How to solder an SMD component with a "pad" on the bottom?

Question

I am getting a PCB manufactured for a project that I am working on. One of the parts, the A4950 motor driver (datasheet), has a "pad" on the bottom, which is meant to be soldered to GND of the PCB for thermal dissipation. I am only ordering a small quantity of the PCBs, so it would not make sense for me to buy some sort of PCB assembly service. I am planning on soldering the components myself.

I was thinking about the soldering, and I am unsure how I would go about (using a soldering iron), soldering the pad on the bottom. Is this even possible to do by hand?

I was thinking maybe I could manually apply some soldering paste to the PCB, but I'm not sure whether that is an appropriate use of solder paste or not.

How can I prototype an IC with an exposed pad on the bottom?

Answer 1

The absolute best way to do this is to preheat everything with a large high flow hot air source or oven. Apply paste first, if you have it, or a little bit of wire solder to the pad. Then pre-heat. The pre-heat temperature is around 125C or so.

Once everything is heat soaked at 125C, apply localized hot air directly to the part to be soldered and immediately around it. The temperature should be hot enough to melt the solder, but not overheat the part. A lot of cheap hot-air equipment has poor temperature setting and indication. So you may need to experiment. If the solder melts extremely fast, it is too hot. If it melts in about 10-45 seconds, that is probably good. If it takes a full minute, then it should probably be hotter. Often, you will notice the part kind of self-aligns itself and snaps into place when the solder is all melted. This is a good indication that it is hot enough.

Small parts will probably reflow much faster than large parts, and may not need as high a temperature either. Your first efforts may not work well. So keep track of the time, temperature and results. Once you find a winning recipe, stick to it.

If you don't have any way to pre-heat the whole board, then you can just do it the way Arsenal says. If you are repairing a board that went through a reflow oven, keep track of time and temperature when you remove the part. This will give you a good idea of time and temperature required to install the new one.

For large parts, I sometimes don't place them prior to heating. I hold the part with tweezers near the edge of the hot air stream. I use hot air on the pad until I can see that the solder is thoroughly melted, then I place the hot part on the molten solder pad with tweezers. Don't place a cold part on hot solder. The part has to be hot, too, otherwise you will get a cold solder joint. If you do it this way, you can stop heating almost immediately after you place the part. Oh, also, use flux.

Answer 2

One cheap and easy way to do this is to drill a small (50 to 100 mils) hole in the center of the pad on the PCB. Solder the pad itself but not so much it puddles. Solder or at least flux the pad on the IC and solder just the corner pins to the PCB.

Put a 60 watt or so solder iron with a small chisel tip into the back of the PCB and into the hole you drilled. This will heat the IC pad and the PCB pad enough to fuse together. Use a gloved finger to press the IC flat as it fuses to the pad. STOP the instant this takes place. Now you can manually solder or use infra-red or a heat gun to solder the remaining pins.

This works well once you have done it a few times. You do loose some heat transfer to the PCB using this trick, but less chance of damage from cooking the IC or PCB if other procedures last too long.

EDIT: The only time this trick will not work is with multi-layer boards and you know there are traces you might cut through. However IC's that have a bottom pad for grounding and/or heat sink normally have no hidden traces under them. At most there would be a grounding pad with a ring of SMD capacitors around its perimeter. Unless it is very small it is still safe to drill a small hole in the center.

Thanks to @MichaelKaras for his suggestion that if you are doing your own board layout, a 50 mil hole can be embedded in the board that is plated through at the board house. This creates more surface to transfer heat and avoid drilling burrs in the copper if it is done later on. The plate through also allows more heat to be picked up from the solder iron so this step happens fast. Also it allows you to route a few traces around the hole if it simplifies routing.

Answer 3

Here is a way to do it without a hot air gun.

Because the part only has pins on two sides, you can make the center pad longer, like done here for U3. That way you can heat it with the chip in place:

Then pre-tin both the pad on the device and on the pcb, and heat until it melts together. After that you can solder the rest of the pins normally.

Answer 4

If you have soldering paste and an adjustable (air flow and heat) hot air gun available, you can use those.

What I use to do is put soldering paste on pads (I use a syringe with a very fine needle to apply it, it really doesn't need much), place the component as best as possible. It doesn't have to be 100 % perfect, especially if the board has solder resist, as the reflowing soldering paste will allow the part to align itself a bit, but not too much.

