MOAM Industries

Make great things.

Mounting thermocouples

In a recent post we investigated methods of securing thermocouples to PCBs. When reflowing boards it is vital to have an accurate temperature reading from the surface of a PCB and to ensure you are not reading air temperatures instead. The documentation we discussed previously recomended that high temperature solder should be used to affix thermocouples to PCBs for temperature reading, if that is not feasible then aluminium or kapton tape is a possible alternative. So to provide users with an easy method of securing thermocouples that also gives them a variety of affixing options we have developed a small 5cmX5cm PCB which provides a wide variety of mounting options.

Thermocpuple mounting PCB for accurate board temperature references

The board is relatively simple with a variety of conveniently placed holes. The clusters of holes in the corners are to facilitate affixing the board to the wire rungs found in many toaster ovens. The center pads and holes provide a mounting point for the thermocouple itself. As high temperature solder is difficult to find and even more difficult to solder we have included an array of holes so copper wire can be used to tie the thermocouple into place, a small bend of the beaded tip of the thermocouple will ensure it stays firmly in place. Of course you can still use high temperature solder or kapton tape if you desire.

Breaking radio silence…

Things have been moving very quickly here at MOAM industries and we have made some large steps in the past few weeks. Most notably we have almost a thousand PCBs on their way to us from China after our latest fabrication run. The majority of these are for our soon to be released reflow controller kit.

Bottom PCB for the reflow controller, housing the solid state relay and DC supply

The final stages of our kit are now coming together, once we have verified our new PCBs we will start assembling the kits and machining the enclosures ready for sale. It has been a long and expensive process to reach this point, purchasing stock, spending weeks on design and documentation, testing prototypes, purchasing tools etc. However we knew this would be the case, especially considering we decided not to pursue the kickstarter funding route and instead drove our efforts towards having a well-tested and reliable product at launch instead of just an idea. In the next few days we will be documenting our design and construction process for creating this style of kit and sharing our experiences for others. We will also be releasing more details on the controller and its capabilities now we are in production of the release revision.

Attaching thermocouples to PCBs

One of the challenges when reflow soldering is having the thermocouple report an accurate board temperature. For this, the thermocouple must be in physical contact with the board. Up until now we have made do bending a thermocouple through the holes of our PCB reference board.

This technique doesnt fall into the catagory of a reliable, repeatable solution so instead we have been looking for alternative options. Thankfully kicthermal have done some research into this and provided some solutions for attaching thermocouples to PCBs

http://kicthermal.com/paper-a-articles/261-a-comparison-of-methods-for-attaching-thermocouples-to-printed-circuit-boards-for-thermal-profiling

So it looks as though aluminium tape secured with kapton tape is the quick and easy method or for solid, reliable connections, high temperature solder to connect the thermocouple bead to a PCB. In the near future we may look at releasing some thermocouples already high temp soldered onto small pcbs, ready for placement into reflow ovens and skillets.

Good news everyone

 


It is indeed possible to succesfully solder small VQFN packages with a limited number of tools and without hot air. It appears the guide we linked too the other day is currently down however we will be posting our own abbreviated guide in the very near future.
We used a few simple, relatively cost effective tools:

  • Fine tipped soldering iron (we used a simple atten soldering station)
  • Solder paste
  • Tweezers
  • Tissues (lint free cloth would be better)
  • Isopropyl alcohol
  • Solder wick
  • Microscope ($30 USB variety works fine)
  • Reflow controller prototype
  • Thrift store frying pan
  • continuity tester

The process is relatively straight forward however time consuming, a little luck doesnt hurt either. The basic principle is to apply a little solder to all pads, both on the chip and the pcb so when the boards reaches a reflow temperature all of the pads have the required ammount of solder and there is no paste on the soldermask to potentially cause shorts. The one exception is any thermal pads in the middle of the chip, they should just have solder applied on the chip, not the PCB. This was easier to apply directly with wire solder, simply apply enough to ensure the whole pad is covered but not buldging, looking side on should show a very small rise in the centre.

Tinned pads on PCB ready for soldering

The guide we linked to recomended tinning a soldering iron then transfering this to the pad when it is heated. We found this process to be very hit or miss so instead we worked with solder paste. A thin smear of paste over a line of pads then moving over the pads with the iron melting the solder and ensuring it beads onto the pads. Then wipe away the excess with a cloth and alcohol. Whilst this sounds relatively straight forward any mistakes can be time consuming to clean up. A microscope allows you to get a clear view of the surfaces and check that each pad has some solder (you dont want it flooded as it is likely to cause a short later).

