SMD soldering and professional PCBs

I thought I would show some photos of my latest project and share some links of my favourite PCB suppliers as it is becoming high time that the regular hobyist needs to "take the plunge" into using SMD components if you are going to want to keep up with the new range of chips on the market.

With picaxe chips offering I2C and SPI, it would be a shame not to use them on the myriad of ICs that are only available in surface mount. Hence a quick tutorial on how I tackle the SMD dilemma.



Here is a board that I designed in Protel Autotrax. (free download), and had made by www.pcbcart.com (US$14 each for 10 boards including shipping to Australia from China)
It's a SMD Picaxe 28x1, which reads a precision 16 bit ADC IC and then sends the reading via optocouplers to the RS485 SMD chip (MAX487). Its used to detect a photodiode and measure accurately the level of ambient light.




The smallest SMD part is a precision voltage reference IC which has less than 0.25mm clearance between pads. So to solder these without trouble, I used solder paste, found at Jaycar part number NS3037 AUD$29.95

But to ensure the solder paste did not go everywhere, I purchased a "solder paste stencil" from www.polulu.com (USD$49 inc shipping to Australia)



I emailed them the DXF file of the pad layer, and they mailed me a mylar stencil. It's essentially an overhead transparency that has all of the pads laser cut into it.
They say they need Gerber files but they were able to use my DXF which was produced by using a program called pcbtodxf.exe which is available as a free download from the same site that hosts autotrax.

I used a standard paint-scraper spatula to spread solder paste over the stencil board, then used tweezers to place the components onto the pads.

Sparkfun has a great tutorial on how to do this. http://www.sparkfun.com/commerce/present.php?p=Stenciling

Then I used an old "George Forman" grill to reflow the PCB.



Set to its maximum setting, it gets up to 230 degrees C. This is enough to reflow the PCB.
I waited till it got to this temperature before carefully placing the populated board onto the hot plate. It took approx 60 seconds to reflow every component. I then used tweezers to take the board off the hot plate, and placed it on the table to cool down. I used the K type thermocouple on my multimeter to sense the temperature of the hot plate.


So total cost may be just a little high for a single board, but if you are making RF repeaters or things like that where you might need 10 boards or so, then the cost per board can get down to under $20.

Pretty good considering you get to utilise SMD chips that you may have put in the too-hard basket, and you get a really professional looking result when you are done.

I hope this has helped others to give SMD soldering a try.

Haxby,

Nicely done! Good tutorial.

PCBcart seems to have one of the more reasonable pricing structure for small boards. I've been looking and some pcb houses charge outrageous amount for set up fees.

I am really impressed with the layout, especially since Autotrax is a DOS program.

Surface mount is not that difficult. Once you get started, you won't go back to through the hole chips.

Myc

Heres some pics of my reflow attempts using an electric frying pan:







Not very scientific yet - stick the board in the pan - heat it on 3 for a little while then CRANK IT UP TO FULL - wait till it reflows then pull it out to cool.

I'm pretty happy with the results so far! Next step is a *proper* controller that ramps up & down

BTW - frypan is a "Sunbeam" brand 15" electric with GLASS lid - bit flash but lets you see whats going on without sucking all the vapours! Bought on special at Target for $60

What does the temp profile look like with this method? Is it anywhere near the recommended profile?

There seems to be lots af data (reasonably consistent) from the chip manufacturers and here;



Time and Temperature seem to be pretty tightly specified!

Thats the kind of profile I'll be aiming for when i get my controller running - these where a test run to see if the pan achieve suitable temps.

My manual method was to *try* a duplicate that as roughly as i could without a thermocouple:

Turn on
Heat to "3" on dial and wait 60 seconds
Turn heat to FLAT OUT - wait for reflow - then heat OFF and allow to cool a little before removing from pan to continue to cool on a rack or wood block.

When my controller arrives I'll be aiming for something more scientific!

I know its *sacriledge* - but I am using one of these controllers:

http://www.thesiliconhorizon.com/reflow.htm

Its all about time.....too much other on my plate!

I take a less scientific approach. I use a George Forman grill retro-fitted with a K type thermocouple.

I turn it up to 220 degrees and wait till it gets there.

Then I place the board on the tray, wait about a minute to 90 seconds while gently moving the board around and around to stop any hot spots/cold spots developing.

Then once all components have reflowed, I take the board out with tweezers immediately and place it on a piece of dry paper to cool down.

I figure you don't have to pre-heat a joint when you are using a soldering iron, and there is much more thermal stress when hand soldering SMD components.


Sparkfun.com have a great tutorial on reflow stuff and one of their biggest mistakes they admit was to purchase a commercial reflow oven. They are using a baking oven with better results!!

A few more pics from another board i did today. I did this board with the lid of the frypan OFF (damn auto focus) and it didnt work as well - much quicker and even results with the lid ON (yay for glass lid!)

Bare board ready for pasting:


Pasted applied - its amazing how rough you can get away with! (just applied with syringe & nozzle)


Parts loaded:


Completed reflow:

That is really neat Stocky.

I just hope others appreciate your steady nerves and the benefits of Solder Resist.

Thanks Dippy - I think the paste/reflow method is actually very forgiving in terms of a steady hand.

for example - check out the cap near the top LH corner in this video - its starts off very wonky then gets pulled into alignment by the solder!

Very nice stocky. The video shows all. I'm tempted to try this myself with some SMD's I have at home.

Do you have to be careful not to get the chips too hot? Or are all chips happy at 230 degrees ish?

Andrew

they are not that hot for very long - see an earlier post for a typical thermal profile of the process.

Also rather than heat being applied unevenly to he devices the WHOLE device is evenly heated which must reduce thermal shock as well

Another example - different board this time.
Also managed to get the camera to focus with the lid on the frying pan this time!
You will notice in the video the much faster and more even reflow when the critical temperature is reached. I put this down to more heat being applied to the top of the board due to reflection from the lid and the hot air of the pan being contained by the lid as well.



A batch of completed boards:

Nice SMDing - might try it - got a few bags of SMDs lying about. Thanks a lot for the link to PCBCART too - ordering a batch now...
Thanks!
73 de MI6TUX

Here is one of the best videos oj the web for sm
t soldering, that I have seen. It covers all the bases and formats.

http://www.curiousinventor.com/guides/Surface_Mount_Soldering/101

No, it is notmine

Another suggestion

About 10 years ago I was faced with soldering surface mount parts as part of a support activity for Rosemount instruments.

I was at my wits end but got some great advice which developed into a simple method.

I placed the board with solder paste and the ADC converter in place on a lab hot plate (somewhat like the system with the griller) but with one difference, I kept the temperature well below the melting point of the solder. I had hung a quartz lamp just above the part. When the lamp was turned on, the added heat melted the solder in an instant and the heat and lamp could be turned off. You might want to try it. It works.

Tom

I think both those videos have board with a solder resist mask.... it's not too much more difficult to undertake SMD soldering on home made boards but the mask seems to help in all that I've seen vs. the experience I've had without.

Thanks for the references

best regards,
Colin