Good practice when designing a PCB and stencil?

While I generally use nothing smaller that SOIC chips or 0603 passive components, I have to design a PCB with a WSON-14 IC with tiny 0.5mm pin spacing and with an exposed pad.

While I generally use a syringe to apply solder paste I thought that, for such a small beast, I should better order a stencil with the PCBs.

Since this will be the first time I order and use a stencil, I wonder if there is any good practice to ease the alignment of the stencil and the application of the solder paste.

For what it’s worth, I’m using Kicad to design the PCB. The PCB is tiny (0.4"×0.7"), and I consider ordering to a low-cost Chinese factory (~$10 for 10 PCBs, ~$15 for a stencil).

• You can solder .5 mm pins with a soldering iron and fine point tip. Or, what I sometimes do for this pitch and finer is to use a soldering iron to pre-load solder on all the pads. Apply paste flux liberally. Place the part, using the paste flux to keep it in place. Apply hot air until all the pre-loaded solder melts and wicks to each respective pin. – Olin Lathrop Mar 27 '17 at 10:47
• Get some Kapton tape. It is resistant to high temperatures and will survive soldering operations so you can use it to stick down your PCB and then on top of that align your stencil and stick that down too. Then bung it in your reflow oven or whatever you plan to do. However 0.5mm pitch isn't that difficult to manually solder if you have a fine iron tip. It helps to have a good magnifier handy. – Wossname Mar 27 '17 at 10:55
• @OlinLathrop I have the opportunity to use a reflow oven; don’t you think it is easier and less error-prone that hand-soldering with a fine tip or hit air? Moreover, that chip has an exposed pad which make things more complicated… – user2233709 Mar 27 '17 at 12:07
• @Wossname I must be missing something… Are you suggesting to take the stencil in the reflow oven? – user2233709 Mar 27 '17 at 12:09
• If you have a reflow oven, then just use the normal process. You get a stencil for the whole board, not individual parts. Your question makes no sense if you've got the proper processes available to you. Do it like you would any other board. – Olin Lathrop Mar 27 '17 at 12:11

well for easing the alignment I usually make some two '+'(plus) signs at two sides of the PCB I draw those plus signs as multi-layer in Altium designer so it will be on the stencil too. one more thing is using a transparent stencil so that you can see the other side and If you are making lots of PCBs, it is profitable to buy a stencil than to make one but remember to ask the operator not to cut the PCBs to pieces so that you can put the components all at one try it is a lot faster

another way is that you can also just export your pins as a CAD file and send if for a laser cut on a 0.5mm It will cost you much less.

• Thanks for you suggestion, the two + signs look like a good idea. I have no idea what kind of stencil (transparent plastic or metal) the cheap Chinese factory provide; any idea? As for your last suggestion, what do you call “a laser cut on a 0.5mm”? Do you mean on a 0.5mm-thick metal sheet? Isn’t it much too thick? – user2233709 Mar 27 '17 at 13:38
• I suggested that you go and ask a Laser CNC Engraver to make you a stencil with some kinda really thin material like transparent sheets available I'm pretty sure they help you with the stencil it would be precise and really cheap – AHIVO Mar 27 '17 at 14:38
• For what it’s worth, I received the PCBs and the stencil I ordered. I thought plastic stencils were less expensive so I would receive a transparent plastic stencil, but I received a metal (I guess steel) stencil. Fortunately, I had drawn two “+” signs, and that helped a lot to align the stencil. Thanks for the suggestion! I accepted your answer. – user2233709 Apr 27 '17 at 18:48

You won't need any special features for visual alignment. You'll definitely be able to tell when your stencil is aligned.

As far as Kicad goes. You shouldn't need to do anything special in KiCad to make a good stencil file. Just check your gerbers to make sure all your layers line up the way you expect them to.

Many hobby stencil kits will include everything you need to stencil your board.

A common setup is to send you two "L" shaped pieces of FR4 to hold your PCB.

1. The idea is that you tape the big "L" down to your work space
2. Seat your PCB in the inner corner of the "L".
3. Brace your PCB with the little "L", and tape that down.
4. Visually align the stencil to the PCB, then tape the flat edge down.