SMT Reflow Soldering Using Kapton Sheet Solder Stencil

I run a small Electronics design and retail company. I specialise in producing working prototypes from a concept, handling all aspects of schematic design, PCB layout, manufacturing the prototype and writing any micro-controller firmware. I hope people who read these guides will feel inspired to try some of the trickier aspects of electronics, it's not all magic and high tech - you can make some amazing designs by hand with some care and practice!

Surface mount soldering is not as tricky as some people make out - provided you have a good PCB design, with a correct solder-mask layer, and follow some simple rules and methods. I hope to make this easy.

There are three critical layers of interest regarding the actual PCB design; that of the copper, the solder mask and the paste mask. The first two are physical layers of the PCB, and dictate the PCB land (where the solder contact will be made) and the constraint of the soldering (to manage solder bridges, and other things) The paste mask layer is used to create the paste mask, which is typically a thin metal sheet, that is often laser cut these days.

This metal mask is used for a simple silk-screening process, where liquid solder paste is pushed through the mask (or screen) using a swipe of a metal blade. This process is also usually automated on the production line.

For prototyping needs, we don't need the expense and long-life of a metal screen, so recently it's been possible to buy solder stencils made from thin kapton sheet, using various web sites, for a very low price. These are still laser cut, so have excellent precision, just will wear out quickly. But we don't care about that :)

Update:- The pictures have lots of extra captions and information - so click on them and scroll through the sets!

Step 1: Prepare Your Data

I'm assuming you already know how to make a PCB design that is correct for manufacture, and have sent the gerbers, received your bare PCB, and it is correct, with a viable soldermask for the components you intend to solder.

The next step is to generate the appropriate data set for the solder paste stencil manufacturer. Recently I tried OSHstencils, so I can show you how easy it is to prepare the data in the format they need.

OSHstencils make a big deal about not including the board edge cut in the paste layer (presumably, because it's a waste of laser time, and could make the mask bigger than needed) I use KiCAD for my designs, so in the 'plot' dialog, you just need to select 'Exclude PCB edge from other layers' option, and provide three separate gerber files, one for each PCB side paste layer, and a third one containing the 'edge cuts' as a reference. Plot those layers, load them into gerbview (from the KiCAD main window) and check the data contains just the paste layers you expect, without the edge cuts. Login into OSHstencils, upload the data and check it looks correct in their web preview, and make your order.

Step 2: Gather the Materials

Aside from your PCB and stencil set, I would have this list of things ready to go:

• Solder paste. I recommend using leaded solder paste, from a reputable brand. Ensure it's 'in-date' and has been stored at the correct temperature. Failure to do this will reduce chances of success.
• Spreader / solder paste squeegee. A 'proper' metal blade squeegee will be best, but the free credit card 'spreader' you get from OSHstencils will work reasonably well, with some care.
• Isopropyl Alchohol (or propan-2-ol) and lint-free cloths or 'technical wipes' for cleaning. You need to do plenty of this.
• Kapton tape to affix the board to something immovable and to attach the mask to the PCB. Kapton is the best to use, as it is relatively cheap, pretty sticky, and very resistant to heat. PET tape is an alternative, but is harder to source and more expensive.
• A PCB holder or clamp can help if you have one.

Step 3: Cleaning

Clean the PCB with IPA soaked into a cloth (just a little) If you have a microscope or decent magnifying lens, now would be a good time look around for any stray fibers which might interfere with the solder pads.

Personally, I cheat, and use an ultrasonic cleaner (one sized for home use, cleaning jewellery is just fine) filled with a little IPA. This allows the board to be totally submerged in solvent, and will remove all traces of solder flux and dirt. Just make sure not to cook it for too long, as the alcohol has a low boiling point, and will start to fume excessively.

I don't need to add - do this in a well ventilated area!

Clean the paste mask, both sides, and again inspect for anything kicking around which might interfere with the solder paste.

I believe one of the key points to electronics is cleanliness - a shiny clean joint is a good one. You can promote good soldering, especially with SMT and re-flow, by keeping everything pristine and free from stray fibers, hairs etc.

Step 4: Attach the PCB and the Mask

Attach the PCB to something that won't move when you don't want it to. I just used a slab of wood, and an ESD box to jack the board up, as there were already a few components on the other side. The slab of wood is small enough, to slide around under the microscope, but heavy enough to not move when I don't want it to.

I attach the mask to the PCB from two opposite sides, to keep a little tension on the mask to help it stay flat. Don't pull too hard, else you risk tearing it.

Step 5: Apply Some Solder Paste and Spread It

Let the solder paste warm up to room temperature. Hold it in your hand to speed this up.

Apply a little solder paste. You don't need a massive amount, but too little will result in most of it sticking to the spreader (a squeegee may work better) Spread it roughly into the area of the mask where the openings are. Using gentle pressure, push some of the paste over each of the openings.

