times.
I would guess that vias inside a SMT pad would make a bad soldering, but I'm no expert on this so I can't tell for sure.
 

mikem

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I have been in situations before where I had to consider this and in most cases the process engineers (programming pick'n place machines and reflow ovens etc.) recommended against it. I think the reason was to do with solder being absorbed by the hole in the via and possibly causing an inferior solder join at that SMT pad. In a SOIC type package this may not be so drastic but say a TQFP ... SMT pads are much smaller. Also, smaller SMT pads with holes in them are less mechanically robust. It just seems like opening up a can of worms if you decide to go with "via in pad" to manufacturing ... Use extra layers if space is that critical !
 

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