Then I use low air flow (the part could be blown away) with around 350 to 400 °C and try to evenly heat around the part. At some point the solder paste will start to reflow at the pins. To get the bottom pad, it needs a bit more heat, so I keep going for a few more seconds around the chip.

If there are small parts (decoupling capacitors for example) in close proximity to the chip, be ready for them to fly away or tombstone on you.

So after you are done, inspect the board closely for any shorts which might occur during this procedure - it is not uncommon for me at least.

This method puts thermal stress on the PCB, so after around 4 or 5 attempts the PCB will show signs of degradation and I usually use a new one.

Answer 5

[disclaimer: This technique is proposed only for one-off prototypes.]

One time, I had to solder an SOIC chip with a thermal pad to a 2 layer board. I didn't have to use solder paste. Here's how I did it.

1. PCB layout. The bottom layer of my the PCB served as a ground plane. I've added vias under the IC, which connected the thermal pad to the bottom layer ground plane. The main purpose of the vias was to conduct away the heat dissipated by the IC. The same vias can conduct the heat needed for soldering.

2. Solder the accessible gull wing leads on the outside of the IC. This will hold it in place.

3. Optional, but very helpful. Apply "bulk heat" to your PCB. You can use an a oven. Even a household hair dryer might work for this. [I use an industrial heat gun, which is an overgrown a hair dryer.] The purpose of bulk heat is to reduce the amount of "topical heat" which you will apply with a soldering iron in the next step.

4. Precision heat. Turn the board over. Stick the soldering iron at the the via opening on the bottom side. Feed the solder and flux into the vias generously. The solder will flow through the vias to the thermal pad, where it will make electric and thermal contact.

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¹I've used the old-fashioned leaded solder for step 5. It has the lower melting point than the modern stuff.
²If you have a choice of tips, use a medium or large tip for step 5.
³If your board has got inner plane layers, it will be harder to make this method work.

Answer 6

Hot air gun and lots of flux. Another method that I have used to solder these parts with a soldering iron is put a few vias on the thermal pad and solder it through that. It's not the best method but it good enough for prototyping.

If the power dissipated in the part is low ( like 1/3 or 1/4 ) of the rated dissipation capacity, you might be able to not solder the pad at all (unless it's also used for grounding or an electrical connection, which for a lot of parts the thermal pad is connected to a pin and the pad).

Another option if the electrical connection is not needed with the thermal pad on the bottom is to put a heatsink on the top for prototyping (sometimes even a block of aluminum will do, anything to increase the surface area to the air).

Answer 7

To solder pads that are under the component you sadly can't use a soldering iron, you need a heat gun or better a station. ....and plenty of Flux. Hope this answers your question.

Answer 8

Hot air gun, solder paste and flux is the right answer as others posted. However I would like to add precisions to the temperature to be used. Pre-heat at around 120C for one minute, then gradualy increase heat by 10C steps every 5 sec until you reach 240C or 250C (for larger parts). Then count slowly until 5 and start decreasing temperature step by step. decreasing can be done faster. Back at 125C you can turn off the hot air. DO NOT heat at higher temperature than that! Your part and the PCB and the other parts around will melt. In the datasheet the maximum reflow soldering temperature and time should be written. Do not exceed them. If you don't have a regulated air gun and can't have one, you can try playing with a digital thermometer but it's much more difficult and less reliable. I strongly recommand buying one if you do more than 10 pieces. The air gun can also be used to weld or repair plactics, solder contact sleeeves and other things.

Answer 9

A very sloppy but existing way to do it is to have the pad on the board slightly bigger, solder a short thin wire to the component itself then after placing the component, solder the remaining of the wire to the pad. This will raise the component a mm or so away from the board, you can push some heat-conductive glue under it. :) I can see the flinches on faces and I understand completely, but it can actually work, takes care of electrical connection and heat, and does not require an air gun.

Answer 10

This is possible to solder by hand if you design the bare board with a hole through the board that is large enough for the soldering iron tip to fit but you will also need to have a ground pad. Take a look at the picture for reference. I highly do not recommend doing this but if you are only building a handful of boards then this will work. If you ever decide to get the a larger volume, remove the hole and hire a CM. The image is of a dfn with a ground pad.

Use this link for image.