Here are a bunch of tutorials for manufacturing small to medium scale (10 - 10,000) piece surface mount electronics.
Solder Paste Storage
If you want to do precise, fast, and fine pitch SMT assembly, you'll have to switch from 'wire' solder to 'paste' solder
Wire solder is what most people start with, it comes on a roll:
When doing SMT work, you can use thin wire but often times even that isn't good enough, you need to use paste! Paste comes in tubs of 1/2 - 1 lb or so. The paste has consistancy of smooth peanut butter and is made of ball of solder suspended in flux. As the paste is heated in an oven the solder melts and the flux burns away leaving a solid solder joint.
We may have a future tutorial with more details about choosing paste and solder but for now we will move on to what this tutorial is about which is how to store solder paste.
The problem with paste is that the flux can evaporate off, leaving the paste 'old' and 'dry'. It won't screenprint as well - you'll have difficulty with bridges and getting clean deposits. Paste should be kept cold, but not freezing. Kester suggests 0-10 degrees C (32-50 degrees F).
If you can get your hands on a cube fridge, we suggest using that - make sure that no food is stored in there as paste is toxic and it gets everywhere. If you don't have space for a cube fridge (like us) here are two solutions we found.
We picked up this fridge from thinkgeek, but you can find 12V camping fridges at most appliance stores. It draws about 60W and has a temp display on the front. It tends to keep stuff inside at 45 degrees F or so which is perfect for us.
One thing to watch for with these is that they condense water during the summer months so you may want to keep a rag on the bottom to soak up water and wring it out once in a while.
DIY Solder Paste Stencils
Thanks to Ryan O'Hara at Ohararp.com for this information, he provides a stencil cutting service and is recommended!
Supplies you'll need:
- A laser cutter
- Kapton film, I like the 2 mil thick 1 ft square sheets from McMaster-Carr
- Solder paste such as Kester No-Clean
Software you'll need:
I usually use the Dimension layer info to make a jig for silkscreening by cutting out the PCB outline in a 0.062'' (1/16th) clear acrylic sheet
Here is a LFCSP 16 (4mm on each side) cut out of 2 mil kapton as above.
If you want to make a lot of PCBs using SMT technique, its key to use reflowing instead of soldering - so that the entire board is 'soldered' at once. But to do that you'll need to deposit paste precisely on the pads. For starting out, you can use a DIY stencil such as a laser cut kapton/mylar sheet (low cost) or depositing the paste by hand using a syringe.
However, if you ever decide to make a few hundred boards especially those with very fine pitch type parts (say 0.4 or 0.5mm pitch) it may be time to move to a framed stencil!
With a framed stencil, the thin stainless steel sheet is laser cut and then stretched into a solid cast aluminum frame. Its less likely to have misalignment because the sheet cant slide around
If you have the right equipment you can use unframed metal stencils and stretch them into a frame yourself, but unless you're a board fab house its unlikely that this is cost effective.
We get our framed stencils made by stencils unlimited . Its pretty easy to make a stencil, just export the Cream Top (or Bottom) layer from your PCB layout software and upload it during your order. They will calculate the best stencil thickness (you want a thicker stencil for large-pitch parts and thinner for fine pitch so an average is taken)* and ship it the next day.
If you have a board fab house with stencil-making capabilities, you can also ask them to make you the stencil. Don't forget to have a tiled Cream gerber if you are having panels made - so if your design is tiled get them to tile the GBC file for you!
Solder Paste Syringes
If you want to do small scale prototype SMT manufacture, it may be faster to just deposit paste by hand onto the PCB instead of getting a stencil made and shipped. We use this for our prototypes and its very fast once you get the hang of it. Instead of stenciling paste on, a small syringe of paste is squeezed onto each pad, sort of like a cake decorator (but with toxic metal).
After the paste is deposited on the pads, each component is handplaced with tweezers. Then the entire board is reflowed. you can use hot air but a plate or oven works best (we'll have a tutorial about that some day)
This technique works for both lead and lead-free PCB/parts/paste but of course leaded solder is easier to work with.
You'll also need a plunger and tips. You can get a huge assortment of different tips from McMaster . For every day use we like using the 20ga pink plastic tip from the shop above - best for larger parts like SOIC, passives and some larger pitch TQFP.
