Post History
Without the images or information elsewhere not in this question, it's hard to understand what you are trying to do. I can therefore only discuss the topic generally. It seems you are asking abou...
#2: Post edited
by
Olin Lathrop
·
2026-07-18T13:22:07Z (2 months ago)
- Without the images or information elsewhere not in this question, it's hard to understand what you are trying to do. I can therefore only discuss the topic generally.
- It seems you are asking about issues related to having a via directly within an SMD pad.
- First, why do you want to do that? Is it for higher board density? For greater heat transfer from the part? Something else? To decide whether a solution is "good" requires knowing what criteria "good" will be judged against.
- 0.3 mm is only 12 mil, which is a very small via. Most board houses will charge extra for something that small. Unless you have a very dense design, sticking with 20 mil (0.5 mm) is a good design rule. That is usually just above the extra-small limit for vias for normal board houses.
- I wouldn't put vias in pads for small SMD parts. For example, while an 0805 resistor can have pads large enough for a small via inside them, I'd worry about uneven heating and possibly uneven surface tension of the molten solder pulling the part away from its intended position.
- Larger parts, like a 64 pin TQFP for example, might be able to handle the asymmetric surface tension, but have pads too small to support vias.
- That leaves large parts with large pads where vias in the pad make any sense, like in the collector pad of DPAK transistor for example. I've never done that, but I don't see any major reasons not too.
- Definitely consider the manufacturing process. Have a discussion with your assembly house first. Consider that the solder paste stencil will need to be open over the via, because it is surrounded by the pad that will need solder paste applied to it. That means paste will be forced down the via hole during the stencil process. That could be a problem, or maybe not. Again, talk to your assembly house.
- Once the part is placed onto the pad, the pin is wetted with the solder paste, which should cause normal soldering on the pad where there isn't the via hole. That should be fine in most cases. Of course for high-current designs you need to make sure that there is sufficient pad area, and that the via diameter is large enough for whatever current will go thru the via.
- During soldering, there again could be a problem because the solder on the pads with vias may melt at a different time than the solder on normal SMD pads. That isn't by itself a problem, but you have to consider how the part is pulled by solder surface tension on the solid pads while not being balanced by the pads with holes in them.
- I have never done this and therefore never looked at this in depth. My reaction is that it should be doable, but there are some issues you need to consider carefully when doing it.
Again, though, it would help to know what problem vias within SMD pads is intended to solve.
- Without the images or information elsewhere not in this question, it's hard to understand what you are trying to do. I can therefore only discuss the topic generally.
- It seems you are asking about issues related to having a via directly within an SMD pad.
- First, why do you want to do that? Is it for higher board density? For greater heat transfer from the part? Something else? To decide whether a solution is "good" requires knowing what criteria "good" will be judged against.
- 0.3 mm is only 12 mil, which is a very small via. Most board houses will charge extra for something that small. Unless you have a very dense design, sticking with 20 mil (0.5 mm) is a good design rule. That is usually just above the extra-small limit for vias for normal board houses.
- I wouldn't put vias in pads for small SMD parts. For example, while an 0805 resistor can have pads large enough for a small via inside them, I'd worry about uneven heating and possibly uneven surface tension of the molten solder pulling the part away from its intended position.
- Larger parts, like a 64 pin TQFP for example, might be able to handle the asymmetric surface tension, but have pads too small to support vias.
- That leaves large parts with large pads where vias in the pad make any sense, like in the collector pad of DPAK transistor for example. I've never done that, but I don't see any major reasons not too.
- Definitely consider the manufacturing process. Have a discussion with your assembly house first. Consider that the solder paste stencil will need to be open over the via, because it is surrounded by the pad that will need solder paste applied to it. That means paste will be forced down the via hole during the stencil process. That could be a problem, or maybe not. Again, talk to your assembly house.
