Pin-in-Paste Hole Stencil Printing

We want to use a 8mil thick stencil to deposit paste into thru holes in a 93mil thickness board. The stencil aperture is 12mil wide. Using a squeegee to spread the paste over the stencil, are we going to need to do multiple printings in order to fill the holes? How can we maximize the paste release?

E.P.

Expert Panel Responses

In solder paste printing we need set correctly set printing process parameters

solder paste thawing process min 3-4 Hours ,keep in normal room temperature
mixed the solder paste at @ 300rpm for one minute.
Purpose of Paste mixing is that Metals alloy mixed with flux, that results Good solder printing quality and paste deposit correctly through the stencil aperture.
stencil aperture should be cleaned no hole block or not any dent on stencil.
while paste printing squeeze blade should be tight.
paste printing speed 3 to 4 mm/sec
printing presses on squeeze 3 to 4 bar
auto stencil cleaning frequency after b Two cycle
wiper roll load in correct direction
Stencil auto cleaning with wet /Dry /vacuum so benefit stencil aperture should clearly open.
there is no possibility of stencil aperture not black.
stable the printing process parameters. when we get good results with uniform paste height should be Min 80 micron to 180 micron
printing process parameter variable and it's depends on board and stencil thickness.
Support Table pins or block are align * according to area of printing board.

* This much information from my end
Please check your set printing parameters one by one to make good results.

Raju Wagh
Senior Engineer SMT
Advance Power Display system Ltd Mumbai

Mr. Raju Wagh has worked in the electronics industries for the past 20 years focused in TV manufacturing, mother boards, mobile, LED lighting, power supply manufacturing and more. His specialty is in SMT production and processes

The first question to consider is what is the criteria for hole fill? Are you trying to achieve 100% fill of the hole with solder? If so, then the solder paste print must deliver enough solder paste to fill the hole minus the volume of the component lead. That means that 200% of the volume of the hole (minus the lead) is required, because solder paste is roughly 50% by volume solder metal and 50% by volume flux. If you plan to achieve this with one print, then the stencil aperture must be opened up to print paste on the surface of the circuit board with a large enough area to give the appropriate volume of solder paste. Then the next consideration is whether there is room on the surface of the circuit board to print that much paste around the holes. Without knowing the hole diameter it is difficult to estimate what the paste print may look like.

It is possible to fill the holes with solder paste by running multiple print cycles. This would be best done to using a stencil designed to print only the hole patterns. Then it would be a process of trial and error to determine how many prints to use to fill the holes and have paste starting to come out of the bottom of the holes.

Paste release can be maximized and hole fill optimized by doing the following:

Use a 45 degree squeegee angle rather than the standard 60 degree squeegee.
Decrease the print speed to 10-20 mm/sec to maximize time of the squeegee forcing paste down the holes.
Use a stencil coated with a ceramic type nano-coating to optimize release from the side walls of the stencil apertures.
Use a type 4 or 4.5 solder paste to increase the volume of solder paste released from the stencil.

Tony Lentz
Field Applications
FCT Assembly

Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

This is a good question... it does not have a clear-cut answer, but I can provide some detailed guidance. First, remember that your paste is only about 50% metal by volume even though it may be 90% metal by weight. That means that if you fill the entire hole with paste, you'll still wind up with far less than 100% solder fill after reflow.

You can supplement volume by printing a larger area on the top surface, but you will be limited there. Also, your ability to fill the hole will depend strongly on the hole diameter. Below about 0.040" diameter, it will become difficult to fill, and above 0.070" diameter, you may fill too effectively, and have trouble with paste extruding out the bottom. Finally, the shape and cross section of the lead has a big impact.

I suggest taking the following approach:

Calculate the volume of the hole, minus the volume of the lead
Multiply this by two. This is the required paste volume for 100% solder fill
Estimate the percent of the hole you can fill with paste. This is best done by experiment; fill various size holes and look at what your hole fill percentage is. Note that more than one squeegee pass will not normally improve thins much.
Calculate the fill volume, and determine whether you need additional paste; if so, see if you can print it on the surface and let it reflow into the joint

It can be very difficult to achieve the necessary metal volume to meet IPC Class 3 standards. Class 1 and 2 are more easily met. If your application is Class 3, you may need to consider developing some supporting evidence that you do not need the 75% fill. With a 93-mil thick PWB, you normally do not need 75% fill for strength or reliability, but all depends on the specific component and application.

Fritz Byle
Process Engineer
Astronautics

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

If you have a positive pressure squeegee head (self dispensing) you might get away with slowing the travel or pausing the travel to allow for Thru-Hole filling. However, a dispensing machine would be your better, if not best option.

David Cormier
Engineering Manager
Circuit Technology Center, Inc.

Manufacturing Engineer of 20+ years. Involved in Industries relating to all sectors of defense, Commercial product Industries, RF - Microwave and Semiconductor industries. Vast knowledge and experience relating Mil-STD’s, IPC-STD’s, EAI-STD’s, GEIA-STD’s, J-STD’s and MIL-PRF-STD’s.

Trying to fill the PTH with paste is typically the last thing you want to do. The reason I say this is when you insert the lead it pushes the paste out and you will have paste in the placement equipment and the oven when is goes through preheat.

Paste in hole has been done for years. The first calculation that needs to be done is the volume difference between the through hole and the pin. Once this is established use a split pad that partially covers the PTH but leaves most of the paste on the solder mask. Calculate the aperture openings base on twice the volume you need. As paste that is 88-90 metal load (depending on alloy) is approximately 50% by volume metal. This is due to the difference in weight verse density of the components in solder paste The balance is flux.

Capillary action will draw the solder down the hole. This is provided that there are not competing wetable surfaces near the print. Experiments have seen that up to 250 thousands of an inch long pads will flow down the hole with out leaving solder balls behind. These pad do need to be on contact with the hole. Print about 30-50% across the hole on opposite sides.

Karl Seelig

Deck Street Consultants

In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

Lacks info regarding the type of paste and the finished hole diameter but a vacuum plate to help pull the paste into the holes is typically used for this type of application. Suspect multiple passes/printings would be required to completely fill the holes. Better yet there are machines built specifically for via filling that apply paste under pressure on one side of the panel while pulling vacuum on the other side. This process doesn't use a stencil either. The paste is applied directly to the panel, excess is scrapped off, panel is baked to cure paste than any remaining paste is sanded off. Trying to use a Stencil presents several problems:

Typically drill programs need to be scaled to follow panel movement and maintain internal registration. Unless the stencil is drilled with the same scaling the holes will not line up and it doesn't take much to miss-align .012" holes.
Unless there is intiment contact between the stencil and the panel the paste will push out between the two and make a mess
Using an 8mil thick stencil would leave an 8mil tall "button" of paste above the panel surface which would also be difficult to remove.

Amit Bahl
Director of Sales and Marketing
Sierra Circuits

Amit Bahl started to work at Sierra Circuits in 2006 where he formed strong relationships with his customers working with them on flex PCBs, HDI, controlled impedance, etc. In 2009, he was promoted Director of Sales and Marketing.

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