VQFP Hand soldered package using solder paste

The next step is to reflow the board. This is where you may need hot air if you are just working on the one board component. In our situation we were working with a virgin board and this was the most complicated component (the other are mostly 1206 package passives) so we alligned the package (another time consuming task) ensuring that all pads were alligned on all sides within half a pad. This we inspected with the microscope and nudge appropriatly. Next time we repeat this process we would probably use a compong like Blu-Tak to apply a small adhesive force and stop the component moving too much when nudged. Finally reflowing the board to set the solder, a few small nudges during the reflow stage should ensure the part is properly alligned and reflowed as it snaps back into place.

From here most of the work is done, final checking and some additional soldering if required. We found that solder wick can remove brdiges provided they are external and accesible. Power pins had a little extra solder applied to them using the tinned iron method and all pins were tested for shorts with a continuity meter. The probes on my meter were not fine enough for my liking, dulled from years of use. Instead a small piece of solid wire cut at an angle and held in aligator clips workes very well.

Soldering QFN and VQFN parts without a stencil or solder paste

The recipe of the day here at MOAM is to work on a motor controller breakout, a test bed for another project. The module is an L6206Q which is a dual 2 amp per channel motor controller with current limiting and thermal cut out. They are a very flexible motor controller and one we are interested in working with in other projects.

This is our L6206Q development board, try not to judge our slightly obscure header labeling, it is an internal product after all.

So several weeks ago we developed a small testing PCB and had them made through Seeed studios fusion PCB service. Now it is not really cost effective or terribly easy to get solder stencils made (are you listening seeed!) through another manufacturer although i belive there are some prototyping services that offer a stencil with their pcbs, they are just a little too expensive for our liking at the moment. So we somehow need to solder a VQFN package with a 0.3mm pitch. Our tools are a $25 toaster oven, our reflow controller prototype and a mediocre Atten soldering station and we are hopefully going to follow this very informative guide we have found online http://store.curiousinventor.com/guides/Surface_Mount_Soldering/QFN/
Now unfortunatly we dont have any hot air reworking tools (they are on a very extensive list) neither do we have a warming plate so the first step of the day is to go and buy and electric skillet and build some extra functionality into our reflow firmware to help it function as a warming plate, fingers crossed!

MOAMindustries is infected for the day. Quarantine protocols engaged

AsĀ  a result of a viral outbreak of a mutated viral strain that causes the dead to re-animate and seek out human brains (some may say that we are at no risk for this reason) MOAMindustries is enforcing a strict quarantine protocol!

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Fitting tests for PCBs

As we move towards the final version of our reflow controller kit we needed to test the mechanical fit of our boards. This ensures that all of our holes are alligned properly, and components fit with the appropriate clearance. This is a really useful step because when you are working on a screen where sizes can be blown up 1,000% what appears to be adequte clearance can easily be misrepresented.

With Eagle the easiest method is to select the appropriate layers. Typically tPlace, bPlace, Milling, tNames, bNames, pads, vias and holes. This ensures all of the mechanicaly important layers are visible whilst the traces are not, they typically confuse and over power the drawing.

Output image from Eagle CAD

To output the drawing simply use the print function under File>Print. Ensure the scaling is set as 1 then select your output destination. As our printer is not connected to our development machine (long story) we output the file to a PDF then print from a laptop (again ensuring that no scaling or “fit to page” are selected).

The next step is brought to you by your year 6 craft teacher and all the training she provided in cutting along the lines. Scalpels are the easiest way to cut along compond curves and also make it easy to cut out the associated holes.

Doing this process really helps you get a feel for a PCB without spending hundreds on prototype boards, it also helped us pick up an issue with our enclosure. When we ordered our checked the sizing was appropriate with a drawing provided by the manufacturer. Off this drawing we designed all of our mechanical fixing points, as it turns out the drawing from the supplier was out by almost 2mm in one dimension which pushed all of the bottom fixture points out. By working through this process we picked up a potentially costly mistake and got a better feel for our product. As a final note this method is also useful for checking the allignment of other fixtures such as voltage regulators, our solid state relay and header terminals.