The sole purpose of an SMT stencil is to transfer solder paste to a bare circuit board. A stainless steel foil is laser cut creating an opening for every surface mount device on the board. Once the stencil is properly aligned on top of the board, solder paste is applied over the openings (making a single pass, using a metal squeegee blade). When the stainless steel foil is separated from the board, solder paste will remain, ready for placement of the SMD. This process, as opposed to hand soldering methods, ensures consistency and saves time.
Stencil thickness and aperture opening size control the volume of paste deposited on the board. Too much solder paste causes solder balling, bridging, and tomb-stoning. A lack of solder paste creates insufficient solder joints. All of which compromise the electrical functionality of the board.
Proper stencil thickness is chosen based on the types of devices being loaded on the board. Component packages such as 0603 capacitors or 0.020” pitch SOICs, will require a thinner solder paste stencil than larger packages such as 1206 capacitors or 0.050” pitch SOICs. Solder paste stencil thickness ranges from 0.001” to 0.030”. We have found that a 0.004″ provides the most flexibility for prototype stencil applications. Typical foil thickness used on the majority of boards is anywhere from 0.004” to 0.007”.
We offer two types of material for our prototype stencils. We recommend our Kapton stencils for a quick turn and low cost prototype stencil. We recommend our Stainless Steel for a more durable, but slightly more expensive, stencil with extreme durability.
Why would I need a PCB Stencil?
Stencils replace hand soldering of surface mount devices, and the inconsistencies created by hand soldering. They allow for direct placement of solder paste to the surface to be soldered.
What are home base apertures and why would I need them?
Home base apertures are used to control solder balling and tomb-stoning of chip components such as 0603’s and 0805’s. The shape is generated by taking a percentage of the length of the pad, and percentage of the width of the pad to create a partial triangle where the points face toward the center of the component body. The resulting shape resembles a baseball “Home base” which is where its name is derived. The most commonly utilized percentage for lenght and width is 50%.
What are Trapezoidal Apertures and why would I need them?
Trapezoidal Apertures are created by distorting the beam on the laser, creating a draft in the aperture wall. Trapezoidal Apertures are effective if you plan on a long dwell time between printing of solder paste and placement of components. The taper in the aperture wall causes the paste to sit on top of itself, rather than sliding out the sides. The shape of the trapezoid can be seen in cross section of the stencil opening.
Example Use Cases From Our Customers
8×8 Matrix Pre-Production Prototype
EEVBlog’s Great Stencil Demo
Curious Inventor’s Detailed Tutorial
Sparkfun Learn to Surface Mount Tutorial – Using our Stencils
JVS PAC – Homebrew Production
Oleg On reflow soldering
Kapton Stencil From OHARARP Review – eRaviv Part 1
Solder Paste Tutorial – eRaviv Part 2
SFE – Kapton Solder Paste Stencils
Project Ubertooth – Hardware Build Guide
Camera board and Kapton stencil | Flickr – Photo Sharing!
How to assemble an SMD GPS Device – OHARARP LLC
[…] ordered a Kapton stencil from Ohararp.com since the bq25570 comes in a very fine pitch QFN package. Squeegeeing the solder paste onto the […]
[…] assembly of our products starts with a bare PCB. Solder paste is then applied using a stencil and squeegee. We currently use primarily NC676 solder paste from FCT Assembly, but also use […]
[…] The process of applying the solder paste is very similar to that used for screen printing. We start with a laser cut kapton stencil with holes cut where we want the solder to be placed – which is on all the pads intended for surface mount components. While we would love to have our own laser cutter, the stencils are made for us by OHARARP. […]
[…] should have the PCBs and stencils any day, and will then build the first real boards. Watch the blog for the official announcements […]