044 :: stepsticks concentration camp

After picking up some custom supplies from Element14 (Farnell) and hunting for some real flux from the local electronics shops, it is time to continue torturing the stepsticks, to boldly go where no stepsticks has gone before.
Exhibit A, a stepstick from reprapdiscount (China), driver = A4988. Rsense removed for testing (suspected from a test in post #003 that Rsense is not 0.05ohm). Using a rather accurate DMM with 0.01 ohm resolution (see post #012, DMM UT-61E by Uni-T), +/- error of 0.01 ohm. we find that the measured resistance flickers between 0.01 to 0.02 ohms, we can therefore conclude that reprapdiscount did not even use the stipulated minimum 0.05ohm Rsense components, like EEVblog dave would say ... "WHAT A LOAD OF TURD !!!". So folks, if you buy from reprapdiscount, there is a chance that some components are not what they are suppose to be. @ 0.05ohms, we are expecting the driver to drive a maximum of 4amperes. So in order to tune the current drive to within normal performance values, you are limited to 0.8 volts of Vref (based on a maximum of 1.6v Vref). If you have stubby fingers like me and managed to destroy 4 of them in 3 months, you might run into what is displayed in this next picture. The rear heat transfer pad, again, it is not very well done. I wonder if genuine Pololu drivers have the same problem in interfacing these heatpads, cos once the chip goes in, you can never know if you have perfectly reflowed solder over the entire pad. Anyway, this is not the only piece i have discovered with such a problem. As per datasheet, the ic package is suppose to deliver 1A IF the heatpad is properly sinked. So now, who wants to pry open a Pololu to see if they are perfectly sinked?
There are at least 10 resellers on Ebay peddling stepsticks, I have the clumsy luck of buying from a few as i keep on destroying stepsticks in my horrid experiments.

These commandos died ...

These survived. 

Amongst the things I tried today, I also took the opportunity to find out why leadless soldering is such a tough nut to crack. Over at EEVblog, I managed to spew some questions on solders and fluxes. But its all mostly the same reactions. So I popped the wallet and got me a 217degree melting point leadless solder 0.5mm (made in Germany) and a tin of German flux (which looks suspiciously like a China rip off). If you look at the picture, there is absolutely nothing on the tin which says what the heck grease it is made of. With the combination of a relatively accurate temperature controlled soldering iron, i dropped a few blobs of solder on the verboard for a try.

From left to right, ignoring the leftmost half broken track. the 6th track is the leadless done at 240, 280, 310. Even when coaxed with extra flux, you just cant get them to shine or flow like a 60/40. I am inclined to assume that this 0.5mm lead do not have enough flux within to do its job and that it will need a strong special flux that will make the leadless work like a charm. Having said this and googling some more real information on what others are saying about the leadless, it seems those fluxes will give you more problems than leaded solder ! Conclusion? Even at 320 degrees, which is 100+ above the rated melting point, leadless just doesnt seem to want to work like the familiar 60/40s. I have yet the guts to try up to 400degrees, but it pretty much is making any soldering process more tedious.

Now with the extra effort required to melt the leadless, having a glob of solder that refuses to melt and flow freely, you then try to increase temperature to melt it. But then disaster strikes, SMDs and their small copper track dont survive high temperatures very well. Reference the picture on the left, note the SMD marked "000" from "reprapdiscount", and R20s from suppliers who made it correctly (the R33 and R15 are my own stock)

So much for leadless solder, but no matter how many negative reviews there are out there, curiosity still kills the wallet for many of us.
Lets see if Felder Germany will reply to me and tell me what is in their flux. Meanwhile, its shopping time again, more stepsticks and a hunt for some 0.5mm kesters.

*edit* 9 days later, nothing from FELDER ! 

*edit* i forgot to update that FELDER replied, and this tin of flux is genuine ! HIP HIP HURRAH !!! TYVM FELDER for your replies. ( i also made several inquiries to GOOT, they were also very responsive, i'm just glad that there are really hard working folks out there making everyone in the EE world happy)


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