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Old 3rd November 2013, 05:13 AM   #1
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Question SMT connection to copper pour?

Thru-hole connections to wide PCB traces like power buses usually have a thermal pad to make it easier to solder.
What's the practice with SMT components?
Typical application would be an SMT emitter resistor to a wide output trace or a substantial SMT capacitor between earth and the positive rail.
Most of the SMT recommendations I can find are oriented to much smaller applications.

Best wishes
David
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Old 3rd November 2013, 07:46 AM   #2
peufeu is offline peufeu  France
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Thermal relief does make SMD easier to hand-solder. Without it, if the pad is part of a large copper pour, you'll need a decent tool (like 50W soldering iron with temperature control), with a 20W iron and a skinny tip, it'll just get stuck.

Same for sticking vias in pads.

But it has less inductance and resistance. A few mm of skinny trace is enough to make a decoupling cap between power and ground planes useless, so, your choice.

If it's a part which will disispate some heat, like a LDO or a power resistor, the copper pour becomes a very useful heat sink. In this case, using thermal reliefs defeats the purpose...

If you use reflow soldering in an oven, it heats the whole board, which makes it less an issue.

Also many SMT recommendations aim to have a smooth volume production, for example using via-in-pad cools the pad during soldering and sucks up the solder, which is why it's not very common. The vias usually need to be plugged first with ca conductive filler. For DIY, you usually don't care about that, if that particular pad takes a little longer to solder by hand or needs a little bit more solder, who cares.

Last edited by peufeu; 3rd November 2013 at 07:50 AM.
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Old 3rd November 2013, 10:56 AM   #3
mcd99uk is offline mcd99uk  United Kingdom
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Agree with this. SMD soldering is something I do everyday at work.

Having a 50W soldering iron and using plenty of flux there is no problem with large copper areas.

You can use a normal cooking oven for SMD. Just need to use a thermocouple in there to make sure you are using the correct process temperatures.
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Old 3rd November 2013, 11:03 AM   #4
AndrewT is offline AndrewT  Scotland
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Does one place the 50W iron on the copper plane next to the chip's solder heatsink?
or
on top of the chip's solder heatsink?

What if there is no exposed land next to the chip's heatsink lead?
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Old 3rd November 2013, 01:08 PM   #5
Bigun is online now Bigun  Canada
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To avoid a cold joint you want to ensure that the chip's solder heatsink and copper plane are both hot enough for the solder to flow properly. If there's no exposed land I'd suggest first pre-soldering the land area to cover the pad with a reasonable thickness, then place chip over it and heat the combined area through the heatsink to melt the solder. Usually it isn't recommended to do this and instead to use only fresh solder but I've not found a way to avoid this method if the landing area is (improperly designed) hidden by the part. On the other hand it appears to work well.

If the landing area is larger than the part then I usually treat it like a through hole joint and place the tip of my iron on the landing area but then slide the tip up against the lead of the part you are trying to solder so that you heat both the landing area and the part at the same time. Soldering the larger SMT parts can be as easy or easier than through-hole parts, requires no lead clipping, uses less solder and leaves less flux residue plus is easier to clean.

I don't glue SMT parts before soldering them so in some cases I might choose to 'tack' a part on to the board lightly soldering one of the leads to it's pad with a 'touch' of the iron and then carefully solder the remaining leads one at a time, then come back to the first one and solder it more thoroughly. This works well with power transistors where you tack the small signal leads to hold it before soldering the heatsink.

It's bad practice to use pads that are undersized for soldered heatsinks - check the data sheet for the part - the pad area is an important and integral part of the heatsinking and needs a large enough area to work. In my TGM5 amplifier (thread around here) I even added additional pieces of copper to the heatsink of SMT power devices to increase copper area and heatsinking without using up additional board area.

You still need to be careful you don't overhead a part whilst soldering it which is where a powerful iron helps because it allows you to complete the joint quickly.