When all the holes look covered, make one final clean sweep with constant pressure. This will push the paste firmly into the openings, the excess will be ejected out of the top, and be swept away with the spreader. Don't push too hard as you could rip the paste out of the holes or even tear the mask. This takes a little practice. If there are areas that are still showing copper through the holes, just give it another sweep, in a different direction.

Step 6: Let It Rest, Cleanup and Prepare for Placement

I leave the solder paste a little while, say 15 minutes, just to let it all settle before touching it. I use this time to cleanup my masks with IPA, inspect for fibers and damage, and put them away somewhere clean. You can store them in an envelope when not in use.

It is also a good idea to lay out all your parts, ready for picking and placing. I just tape them to the work area before opening the reels. That way, you don't loose parts on the floor.

If you do drop some parts, and need to recover them - check them for cleanliness - wash them in IPA if needed and blow off the excess! Dirty, or hairy parts will cause problems when they are this small!

Pick out the exact quantity you need - placing them either to one side, or onto the PCB to the side of the solder lands ready for placement.

As soon as you're done picking parts, seal the reels with tape and put them away.

Remember! Modern electronic parts are environmentally sensitive, so if they came packed in a sealed dry-pack (you know - the little bags of silica gel) put them straight back, seal it up and store it somewhere warm and dry.

These sensitive parts will have an 'MSL' (Moisture Sensitivity Level) rating, which determines how to store them, how long they can be out of the dry-pack, and how long to bake them prior to re-flow soldering. If in doubt - read the datasheet for the part, or consult the manufacturer's website on the subject. Sometimes (and to be honest this is only really massively relevant for mass-productions) parts can explode (we call it pop-corning!) under the heat of re-flow, after they have absorbed too much moisture!

For prototyping, we don't care so much, just handle them with a little care, and try to keep them dry and you'll be fine.

One final point - if you're really worried or unsure how a part (especially an expensive one) has been stored, you can always bake out the moisture in an oven. Typically this would be for 10 or more hours at 125 DegC. This will vary from part to part!

Step 7: Place the Parts!

This is the hard part in my opinion, as you need good eyesight and steady hands. Just take it slow until you have enough practice, and you'll find that it's perfectly possible to hand place pretty much any part, even small ones like 'chip' resistors.

A binocular microscope will help for smaller parts, even just allowing you inspect the mess you just made!

Clean your tweezers with IPA (I just slosh them in the ultrasonic cleaner for a few seconds) then blow off the excess. Small parts will stick to the tweezers annoyingly, if they are not perfectly clean, so spend the time.

Please note, the solder-mask design you made is crucial here - not the initial placement, or even the solder paste masking. So long as you don't have a huge excess of paste, and the parts are roughly placed central over the lands, the surface tension of the molten solder during re-flow will do all the aligning for you - it's magical to watch. One day I'll try to film it.

If you're placing parts with multiple close contacts, such as QFN or QFP, SOT-23-6 etc, just ensure the part is placed close to the correct pads - the re-flow process will pull the part into alignment, but it will be affected by the accumulated effect of all contacts, so make sure it's not too far out, or it might move to the next position and that would be a shame!

Step 8: Re-flow Solder

This is the business end, and what all the preparation is for. I always find this a bit of an anti-climax as it takes a bit of time to get to this point, and the actual soldering is over so quickly!

For small prototyping runs, I just use a hot air station for re-flow soldering. You can also use a re-flow oven for this (I have access to one, at my local hackerspace (Southampton Makerspace) but I didn't need it for this design, plus it has SMT parts on both sides, so you can't really use an oven for this anyway (well to be precise, I could, but only for one side - so there's no point bothering!)

The key points for this are

• (Optional) Pre-heat the board with a PCB pre-heater if you have one.
• Set the air temperature as low as you can for the solder paste you have - lead free solder-paste is harder, and you need to go hotter. You don't really need to do this for prototyping - obviously it's mandatory for manufacturing now, but that's not relevant here!
• Set the airflow low too - too high and you risk blowing the parts out of alignment. It will just take a little longer to get up to temperature, but not excessively so.
• As soon as solder paste has melted and the flux is flowing (i.e. doing it's job) the part will be pulled into alignment magically. Make sure to heat the part evenly, so all its pads have molten solder at the same time, else it might not work very well.
• Take the heat off ASAP - you don't want to over-heat and damage the part!

Inspect the joints afterwards - they should all look clean, smooth and shiny. Any evidence of graininess, is solder paste balls that have not melted - just re-heat that part a little more to get it to flow.

Don't worry about perfection - with hot air, by hand, you won't always get absolutely perfect alignment - just get it good enough to work, without overheating and destroying anything!

A re-flow oven will do a better job of heating the board evenly, so if you have access to one, and parts only on one side, then consider doing that!

Step 9: Admire Your Work

After soldering, I tend to throw the whole thing back into the ultrasonic cleaner, with IPA. This will dissolve all the excess solder flux, and reveal any bad joints and problems. Plus it looks nice too.

Look out for more articles from me - I will be writing a lot more soon :)

If you need professional help with your prototyping, I may be able to help you get from concept to schematic capture, PCB design and prototyping. Just contact me on my company email!

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