- Once the part is placed onto the pad, the pin is wetted with the solder paste, which should cause normal soldering on the pad where there isn't the via hole. That should be fine in most cases. Of course for high-current designs you need to make sure that there is sufficient pad area, and that the via diameter is large enough for whatever current will go thru the via.
- During soldering, there again could be a problem because the solder on the pads with vias may melt at a different time than the solder on normal SMD pads. That isn't by itself a problem, but you have to consider how the part is pulled by solder surface tension on the solid pads while not being balanced by the pads with holes in them.
- I have never done this and therefore never looked at this in depth. My reaction is that it should be doable, but there are some issues you need to consider carefully when doing it.
- Again, though, it would help to know what problem vias within SMD pads is intended to solve.
- <blockquote>It is only to make the routing easier. No, my designs are not high density.</blockquote>
- <blockquote>I intend to stick to 2512s mainly, and if it is necessary, I will not go below 1206 packages.</blockquote>
- If the point is to make routing easier, then smaller parts will have greater return than putting vias inside instead of next to some pads. There are many more available parts in 0805 and smaller, and 0805 is still easy to work with manually. That's what I use unless there are specific reasons not to.
- Only use large parts, like 2512 and 1206, when you actually need the physical size. That could be because of needing higher power dissipation, more capacitance at a particular voltage, or higher maximum voltage. Some standards, like Intrinsic Safety and medical, may also require minimum spacings that only larger parts can meet.
- <blockquote>Well, this project is only for my thesis. No product development. I alone would do the soldering or any kind of rework. No reflow soldering.</blockquote>
- In that case, I'd stay away from vias inside pads. The extra heat sinking to the other side of the via will make manual soldering difficult. You can only heat the edge of the pad, but the via will be sucking away heat right in the middle of the pad.
- Unless you've got experience with vias inside pads (which I don't) and know it will be OK, I'd stay away from it, especially for hand soldering.
#1: Initial revision
by
Olin Lathrop
·
2026-07-17T19:23:13Z (2 months ago)
Without the images or information elsewhere not in this question, it's hard to understand what you are trying to do. I can therefore only discuss the topic generally. It seems you are asking about issues related to having a via directly within an SMD pad. First, why do you want to do that? Is it for higher board density? For greater heat transfer from the part? Something else? To decide whether a solution is "good" requires knowing what criteria "good" will be judged against. 0.3 mm is only 12 mil, which is a very small via. Most board houses will charge extra for something that small. Unless you have a very dense design, sticking with 20 mil (0.5 mm) is a good design rule. That is usually just above the extra-small limit for vias for normal board houses. I wouldn't put vias in pads for small SMD parts. For example, while an 0805 resistor can have pads large enough for a small via inside them, I'd worry about uneven heating and possibly uneven surface tension of the molten solder pulling the part away from its intended position. Larger parts, like a 64 pin TQFP for example, might be able to handle the asymmetric surface tension, but have pads too small to support vias. That leaves large parts with large pads where vias in the pad make any sense, like in the collector pad of DPAK transistor for example. I've never done that, but I don't see any major reasons not too. Definitely consider the manufacturing process. Have a discussion with your assembly house first. Consider that the solder paste stencil will need to be open over the via, because it is surrounded by the pad that will need solder paste applied to it. That means paste will be forced down the via hole during the stencil process. That could be a problem, or maybe not. Again, talk to your assembly house. Once the part is placed onto the pad, the pin is wetted with the solder paste, which should cause normal soldering on the pad where there isn't the via hole. That should be fine in most cases. Of course for high-current designs you need to make sure that there is sufficient pad area, and that the via diameter is large enough for whatever current will go thru the via. During soldering, there again could be a problem because the solder on the pads with vias may melt at a different time than the solder on normal SMD pads. That isn't by itself a problem, but you have to consider how the part is pulled by solder surface tension on the solid pads while not being balanced by the pads with holes in them. I have never done this and therefore never looked at this in depth. My reaction is that it should be doable, but there are some issues you need to consider carefully when doing it. Again, though, it would help to know what problem vias within SMD pads is intended to solve.