SMT Stencil FAQs

Question: Should the solder paste layer pads be smaller than the smt pads due to swelling of solder paste? If so, how do I determine the % difference?
Answer: The solder paste layer pads should be approximately 10% smaller than the SMT pads.

Question: What is the difference between a solder mask Gerber file and a solder paste Gerber file?
Answer: The solder mask Gerber file has all of the mask openings including surface mount, component holes, and vias. The solder paste file has just the surface mount pads and usually they are smaller (e.g. a 50 by 50 mask opening will translate into a 48 x 48 paste opening).

Question: What is ID?
Answer: The ID refers to the inside dimensions. The ID on a 12x12 frame is 11.8". Printable area for a 12x12 is 9.5".

Question: What is squeegee view?
Answer: The squeegee view refers to the side of the board that paste is to be applied to. If you are looking at the image, and that is the side you want to apply paste to, that is the squeegee view.

Question: Can you use ExpressPCB file(s) to cut a stencil?
We can not cut a stencil using ExpressPCB proprietary file(s). However, ExpressPCB will allow you to export the Copper Gerber file in DXF format. We need to convert this file to a Solder Paste Gerber file. There is no conversion fee.

Question: What computer files do I need to send you to make a solder paste stencil?
We prefer the Solder Paste Gerber file to cut the stencil. If you do not have this file you can send us the Copper or Solder Mask Gerber file which we can convert to a Solder Paste Gerber file. The pads in the Solder Mask Gerber files are usually larger than the ones on the Solder Paste Gerber files. If you provide the Solder Mask Gerber file you need to provide the reduction information. If you are ordering a bottom side Prototype Stencil you need to include the Silkscreen Gerber file for the bottom side.

Question: I am interested in knowing more about your framed laser-cut stencils to use for screen printing, such as mesh tension and wire/thread count, estimated impression life, durability of mesh to stencil bonding.
Tension is 38 newtons per square centimeter. Thread count is 60-125. Print life varies drastically from customer to customer, depending on squeegee pressure, under board support, and handling. Average usage is 20,000 cycles. I have also seen with proper care usage can be 100,000 cycles.

Question: What guidelines do I need to follow to specify the correct thickness of a stencil when using 0402 components, or is this something you recommend during fabrication?
Stencil thickness is determined by the desired paste height. Typically the paste height is 1 mil less than the stencil thickness. For example .006 thick stencil would deposit .005 paste height. So choose the thickness by the paste height you desire.

Question: Are all 20x20 frames extruded?
No, 20x20 are either cast (solid) or extruded (hollow). So he needs a 20x20 extruded frame. Tube means extruded (hollow) and all 29x29 frames are extruded.

Question: Is the Tacky flux conductive
Tacky flux does not have any metal therefore is not conductive.

Question: What is that tacky flux used for?
It is used for rework and BGA repair.

Question: What else can I use to clean stencils besides stencil wipes?
We recommend using IPA to clean stencils. It should be electronic grade IPA at 99%.

Question: Can work with data in PCB123 format?
We make stencils for PCB123 customers all the time. After you place your stencil order you need to contact PCB123 and ask them to generate a Solder Paste Gerber file for your board and email it directly to us. I believe they will charge you a small fee to do that. Please let PCB123 your stencil order number.

Question: How should we ship solder paste?
In the summer months the solder paste manufacture recommends shipping the solder paste using overnight delivery. We can ship your order overnight if you authorize an additional $20.00. Otherwise, we can ship it ground but we do not take responsibility if the paste gets damaged.

Question: What’s the difference between solder paste and tacky flux?
Tacky flux is the material that contains the flux that cleans the surface where the solder joint form. The tacky flux does not contain any solder (metal). Tacky flux is mainly use in rework. Solder paste contains both flux and metal.

Questions: What does the 20"x20" stencil size on the website refers to?
The 20"x20" stencil size on the website refers to
1. the standard 22 1/4" x 22 1/4" cast (solid) framed stencil
2. the 23.5 x 23.5" extruded (hollow) framed stencil