A while back, I bought a vacuum pickup tool as such things promise to make SMD work easier than with tweezers. The pickup tool is circular which makes it easier to rotate in your fingers and it picks up from the top, so you can’t accidentally dip it in some of the solder paste making it sticky.
I bought one of the ubiquitous “Double type 12000” pickup tools off ebay. They’re cheap (£20 including shipping), and much like the 852D+ rework stations, they seem to come in a variety of brands which have nothing different except the label. If you like yellow, get the “VAC” brand one, but if you prefer pink, get the “Cosmo” brand one.
They comes with two suction ports, two pickup wands and a selection of tips and rubber suction cups.The wands are connected via transparent hosing which looks like that aquarium hose, but is rather thinner walled and much more flexible.
A slightly out of focus and badly lit picture of the tool in question.
I tried to use it it and… it sucks (sorry!) but isn’t very good. There’s a small hole on the pickup tool: cover it with a finger and you can pick up things. Uncover it and they drop off. There Are two problems with that. Firstly, I found it completely unintuitive, since the operation is “backwards”. I dare say you could get used to it with sufficient use. Secondly moving your finger disturbs the position slightly, so just as you release, the position goes bad, which is the worst time for it. As a result, I found it unusable and went back to tweezers.
Apparently the good ones operate off foot pedals, so it’s time to add foot pedal control to it. So, I got a foot switch and a couple of cheap solenoid valves and a T adapter off ebay. One claimed a working pressure of 0.02 to 0.8MPa, which seems a bit too high for this pump, the other had no specs. But they were only a couple of quid, so I figured, what the hell?
Aside: every time I buy some of these random goods from China off ebay I’m astonished that it’s worth someone’s time to manufacture a valve/footpedal, sell a single item, package it up and send it half way around the world for £2, when the manufacturer, vendor, ebay, creditcard processor and post office all need their cut in order to turn a profit. The efficiency of the system is mind boggling.
The idea is to connect the solenoid valve to the hose such that opening the valve exposes it to the atmosphere, killing the partial vacuum inside. So, I connected it all up (minus the foot pedal which is yet to arrive)…
Valve connected to the tool. Yep, the hose is bodged into that port with tape.
Oddly enough it kinda-sorta works. By powering off and on the solenoid, I can pick up/drop quite large things. However I can only pick up small things, I can’t drop them again. This implies the internal resistance of the valve is very high so that it lowers the pressure a bit, but not enough. Time to delve in and figure out why. Firstly the valves in question:
The two valves.
These valves are really very similar. The solenoid are almost identical, but not quite (one’s 12V, the other 24V as it happens), and the connection from the solenoid to the valve body have the same form factor, so they’re interchangeable.
The valve itself is a simple, cunning design. It has an internal rubber diaphragm with a rigid plastic backing that serves the dual purpose of acting as the switching component as well as sealing the two halves of the valve body together. The core of the solenoid is inside, the sealed part. One of the two valves has an additional spring (shown) holding the diaphragm down weakly, in addition to the spring weakly pushing the solenoid core out. Note the arrangement of the valve internals: the fluid enters from the right and exists from the left. There are also two very small holes in the diaphragm. The top of the solenoid core is squishy plastic and when off, the core seals the middle hole by pushing against it.
Now imagine there’s pressurised fluid coming from the right. This leaks into the top half of the valve and pressurises it. The outflow is at low pressure, so this pushes the diaphragm down, sealing the valve. Now the solenoid opens. This opens the middle hole and the fluid in the top chamber leaks into the outflow. The two holes are very small, so there will be a substantial pressure drop across them, meaning the top chamber now has half the pressure of the input fluid. Looking at the arrangement of the chambers in the bottom of the valve, this means that the high pressure fluid coming in will push the diaphragm up, allowing fluid to go from the entry to the exit.
This is a very cunning arrangement is essentially an amplifier because the solenoid is a bit weak on its own to do much. It also explains the behaviour seen above. The valve needs substantial pressure to operate. Not only that, it must always drop a fair bit of pressure across the body too, which means it also needs substantial flow rate. The pickup too has neither, but the leakage which happens when the valve is open lowers the pressure enough to drop large parts but not small ones.
The solution is to rebuild the valve to turn it into a needle valve which can operate at very low pressures and flow rates.
Rebuilding the valve.
First, I drilled out the central hole to about 2mm diameter to increase the air flow rate. I then put a 3mm shaft (back end of a drill bit) and filled the top with molten polycaprolacetone (low temperature thermoplastic). The 3mm shaft keeps the hole open, but it’s necessary to wet it with slightly soapy water (soap aids the wetting) so stop the plastic from sticking strongly to metal. I then removed the shaft and flared the opening of the hole. Flaring is important since the core can move side to side by around a millimetre and so it needs to be guided into the hole. I then pushed the solenoid core in gently, so that the top section of the hole matches the profile of the core perfectly. Removing the core, you can just see the lip on which it will rest inside the hole:
Rebuilt valve body. I initially used a pencil to try to flare the opening. The red stuff is the paint that the plastic pulled off the pencil.
It’s also necessary to then drill out the second hole so there’s somewhere for the air to go once it enters the top chamber. The result works extremely well. The solenoid valve can now e used to pick up/put down very large and very small parts without altering the speed of the pump. This indicates the air flow rate when open and sealing when closed are both very good.
The end result is it works pretty well. Once my foot switch arrives, it will be complete and usable.
Edit: Well, that blows!
While playing with, I observes something very interesting. When the valve is open the pickup tip actually blows air out of it! My complete speculation is that while the pump is sucking (it’s oscillatory), little air gets drawn in due to the high resistance. However, a lot of air gets pulled in the valve tube. This air has momentum, so when the internal valve on the pump closes and it stops sucking, then moving air has to go somewhere, and out of the end of the tip is the only option.
It’s very gentle (not enough to disturb SMD work), but it is an amazing effect, and probably will help the tool drop very light things even if they’re slightly sticky.
Edit 2: My foot pedal isn’t here yet 😦
Well, my foot switch still hasn’t arrived and I’ve done every part of the project I can without soldering up my circuit boards.
The solution is one absolutely appalling hack. I made a foot switch out of a sponge with a hole cut out of it, some old circuit boards with a Prym 13mm snap button stud soldered to one, wires, and of course duct tape. It works fine since the 24V solenoid only draws 200mA at its rated voltage.
It’s horrible and looks like something I might have made in junior school. It does work and once compressed to the right depth is remarkable easy to control.
Edit 3: I tried it and it’s fantastic!
The new foot switch controlled pickup tool is a complete game changer compared to using tweezers. I had to solder a board full of closely packed 0402s and 0.5mm pitch DFNs. I couldn’t imagine doing it with tweezers.
Actually I can, and I’m REALLY glad I have this tool now.