EGWU its alive ...

Busy weekend - catching up on sleep. Finally got around to powering up the first prototype. First I had to put away the hotplate and reflow tools, clear away some space and set aside stuff thats destined for storage and mount the scope in a safe area where my 18 month old isnt tempted to get at it.

Initial power up looks good. The three regulator outputs are as expected. Felt around the various Ic's and none felt warm - so looks like there isnt a short I have to deal with, for now

But I didnt get any signal out of the crystal/oscillator inputs to the Cortex-M3. Ill have to investigate this later ... its 5am and I am turning in.

EGWU pcb assembly: attempt #3

Ok here is the deal. I need more than 1 prototype. Ideally Id like to build two more boards before I start board test/bootstrap and firmware/software development. Its been suggested to me that using the heat gun for reflow is a viable process - and it looks interesting since I can assemble the board incrementally.

To that end here is my rig

I had hoped to use this Monday vacation time to finish up (only six more SMD IC's to place) but I am nursing a fever at the moment so here is to another wasted holiday ;( However I am sure Ill wrap this up this coming weekend or even this afternoon if I wake up feeling better

I might just use the hot plate again for prototype #3 (attempt #4). One thing thats going to help cut down on the time to place parts (manual labor) is having sorted the SMD parts (like 0603 resistors, caps, inductors, LED) into the two cases below (which I picked up at a 99c store). I might have even purchased the pill dispensing boxes I see at pharmacies or shoprite. They would work just as well.

Another thing is printing out 10-15 pages of the silk screen layer. On one page I circle all the 0.1uf parts on another: the 2k parts and on yet another: the 4.7pf parts and so on and so forth. For parts which are less than 10 in number I place more than one on a single page. This makes parts placement easier since I know how much of a certain part I need and I can remove and array them before me. pick and place until done before moving on to another part

EGWU pcb assembly: attempt #2

The second attempt was more successful, partly because I had spoken with some of the Engineers and Technicians at my place of employment - who do board assembly. I was told to not trust the temperature readout on the hotplate - instead insert a thermo couple to be sure.

Also from online and youtube vloggers I got a better sense of the temperature profile I should employ. It would go something like this

1. place the pcb (with solder paste and parts in place) on the hotplate

2. fire it up to 100C

    leave at above for no more than ~2 min (this is partly to reduce/prevent thermal shock for the remainder of the procedure)

3. crank up the board to 150-160C 

    this is the temperature where the solder paste "activates". leave it here for about ~1min to 90 seconds

4. crank up the board to 180-190C

    this is the temperature where the solder paste "reflows". you will see it turning a shiny metal color as it transitions. leave it here for no more than ~2 min

5.  remove the board immediately from the hotplate (dont leave it on there while the hotplate cools because the one I have is dense aluminum and retains heat for a long time. in my case i took advantage of the winter weather and set the board by my slight open window in order to ramp down the heat

This particular process went unbelievable well - much better than I expected. And I learned the following

1. between 0 and 100C my hotplate was reading 20C below the thermocouple (Exatech DMM with probe inserted inside the drilled hole on the aluminum block - dont use IR sensor - its not accurate)

2. between 100C and 180C my hotplate was reading 50C below the thermocouple. This of course explains the damage I had done to the first prototype - with board warp from over heating.

3. leaving solder paste on a board for over two days (from a reflow process I didnt do on time) causes it to "cake in" and solidify.

4. following from above ... scraping the board with copper scouring pad under hot water is the worst thing to do. it will leave scratches on the board and peel off the solder resist on the layer.

5. a better approach is to soak the board in 90% to 99% isoprophyl alcohol for 30 to 60 seconds and then use a foam or washcloth to wipe out the now softened solder paste

I had a friend (who had significant experience soldering) touch up some parts for me. Here is the final board with surface mount AND through parts in place. call it MARK I  (Iron man reference)

EGWU pcb assembly: attempt #1

So last week after the anti-static table mats arrived I set about cleaning one of the pcbs and taping two L brackets unto my table before setting the PCB atop it and then clamping down the stencil. I applied the CHEAPO XG-50 solder paste (I bought from China on Ebay)

The second application came out much better than the first. It still took over 3 hours (I would stop and play with my son, grab a snatch, check emails, watch a movie) before I mounted the parts.

Then followed the following procedure (which was wrong) to reflow the board:

Brought the hotplate to 180 degrees. set the PCB board atop it and crank up the temperature to 230 degrees and after it looks like the solder paste has reflowed  - remove the board and turn off the hotplate. 

The right procedure I learned from a colleague at work should be something like this.

Bring the hotplate to 180 degrees (next time measure this with IR temperature sensor). 

Set the PCB board atop it and crank up the temperature to 200  degrees. 

After a minute or so - remove the board and turn off the hotplate. 

Set a fan to cool the board down ASAP

I may still be able to rescue this failed attempt using a heat gun. But I am eager to try again using 

1) lead free solder with low temperature

2) table magnifying glass to better allow me to place components after solder paste has been applied

Assembling The Boards

Originally I meant to contract out board assembly to some technicians on craigslist but I think I want greater control over even this stage of the process at least for the prototype(s).

So in keeping with the age old adage "if you want something done well, do it yourself" I have assembled the tools Ill need.

Soldering SMD parts almost always requires either a reflow oven or a hot plate. There is a $300 electronic reflow oven on ebay. Chinese made, works and doesnt take up too much space. However on youtube and on reddit various individuals have expressed more success using hotplates to reflow solder. Some even use electric skillets and an infrared temperature monitor

I purchased a $100 electronic hotplate on Ebay (YADGONG 946C from a seller in California with two) with enough surface area to handle an 8x8 sized board. Mine is 7.6 x 5.4. I also bought an IR temperature sensor for around $12 ... shipping included for both

Of course I might have need to do rework and I definitely need to hand solder some through hole parts on the board. With that in mind I bought the 2 in 1 SRS 852D+ reflow soldering station. This cost me about $61 on Ebay shipping included

Finally and possibly the most important tool on an Engineer/Inventors lab (outside of a digital multimeter) is an analog scope. My favorite Engineer vlogger Dave Johnson (eevblog on youtube) recommends ditching the fancy usb/pc scopes and getting a nice dual channel analog oscilloscope for starters. So I bought this Tektronix 464 100Mhz Dual Channel scope on Ebay for about $129 with $30 shipping from Rochester, NY.

Various other small tools like hand pump to lift and place SMT, soldering tools and tweezers, rework wire, solder paste, solder flux and enough parts to make a prototype and perhaps an initial run of 10 has been purchased on Ebay, CircuitSpecialists and Mouser


Once the solder paste and boards arrive I am good to go.

Message from Board Fabrication Plant ... boards are done and shipped

Pictures attached ... should be here in about a week and a half. cant wait. Btw
I am currently reading "Makers: The Next Industrial Revolution" by Chris Anderson.

I also got a stencil made since I plan on assembling/soldering this board at home. I bought a Hotplate and rework/soldering station last week on ebay in anticipation

project EGWU complete !!!

Its been a year ... a whole year since I set about designing this board. At lot has happened distractions wize some enormously stressful none of which I am comfortable going into on here.

But I  have finally completed the layout. Actually I finished the layout about a month ago but had been touching up and correcting problems since. Even this week - after submitting the gerber files to the Chinese PCB Fab plant (from Ebay) that will be doing the boards  ... I still had to revise it two more times (the plots not the layout) to satisfy them.

I am off the whole thanksgiving week and I am looking for a much needed "staycation". Ill also use the opportunity to start on the firmware. It will be another two and a half to three weeks before I see the 5 boards I ordered


Sillkscreen layer below