N.B. This is just my approach - I have no SMT training or experience other than applying what I think is common sense to my hobby projects.
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Last edited by Bigun; 3rd November 2013 at 01:20 PM.
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Old 3rd November 2013, 01:17 PM   #6
nattawa is offline nattawa  Canada
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Quote:
Originally Posted by AndrewT View Post
Does one place the 50W iron on the copper plane next to the chip's solder heatsink? or on top of the chip's solder heatsink?

What if there is no exposed land next to the chip's heatsink lead?
Usually I would apply heat on the copper plane, that way the molten solder works as a very efficient heat transfer media.

When there is no exposed plane, it would require using a hot air rework station. I once tried with a heat gun mounted on a stand to help preheat a heat sink before soldering it. It helped but not as much as I had thought it would.
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Old 3rd November 2013, 01:33 PM   #7
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Quote:
Originally Posted by nattawa View Post
...When there is no exposed plane, it would require using a hot air rework station...
I have not used a proper hot air rework station.
It's called a "rework" station but I wondered if it would be a better way to solder to a plane.
Perhaps I could preheat the copper with my paint stripper/hot air tool.
Any one here with experience in either technique?

Best wishes
David
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Old 3rd November 2013, 01:37 PM   #8
mcd99uk is offline mcd99uk  United Kingdom
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I have used the hot air method, it works surprisingly well. You can get mini gas soldering irons which have hot air nozzles. The only problem is the potential for overheating the board and the part.
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Old 3rd November 2013, 04:20 PM   #9
rsavas is offline rsavas  Canada
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Default SMT layout

I bought a sparkfun hot air, it worked much the same as a Hakko that I used in the past. Just have to get the right nozzles for the application.
When I do a copper area fill, I just let it do it the same as a through hole or via. You can adjust the thermal gap width to make more contact to the PTH.
For
Quote:
Typical application would be an SMT emitter resistor to a wide output trace or a substantial SMT capacitor between earth and the positive rail.
For this I do not do a copper area fill/pour, I do a full contact (copper area) with no thermals and then crank up the temp on the weller and put a wide chisel tip on it.
I have a LME49830/LatFet amp in the works and am using Ohmite FCSL R's as ballast R's, so that I can measure the current. The FCSL are on the right side, I have 2 pairs of TO-264 to be mounted on the backside, LME is on the left side. See attached.
Attached Files
File Type: pdf LME49830+LATFET-27-top-Screen Print..pdf (222.6 KB, 15 views)
File Type: pdf LME49830+LATFET-27-top-Screen Print-fill-on..pdf (244.8 KB, 12 views)

Last edited by rsavas; 3rd November 2013 at 04:26 PM.
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Old 3rd November 2013, 07:48 PM   #10
CBS240 is offline CBS240  United States
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The only reason I would include a SMD pad inside a copper pour is to heat sink the device via the SMD pin such as the collector or drain of a transistor. Otherwise I leave a small 'leader' coming from the pour and forming the SMD pad. If you assemble with a soldering iron that is too hot you risk lifting a pad or causing thermal fatigue to the SMD pins and internal bonding wires that connect to the die. If you bury a 0201 or 0402 or something small like a SOT-563 pin inside a pour, you will have trouble soldering by hand for sure. For example, in my EC mosfet amp there are many different size SMD packages used, even 0201 resistors and 0402 X 4 resistor arrays all soldered by hand. SOT-23 is a relatively large component for me.
Attached Images
File Type: jpg DSCF7769.jpg (582.8 KB, 107 views)
File Type: jpg ST120_verII.jpg (1.01 MB, 106 views)
File Type: jpg DSCF7758.JPG (662.3 KB, 101 views)
File Type: jpg DSCF7750.jpg (563.2 KB, 99 views)
File Type: jpg DSCF7772.JPG (579.8 KB, 99 views)
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Last edited by CBS240; 3rd November 2013 at 07:50 PM.
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