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)