For this board we'll be placing a 0.4mm TDFN leadless part. Virtually impossible to do by hand! Using the fine needle, place a blog on each pad and lay a small bead on the TDFN pads. You can also place a small dot in the center of the TDFN pad - sometimes this helps with placement but its a bit of a toss up so try both ways and see which you prefer
They're easy to switch tips, and can power two different soldering stations (we use a hot tweezer on the 'alternate' station). The wand is light and easy to grip.
The only weird thing to watch for is that the temperature is set by the tip, not the station. The station only has an on-off switch. This can be good when you're using only one type of solder, say leadfree (700 degree F tips) or lead (600 degree F tips) but can be annoying if you use both - although we suppose you can just go with the hottest tips.
Metcals are a little pricey but you can pick one up used on ebay for around $200 (thats what we did).
For general thru-hole we like the screwdriver tip STTC-137P (shown on the wand)
For soldering FPCs or reworking a long row of pins, the SMTC-061 (all the way on the right) is very handy
For fine pitch rework we use the STTC-122 (second from the right)
We also like the two long 'hard to reach areas' tips STTC-107 and STTC-140. Great for getting underneath microSD holders and miniUSB connectors.
We also got a hoof tip SMTC-1147 (second from the left) but haven't used it yet since we don't do drag soldering so much.
Hot Air Tools
The nice thing about hot air is that its just air - this means there's no physical 'tip' nudging the part to heat it up. This is great for delicate components. Another thing is that you can heat up an entire area at once, great when you want to rework a small leadless part because the pads are underneath the chip. The annoying thing is it takes a long time to heat up a board especially if there is a ground plane involved. You can try preheating the board if you have a preheater (basically a small reflow oven works fine).
We decided to go with a Hakko pencil tip hot air station. This station has some things going for it: there's an internal pump, its easy to use and it has a fine tip for small component rework. We picked it up off of ebay for about $150 which is lower than new but worth the risk of it not working out. We recommend getting a non-'chinese clone' air rework station since its common for a poorly made one to work ok at first and then die 6 months later because the pump is low quality, or the heater gets damaged. The old 'black box' Hakko's are discontinued and replaced with the new violet/yellow FX series, so it may be easier to get them off ebay for a discounted price
However, we recently decided its not 'good enough' for us so we bought a Hakko 852 off ebay for $400. We'll post up here when it comes in about whether we like it more.
We use the Hakko Hot Air station for SMT rework, especially removing small resistors/capacitor or removing/reflowing ultra-small QFN's or UDFNS.
This is a nice video with a lot of details on using hot air reflow. Its slow but it works!
For prototyping, modding, hacking and reworking, its good to have a range of resistors in your favorite package size. We like the digikey box packs of resistors, and have a set of the 5% in 0603, 0805 and 1206. Sometimes we wish there were more values so we may get the 0805 in 1% for a few values
Either way, having a resistor set is essential!
Pick your poison:
For the 1%, they come in boxes of ranges, pick them up as necessary
0805 1% Resistors (50 of each value):
0603 1% Resistors (50 of each value)
Our favorite use for tweezers is to remove or rework SOIC and such. These are big chips, hard to heat up evenly and quickly but the large PTTC-x06 or PTTC-x07 cartridges make em easy!
For example, I want to remove this 14-SOIC chip:
Thankfully, Adafruit now carries a much more affordable alternative.
New! We've upgraded this microscope to 5.0 Megapixel (from 2.0M) and this one comes with two removable plastic caps to get close ups for a wider focusing range.
If you plug this into any computer, it just shows up as a standard USB camera (we used this for our weekly Ask an Engineer show) and the Windows/Mac software lets you take snapshots using the button on the side of the microscope or direct from the software (so you don't move the camera).
We tried a bunch of different USB microscopes and found this one to be the best combination of optical clarity, usability, and price. It's perfect for electronics hacking, rework, SMT (de)soldering, inspection, and soon you'll find yourself pulling it out to look and photograph all sort of cool small stuff around your lab and home.
Pick up our USB Microscope here.
We took some snaps of this 'scope while looking at a tiny 0201-populated GPS board at 20x magnification: