Adventures with Surface Mount construction

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I’ve been putting off learning how to build PCBs with Surface Mount Devices for many years.

For the most part, I’ve had very little reason to look into SMD/SMT. Most, but not all, of my electronics projects involves the use of 80s and 90s parts which are usually available in through-hole packages; indeed many are only available in through-hole.

But as my plans for MAXI000 involve the use of larger FPGAs which are packaged in Quad Flat Pack I will have to learn how to build boards using Surface Mount Devices, and if I have to use some parts in SMD packaging, I might as well bite the bullet and make the whole board, at least where possible, use SMDs.

SMD offers many advantages, besides increasing the number of pins a device can have. The main one, and really the thing which drives its adoption, is miniaturization. Even projects like mine can benefit from this. MAXI09 had a huge 264mm x 194mm board which was also expensive to get made. I’m hoping the MAXI000 board will be considerably smaller, and thus cheaper.

Instead of jumping in with both feet and designing and building a PCB with SMD parts, I’ve bought several practice boards, specifically designed for learning SMD construction techniques with.

There are many approaches to SMD construction, ranging from entirely by-hand with a regular soldering iron to fully automated with large expensive pick and place machines as used in factories churning out thousands of PCBs a day.

My first go at building up a practice board used my regular iron, with the smallest tip I have. The basic process, for attaching a two terminal component, is as follows:

  1. Apply a small amount of solder to one pad
  2. Place, using tweezers, the component on the board with one end over the pad with solder
  3. Apply the iron to the soldered pad so it reflows, capturing the lead on the part
  4. Apply a small amount of solder to the other pad

This works surprisingly well and is easier then it sound due the solder mask, which contains the solder to the pads. Without any vision aids I was able attach resistors all the way down to 0603 size (60mil by 30mil, or approximately 1.5mm by 0.75mm). 1206 parts were of course much easier. Here’s a picture of the practice board after I’d attached a few parts using this method:

The SOP16 IC was soldered in much the same way: tack a corner pin to the board and then solder each pin. The result is not fantastic by any means, but I believe electrically good enough. One notable effect of using an iron is that the solder blobs have small peaks.

Whilst using an iron and regular solder works and is a useful skill to have, since sometimes correcting mistakes with the iron is the best way, it is not the quickest, nor easiest method.

A second practice board was constructed with solder paste and a rework heat gun. Solder paste is a suspension of metals (typically tin and lead) with flux. It has a consistency similar to toothpaste and comes in pots or ready filled syringes. The general process, if reflowing by hand using a heat gun or rework station, is as follows:

  1. Place, using the syringe, tiny spots of solder paste on the pads for the parts that will be attached to the board
  2. Place, using tweezers, the component on the solder paste
  3. Reflow the solder using the heat gun

Before use the syringe needs a cannula attached:

My syringe has a relatively large 14 gauge or 2.1mm in diameter cannula. Surprisingly it was just small enough to dispense an amount of solder paste suitable for reflowing the 0603 parts, but if I had a board filled with parts that small I would doubtless use a smaller cannula.

The usual approach is to place all the parts on the board and then reflow the entire board, but it is possible, with care, to use the heat gun only on specific areas. The heat gun I bought off of eBay is this one:

These are available from eBay for around £30 and seem to do the job well.

The result was as follows:

Looking closely, you can see that every single part was attached to the board beside a resistor (R33, which flew onto the the floor when being removed from the “tape”), and the SSOP28 IC, which was never included in the practice kit to start with.

In fact, loosing parts is a big hazard when building boards with SMD parts. Resistors and capacitors come on tapes:

These tapes are designed to be loaded into pick and place machines, and whilst the individual parts can be extracted by hand, it is very easy for them to fly off in random directions.

Another drawback with the tiny parts is the markings on them are essentially illegible. Even measuring a resistors value with a Digital Multimeter is next to impossible, with the smaller parts.

Looking back at the practice board, the SOP16 IC was attached by tacking a corner pin with the iron, and then reflowing soldering paste placed along the rows of pins. One or two solder bridges did result, but they were easily removed with the iron. Perhaps the hardest parts to attach were the SOT-23 3 leaded transistors. The pads are absolutely tiny and combined with the fact that my syringe has a cannula fitted to it which is slightly too large it made it very difficult to apply a small enough amount of paste to the pad.

There are two improvements which can be applied to the above process:

  • Use of a paste stencil. This eliminates the manual placement of solder paste and instead the paste is applied through a metal stencil.
  • Instead of using a reflow hot air gun, the PCB is heated in a oven.

Both facilities are available to the hobbyist. Indeed, PCB fabricators will supply a solder paste mask as an optional extra when building PCBs. Basic reflow ovens can be bought for a few hundred pounds, or it is possible to construct one by modifying a small toaster oven. I’m pretty happy with manually reflowing using a hot air gun, but using a stencil for placing the solder paste sounds like a real time saver so I will likely take that route when it comes to producing my first board with SMT components.

In summary, I’m surprised and delighted at the ease with which PCBs with SMT components can be built. I was expecting to really struggle with every aspect of the build process, but it is in fact relatively easy, at least when working with the type of parts on the practice boards. I will, though, stick to the larger (relatively speaking) parts,  mostly because they are harder to loose.

My next little project will be to build a real PCB featuring SMT parts; a keyboard controller for the MINI000 and MAXI000…

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