Quick Comparator--First time using an SMT stencil--A Win!

Time to branch out further into the land of surface mount!  

Viewing videos that feature SMT stencils and hot air guns (here and here for instance) made me want to try this fabrication technique. 

With generous help from this blog's sponsor, PCBWAY, I got some PCB's for an op amp based comparator design I conjured up, an SMT stencil, and gave it a try. 

It took PCBway about 5 days from upload to getting the boards and stencil from China to the US--cool! 

Good news: this technique was really easy, very fast, and I have a new fab methodology at my disposal. 

And! the SMT stencil results came out MUCH better than I thought. Happy with all that.

What was needed: Stencil (still in its shipping cardboard) and SMT-ready PCBs from PCBWAY, MELP SMT resistors from Mouser, and some large unused boards I had lying around. The latter will be used for the masking process.

"Let's get Started"! (how many videos begin that way?) :

I placed the PCB to be populated between old boards I had lying around. Next I taped them down with "Scotch magic tape" which may be my most used fabrication material. 

I positioned the sencil and taped it on only one side:

Board in the middle is a mostly SMT 6HP Euro comparator. The design is untested, I pretty much laid it out with best guesses and intuition--Might work right?

Folded over the stencil:

I used this solder paste--"Chipquik No-Clean T3" (chosen after a bit of research--there were a lot of choices, this one worked....)

Slathered it on boys, I ended up having to use more paste than I thought I'd need....:

No photo but I spread out the paste over the stencil with an old credit card. Then I very carefully and slowly used tweezers to flap the stencil back over, like opening a door.  

Good news, the stencil did the trick--I had neat little pads of paste as needed.

Again no photo, but I placed all the tiny parts with tweezers. The paste was sticky so this wasn't too hard. I feared my parts would blow away when I "hot air gunned" them; fortunately they didn't.

Next I used my trusty hot air rework station (information here--the Sparkfun 303D was relatively inexpensive and has turned out to be a quite useful tool) set at about 350 C and about 9AM on the air pressure pot. This was a guess....but it did the trick.  

As I saw in the videos, upon heating, the paste turned a snowy white, then after maybe a minute, quickly turned a beautiful shiny silver, and each part was drawn via capillary action to its pads. Perfect! 

It was like magic...it worked!!!!

For clean up (I got some paste on the bench--yuck) Testors plastic enamel thinner did the trick....

Here's what I ended up with:

I thought the result came out GREAT! 

This was so much easier than I thought it would be. The entire process, from unbagging to taping to pasting to placing parts to using hot air to solder everything down--took maybe 1/2 hour and yielded professional looking results.

Great! I added the thru hole parts, Euro style power, and powered it up. 

So--did the board work? 

NO!

So much for the fun part, the rest of the morning was spent chasing down stupid mistakes.

I found 3 problems--one due to poor layout, one due to design carelessness, and the third because, well i have no idea why: I had an airwire on the design (so, no traces when I needed a trace) and I missed it. 

Warning to SMT PCB novices like me: "Be careful about via locations!"

When laying out SMT boards, don't put vias too close to parts.  My assumption was that during the soldering process the solder would go right where I wanted want. Not always!  

If there is a solder bridge between an SMT part and the via (there was in my case) you will have a short. 

And, since the via carries a different signal than the component, the short can break the signal flow, which is what happened for me; it grounded out the input signal and the entire circuit didn't work. Nice.

If and when I get this board fabbed again I will fix this by moving the via further from R6.... .

How I fixed the via issue.....

The rest of the problems were not due to SMT fab, rather, bad design choices. I had to lift another SMT resistor and use a kludge wire to get the output to work due to a "your current has to go somewhere" design error.  See R3 in the schematic below. Oops. 

I also added a zener to clamp about 0-7.5V at output. This is optional, for 0-10V systems you could use a 10V zener, or for a rail to rail comparator leave this part off.

Finally I fixed the airwire but putting a solder blob between pins 13 and 14 on the TL084 surface mount IC.  

Overall, debugging, fixing, and modding SMT builds was harder than fixing thru hole PCBs, because everything was tiny, but I laid out the board sparsely and used some thru hole parts such as D1 and D2 because I figured I'd be fixing and modding. I was right.

Here's the working comparator after the mods:

Here's the schematic with fixes:

schematic with fixes....the zener sets output voltage range

Going forward it would be nice to have an inverted output and a few other mods, but this design and build was more about experimenting with SMT stencils than coming up with a fantastic audio comparator. Overall, since using the stencil and hot air soldering elements was easier than anticipated, so I am going to consider it a win. On to other things.

Shameless plug: if you want to help support this blog please check out my sponsor--PCBWAY, link is here. 

They are super friendly and have been soooo helpful..... thanks to Wendy and the gang for helping me get all of this going. Always good to try new things right? 

More fun next time.  See ya.