I have only limited (and not so good to date) experience with soldering SMD but I am designing a project for which some components are only available in SMD packages.

Given that I plan to make a batch of 5 units initially I was wondering if it would be worth getting a solder paste stencil cut?

Does it make placing components easier? I read somewhere that this approach tends to pull the component on to the pads due to capillary action.


I have read/watched a lot of SMD soldering tutorials but there doesn't seem to be much out there by way of a comparison between hand soldering and solder paste stencils. Any advice would be helpful.

 

#1- a hot air gun or DIY reflow oven is a MUST for SMT.. Sure you can do it with an iron but its NOT nearly as easy/fast as with a hot air gun. If you intend to do more than a few components then pick up a cheap-o Chinese hot air gun ($60-80 USD) it will make life sooo much easier.

#2-stencils are great.. But you can just buy paste with a syringe and apply a very small (no thats too big..smaller ) blob on each pad.

 

Thanks mcgyvr, I'll look into a hot air gun

 

many of the prototype pcb houses can make or even include the stencil for free with your order.

 

Spark Fun has a number of tutorials on soldering SMT parts. There is a tutorial on making your own solder paste stencil. They tell how to reflow solder using an electric skillet.


https://www.sparkfun.com/tutorials/category/2

 

5 units is nothing, you can just hand solder that with a fine tip iron. How many ICs in each unit?

If you want to go to the trouble of oven soldering with paste, for 5 units you can just apply paste with a syringe in a line along the pads, it will still centre ok if the IC is placed properly. Afterwards you will need to remove a few solder bridges with a fine tip iron anyway, but that applies even with stencils and hand IC placement so don't let that bother you.

 

If you do hand soldering. It can easy be quite mess using a stencil. As it easy for a beginner (and trained people also) to smear out both the paste and non soldered components. At that time you are really in trouble. You can not use the stencil then the board is half full of components

 

There's a lot of myths about reflowing SMD parts out there, possibly due to this being a relatively new technology, and people tend to look for overly complicated solutions.

A solder stencil does nothing for component placement, it's for solder placement. Once you get the stencil aligned to your board (a possibly finicky process, get a nice flat base and a roll of tape) you then zip out boards. So it may take 20 minutes to align the stencil, then another minute to squeegee out all the boards.

Alternately, you can buy solder paste in a syringe. That can make your thumb want to fall off if you use it all afternoon so either get a dispensing station (our are air powered) or there is a manual mechanical gun out there for syringes. Add a dab on each pad, try to make equal amounts.

I would never use hot air for more then a single component as it WILL blow small parts off your board. (With one part you can hold it in place with tweezers.)

A complete board reflow is always best, that's when the solder by it's surface tension pulls parts into near perfect alignment; a few gentle taps also help.

If your board has parts on both sides you will need some sort of oven. A toaster oven is ideal: ignore all the "I made this controller" videos on YouTube and just set it to a temperature where solder melts relatively fast (do an experiment) if you don't have a thermometer.

Put the board in, watch it thru the glass: when the solder melts and flows give it another few seconds and take the board out. I would have to board on some sort of metal or ceramic carrier.

Heck, I've even reflowed boards on my kitchen stove top. It is NOT rocket science.

 

I would never use hot air for more then a single component as it WILL blow small parts off your board. (With one part you can hold it in place with tweezers.).

I have done a lot of soldering with a pro Weller hot air soldering station with a pneumatic foot operated pump to apply the solder paste. With some experience it is not hard to find the correct temperature/airflow setting so even small components do not behave as confetti on a windy day. And the point is not to secure the component while soldering. And let the surface tension in the molten solder adjust the component to a perfect fit on the pads. Of course large ICs may be hard to solder this way. So using a kind of reflow oven will be best. And you will also get the self alignment here to. Then using the Weller station I applied only paste for one component at the time. At least for the smallest as it was quite easy to mess it up and smear out placed components out during component placing
By the way good point about the stencil. To work properly it and the PCBs must be locked a frame. Made for this purpose. And you will also most often work sevral PCBs in a panel with this method

 

OK, thanks guys, food for thought there. It's some years since I last tried soldering SMD and there is a lot more info out there now so I think I probably ought to buy some cheap components and just practice...