I recently posted about a simple and inexpensive pick-and-place vacuum head that I’ve been designing. With the swivel house connector figured out, I’m just waiting on some hose couplers before mounting it to give it a spin. In light of the pick-and-place vacuum head seemingly in good shape, I thought I’d take a moment to post a few pictures of a prototype solder paste extruder that I started designing this weekend, since I’m hoping to do this automatically as well.
When I first started with surface mount components I just manually dispensed the paste from the syringe, and in the last few years I’ve been using the inexpensive solder paste stencils from places like OSH Stencils to really speed up the pasting process, as well as make it more repeatable and reliable. But as much as I refine my technique, I’m still not great at stenciling paste on larger boards that have lots of fine pitched components, on the order of 0.4mm to 0.5mm spacing. I find that after stenciling I’ll spend a good deal of time moving the paste around with fine tweezers to help prevent bridging (although often there are still bridges), and so I usually end up manually soldering fine pitch TQFP parts, which is very time consuming. For a project in my queue, I’ve designed a board that has about 300 parts (summing to a little under 2000 pads), most of which are 0402 capacitors or 0.4mm pitch leadless QFNs over a board that’s about 10cm on an edge — which is likely beyond my capacity to easily, reliably, or comfortably assemble by hand. This is exactly what these machines were designed for, after all!
I had a good look at a bunch of open paste extruders on Thingiverse to see what folks have been up to, and how this is traditionally approached. I’ve read that high-capacity commercial machines often use pneumatics for solder paste extrusion, but that’s a level of complexity that a lot of the folks in the open source community aren’t willing to get into (myself included), although there are some examples of this approach being used, like the Claystruder. The two more common approaches to open paste extruders seem to be using a lead screw to press on the plunger (like with this simple-paste-extruder or the Printrbot Paste Extruder), or using a belt attached to a vast system of gears to very slowly press down on a syringe plunger with high torque (as with Richrap’s Universal Paste Extruder). I happen to have one of Richrap’s extruders and it’s a beautiful piece of work, but all the gearing make it a little large and heavy for the machine that I’m putting together. The lead screw approach has always appealed to me for it’s simplicity, so I decided to give that a try first. The recent kickstarter for the Voltera Circuit Printer has also convinced me that the lead screw approach can likely be done very compactly, with a minimum of weight, and support quickly changing paste syringes.
The prototype that I put together is more of a sketch in hardware to help me appreciate the issues of paste extrusion, and help hammer out a design. Instead of using a plunger, I’ve used a very long lead screw that acts as the plunger, and has a gear atop with a captive nut to transfer force. The gear is driven by a Parallax continuous rotation servo, which I thought I’d try given that it simplifies the design by having an integrated gear box (which gives it lots of torque), and it can be directly driven by a microcontroller rather than requiring a separate stepper driver.
There are aspects of this design that work really well — the syringe is very easily accessible, and very quick to change. It’s also fairly compact, but I think could be even more compact — the size is largely driven by the orientation and width of the servo motor. Some important bits that I observed with this test rig:
- Resolution requirements: Using this 8-32 screw, it takes about a quarter turn of the nut to start the paste extruding (from rest). But the paste keeps flowing, so it’ll likely need to move (say) a quarter turn in one direction to get the paste flowing, then immediately move most of a quarter turn in the opposite direction to stop the paste from flowing, and get an appropriately small spot of paste. It’s not clear whether I’ll have to move to a stepper to get the required resolution for this back-and-forth traversal.
- Force: Quite a bit of force is required — it does bow the MDF on my prototype a bit at the top, so that plate will have to be more rigid.
- Reversing: Some designs include rails, while others appear not to include them. Given that reversing appears to be not just a convenience for more easily swapping out paste syringes, but also an integral part of the extrusion process to prevent oozing and more accurately deposit precise amounts of paste, some rails will have to be added to ensure that the linear motion is transferred to the screw (and that it doesn’t just spin in place).
Definitely a successful experiment, and plenty to absorb for my next free evening infront of the laser cutter. Hopefully with a few more iterations I’ll have arrived at a very functional, capable, and compact design. Thanks for reading!