How to Gauge Solder Paste Volume

How much solder paste should be applied when running a stencil printing system? Should you print a PWB on the first pass or work the paste a couple of cycles before running the first good board?

C. T.

Expert Panel Responses

The expected solder paste volume should be calculated based on the dimensions of the stencil openings. We would expect the paste to completely fill the aperture and release onto the board with 95% transfer efficiency. You may get greater than 100% transfer efficiency if the board is not sitting flush on the stencil; this can create a scenario where the paste height is slightly higher than expected. Most stencils are going to be 5-6 mils for typical board technologies. Finer feature printing (0201s, micro-BGAs, etc.) may require a thinner stencil (4 mils) for proper release of paste from the stencil. Most solder pastes should be able to be deposited onto the stencil and used immediately without any knead cycles. This depends on the solder paste formula and also whether or not it is stored properly, but in most cases, fresh paste can be deposited onto the stencil and printed without any additional "working" of the paste.

Brian Smith
General Manager - Electronic Assembly Americas
DEK International

Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

The amount of solder paste you dispense onto a stencil for a production run can be varied depending on your production size: If you plan to only run a short number of boards (say under 10) then a minimal amount of paste needed to achieve a nice rolling action would be correct. If you are going to run an extended production cycle, then you would put the maximum amount that will roll properly without loading up high on the squeegees and flooding and spilling to the sides. The minimum amount equals about a 0.75 inch diameter along the whole squeegee length, and the maximum about is about 1.5 inch diameter. Your operators should be trained to watch for "leak out" from the sides of the squeegee, because this can quickly deplete the roll of solder paste in the front of the squeegee causing skip- defects. Also, any paste which remains idle during printing can get hard and dry out faster, so it's better to open the printer periodically (say every 30 minutes) and reposition the side trails to the middle with a spatula, so the paste stays mixed better. It's a good idea to print one cycle before printing a board (use a junk board, or one that will be cleaned off), because the first run often depletes a lot of flux to wet the stencil and also, picks up any microscopic lint/contamination that can load it into the apertures. This is like a "priming" cycle. It also enables you to be sure that the paste is rolling and that the squeegee is setup properly (no leakage, puddles or streaks left behind).

Mark J. Curtin
President
Transition Automation, Inc.

Mr. Curtin is the founder and president of Transition Automation, Inc. Their main product is Permalex Edge Metal Squeegees. They also sell the PrinTEK Ultra-Fine Pitch stencil printers. His background includes 25 years of designing squeegees and SMT printers. He is closely familiar with the SMT printing process, squeegee design, and all aspects of in-line printing machines, double-sided tooling, stencils and solder paste performance.

Initially, you will need a thumb-sized roll of paste. You also want to make sure your bead of paste extends approximately 1/2" beyond your apertures. Strictly speaking, most new pastes on the market don't require a kneed cycle before printing your first board. If you are working with an older paste, a kneed print is probably a necessity, but that's not to say newer pastes won't benefit from the process. Running a kneed print first is never a bad idea to make your initial board printings go more smoothly.

Doug Dixon
Marketing
360-Biz

Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.

Normally you want to put enough paste on the stencil that it forms a bead in front of the squeegee blade between the diameter of a nickel to a quarter. Always good practice to run a couple of dummy boards before actual run.

Bill Coleman
Vice President Technology
Photo Stencil

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

Several guiding rules that I've used:

Always start the job (first print from the back of the stencil towards the front. That will offer you the opportunity to see the solder paste application process.
You've applied enough paste on the stencil as long as in the printing process the solder "rolls" when pushed by the blade and does not "slide" on the stencil's surface.
You should be able to deposit the solder paste on the stencil and used immediately without any knead cycles. If you do not use a paste shaker, make sure that the solder paste is properly / thoroughly stirred.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

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