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SMD Soldering Technique

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The following is a tell all, how to, exposee on reflow soldering using simple tools. In some cases I made intentional mistakes, in other cases I made no effort to correct the mistakes and some cases I got it right the first time. It wouldn't have been very informative if I had a perfect manufactured board, that's not what you need to see, you want to see that you can actually solder a surface mount board and that it is practical to do without tens or hundreds of thousands of dollars worth of equipment.

In addition to the still photos, I have made a short movie of the actual reflow soldering process. The short version is about 3.7Mbyte is size and shows the moment of truth, the point at which the paste turns to solder. For those of you who are not bandwidth challanged the long version is about 27Mbytes in size and gives you a rough approximation of the time and the use of the hot air gun.

One final word about the getting these photos and how to get good results. I think that lighting can make up for a lot of magnification, so I would like to suggest that you get at least a couple of full spectrum light bulbs. These are usually available at a hardware or home improvement stores, but may not be available at other retail outlets. The bulbs I used to light these photos are 13 watt Sylvania brand. I have two mounted over my bench and directed so that they both point at my soldering station.

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In this example the component C1 is a size 1206 part and C2 and C3 are 0603 parts. The smaller parts are fully one half the size of the bigger part. The solder pads are roughly proportional, also. The gauge of the hypodermic needle furnished with Electronic Research Group solder paste is such that making smaller solder blobs is difficult. So one way of applying a smaller amount of paste is to create a cow pie shaped blob, and to apply a slightly larger amount of solder paste is to make a Hershey's kiss shaped solder blob.

click to inlarge

The most important thing to point out in this picture is that the unequal amount of paste on the pads of C2 has lead to something you'll see once in a while. The good thing is that we can generally avoid the problem by careful application of solder paste. Use the minimum amount of solder paste, and apply paste equally to all pads of a component. You won't likely see this problem on larger parts. The corrective action in this situation is to use to remove the solder on the pad that is exposed and then heat the other end with a soldering iron. lay the part over, and then apply a small amount of regular solder to finish soldering the part into the circuit.

C3 in this picture is also a problem and is perhaps the most difficult to repair. The difficulty with C3 (on the right) is that this part is sideways and needs to be set in place again. My method is to wick the solder off of both ends of the component and then load a quarter inch of with solder. Then place the solder wick lengthwise on the capacitor then head the solder wick with the soldering iron and slide the component sideways. Clean the pads of the component and resolder, place a small amount of paste on each pad, then place the part in the paste and use your soldering iron to solder each end of the component.

This part could get by, but if you look at the first picture (above) the hershey kiss shape of the solder paste is more solder than necessary. This is evidenced by the convex solder fillet, the ideal solder fillet would be concave. Use solder wick and remove a small amount of solder and it will be perfect (if you leave it, as is, absolutely no harm will come to the part or board).

Here the three caps have been cleaned up. The C2 was removed and reset. C3 was cleaned up at the far end and straightened out while C1 (the big cap) had a slight amount of excess solder removed from both ends. This is what it should look like.

Board layouts for integrated circuits vary in width so it is a good idea to "dry fit" the part before placing the paste. Take note of whether the part sets on the pads toward the inside, middle or the outside. In this case I, would say that they are approximately between the inside and middle of the pads.

Place a continuous "rope" of solder paste. In this example, the pads are for a SOIC integrated circuit. In the scheme of things this is a large chip with large and widely spaced pads. Use a rather large rope of paste, you control this by dragging the needle rather slowly. When you encounter finer pitch parts, use a proportionally smaller rope of paste, this can be done by dragging the needle a little faster and drawing out the paste to make a smaller diameter rope of paste.

Set the integrated circuit into the ropes of paste. Then press it so that the legs of the part into the paste. It is better to have the paste a little further out than the legs rather than to have paste squeezed under the package, this can lead to conductive paste that has not melted and adhered to a near by pad and component leg.

In the upper right corner, pins 13 and 14 have some excess paste on the pads. That extra bit of solder paste resulted in a solder bridge between pins 13 and 14 (next photo). "Bridging" is an illustration of what can go wrong, but with some care and a little practice these will not be common place after the first or second try. In my opinion, it's easier to clean up the solder bridge, after reflow than cleaning the paste off of the board at this stage.

The solder bridge is removed with a small amount of solder wick and a conventional fine tip soldering iron. Try not to remove all of the solder from the legs only enough to remove the bridge. All of the legs of this device have some excess solder that will not harm normal operation, but being the perfectionist, use some solder wick along the outside edge of the pad to remove and equalize the amount of solder. Other corrective action would be to use less paste in the future.

The solder bridge was removed with a little bit of solderwick and the other terminals of this IC were evened out. Notice the solder is bright and shiny.

Paste is applied to the pads, as usual, but he power transistor presents a little bit of a special case. The power tap is a much bigger than usual pad. Use the same rules, apply approximately the same proportion of solder to the area of the pad.

Place this part, as usual, then press it into the solder paste.

There is basically nothing wrong. It is possible to get it right on the first try. Notice that the solder fillets are all concave.