Surface mount PCBs (Part 1)

If you look at a circuit board today, you’ll see a beautiful array of surface mount chips and components, including very fine 0.5mm or even 0.4mm leaded devices and BGAs.  Some of these ‘exotic’ devices can contain really advanced technology such as high speed ARM microprocessors, flash and high capacity memory, and FPGAs.  If you’re like me, you’ve looked at these boards and wished that you could produce circuit boards of such fine detail at home, and build projects with these exciting technologies.  Well, I’m here to tell you that it’s not as difficult as it looks.

Tools

I’ve been working with 0.1” pitch through hole devices for over 30 years, but it’s only during the past 3 years that I have ventured into the world of surface mount.  The availability of home equipment to create these boards has rocketed into the mainstream.  No longer do you require expensive tools and noxious chemicals.  The main tool of choice is an SMD hot air work station.  I acquired mine on eBay from China where such tools are super cheap.  I opted for a digital display so that I know precisely what temperature the tools are running at.  A quick google search for “digital smd workstation” gives me hits as low as $56 – less than a good quality iron.  I opted for a Yihua 992 dual station, with smoke extractor and temperature controlled iron.  With hindsight, a simple digitally controlled hot air station would have been sufficient as I rarely use the iron.  If you’re on a budget, you won’t miss out by spending $60-$70 on a hot air station.  Just make sure it’s got an accurate temperature control and air speed control.

Yihua 992 Hot Air SMD Rework Station

I also purchased an infra red IC oven, but as I’ll discuss in part 2, that may have been unnecessary.

The other tools that you’ll need:

• One or more craft scalpels (sharper is better)
• SMD anti-static tweezers preferably non-ferrous (sharper is better)
• Desoldering braid
• Cutting mat
• Heat resistant cork table mat
• LED magnifying lamp

Again, don’t spend a lot of money on the scalpel and tweezers, but make sure they are precision tools.  They need to handle components with a body less than one millimeter in size, so sharp is good.  A curve on the tweezers helps with positioning like the examples below.

The cutting mat and cork table mat can be found in your local “Dollar Shop”.

Consumables

We need to clean the board with isopropyl alcohol and cue-tips or a lint-free cloth.  We also need to “prime” the board with solder paste, so start with a no-clean 63/37 leaded solder paste as this is easier to work with and melts at a lower temperature than the unleaded varieties.  A syringe-type packaging with a fine plastic nozzle will allow accurate placement of the solder paste.  Only buy what you need for each job because solder paste ‘goes off’, making it impossible to get the paste through the nozzle.  You can keep it in a cool place to extend the life but after first use the accuracy of application declines due to the paste setting in the needle.  I suggest you buy the smallest syringe you can get, especially when you consider the environmental impact of paste that can no longer be used.  (Don’t throw it in the household garbage!)  Even 15G of solder paste will cover a huge board as you need only a tiny amount for  each pad.

PCB

A good quality PCB is the critical success factor in the whole surface mount process.  The design of a new PCB from scratch is a complex process and will be the topic of future posts.  For now, I suggest you use an existing design.  You’ll need to send your design away for manufacture.  I use OSH Park as they provide a fast, very low cost service and their upload portal makes spotting errors simple.  They also have a great library of contributed designs that you can order from them.  They will provide a two or four layer board with spacing down to 0.005” (5mil) with the appropriate solder mask.  It’s this solder mask that makes the whole process work and keeps the solder from bridging between pins.

Assembling the Board

Some surface mount components are tiny and it can be difficult to identify once they’re out of the packaging,  so keep them sorted and in order of use in their packaging until you place them on the board.  I’ve worked with components down to 0603 (imperial, 1.6mmx0.8mm metric), but smaller components wouldn’t be a problem with a properly designed board and your magnifying lamp.

The first job is to cover the pads with a thin layer of solder paste.  If you ensure that the paste makes contact with the component ‘leg’ then it will naturally wick up into the space between the leg and the pad.  In fact, it’s this wicking effect that is key to the surface mount process.  Even solder paste spread across multiple pads will separate on heating if its spread is thinly enough.

My method involves applying the solder paste individually to each pad via the plastic syringe needle.  A short press of the plunger will start the paste moving and it will continue to squeeze out of the tip for a short while after plunging.  Use this time to dab small drops of the paste on each pad.  A slight push of the plunger will cover three or four pads.  This process may take 10 minutes or an hour, depending upon the complexity of your board.  Most solder pastes are workable for several hours, although they will tend to spread across to neighbouring pads if dolloped on thickly.  If this happens, run your scalpel lightly between the pad to create a small gap to be sure to avoid bridging.

When you’re happy that all of the pads are covered, it’s time to place the components.  This is where the tweezers and magnifying lamp demonstrate their value.  Ensure that polarized components are placed the right way around, according to your design.  Usually the packaging denotes the orientation so you don’t have to scrutinize the component itself.  Placement doesn’t have to be perfect, the surface tension of molten solder will straighten up the components.  The solder paste is slightly tacky and will hold the components in place until soldering.

Now the magic

Once the board is primed with solder paste and the components in place, you’re ready to heat the board.  Place your board on the cork table mat to protect your work surface.  Gentle heat is the name of the game.  Use a medium sized nozzle, set your hot air temperature to 350C and lift the handset from its cradle to start the flow of air.  Set the air speed to minimum. If you do not do this, you’ll blow away the components you’ve just spent the last hour placing carefully on the board.  Hold the handset directly over the component, pointing down about 6-8 cm from the board surface.  If the components start to move before the solder is molten, move the nozzle further away from the board surface slightly and push the component back in place with your tweezers. After about 10 seconds you should see a change in the solder paste as it melts and the components dance into place.  Hold the heat for another second or so to ensure all of the paste has been activated, then move onto the next component.  Your joint should be evenly spread and shiny.  Once all components have been soldered, leave your board to cool for a few minutes before you pick it up.

Inspect your board under a magnifying lamp and if there are any solder bridges use a standard soldering iron and solder braid to wick away any excess solder.  If you have used your paste sparingly, solder bridges are quite rare.

Congratulations, you’ve made your first surface mount board!

Next Time

In my next post, I’ll document my first experience of BGA technology as this was more frustrating (and expensive), but ultimately more exhilarating.

Useful links:

1. https://en.wikipedia.org/wiki/Surface-mount_technology
2. http://store.curiousinventor.com/guides/Surface_Mount_Soldering/Tools