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October 20, 2016

Bottom Terminated Components and Vias

Should the thermal vias of bottom terminated components be tented on both sides to prevent flux trapping and solder wicking, or is it sufficient to tent on the top side only? Or, should the vias just be plugged?


Experts Comments

Unless you are using a dry film solder mask, you are kidding yourself with tenting to prevent flux and other chemistries from wicking into these vias. The vias should be left fully open if possible to allow for cleaning if part of the process OR they should be filled with a non conductive epoxy, planarized and plated over with Copper. The common use of attempting to tent using a liquid solder mask is at best hit or miss.
Gerard O'Brien
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
First and foremost, tenting on both sides without filling is not recommended. There is a high risk of entrapped chemistry which can represent a later reliability risk.

Tenting the side under the component is an option. There is some cleanliness risk, since you still have a rather deep blind hole, but not nearly the risk as double-tented. The level of risk is dependent on the aspect ratio of the via.

Filling and capping is certainly an option, however it represents a significant cost increase, so avoiding it would be desirable. There is at least one other option, the "SMD Window PCB Design" option presented by Matt Kelly et. al., published at SMTAI 2014. This approach leaves both ends of the vias open, but prevents solder from wicking down them.

The above are your options if you absolutely need to eliminate solder wicking down the thermal vias. It's not always necessary to do so, however. We have been very successful on many designs just letting the solder wick, and ensuring that we have enough solder volume to compensate. This is not always an ideal solution, however, and whether it is workable for you depends on a lot of variables that are beyond the scope of a short response.  
Fritz Byle
Process Engineer
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.
Tenting on the top side only will minimize the risk of voiding caused by outgassing as the solder flows down the via hole.
Edithel Marietti
Senior Manufacturing Engineer
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
Single sided via protection -- be it tenting, plugging or something other -- is to be avoided. Such features are consistently rated as NOT RECOMMENDED in the IPC-4761, Design Guide for Protection of Printed Board Via Structures. Single sided tenting turns holes into cups, also known as Flux Buckets (and worse). Via Fill is arguably the best method (less arguably, the most expensive). Double-sided tenting is unreliable (tents break) but double-sided via plugging champions cost and performance. For the latter: Instead of using solder mask for plugging, consider products specifically for via plugging (e.g., Taiyo's LPI Hole Plugging Ink).
Robert "Bob" Lazzara
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.
Amkor has an application note on the topic. There are other papers on the topic as well and how it influences solder joint voiding. As a matter of preference, I would suggest via tenting the top side.
Eric Bastow
Senior Technical Support Engineer
Indium Corporation
Eric is an SMTA-certified process engineer (CSMTPE) and has earned his Six Sigma Green Belt from the Thayer School of Engineering at Dartmouth College. He is also a certified IPC-A-600 and 610D Specialist. He has an associate's degree in Engineering Science from the State University of New York and has authored several technical papers and articles.
The best solution is to fill the vias. Filled vias provide a planar surface that is easy to clean reducing ionic contamination at the board and assembly level. Hole "tenting" or plugging can have some challenges. Liquid photoimageable (LPI) solder mask materials do not truly tent holes in the same way that old school dry film materials once did.

Instead the process results in a partial plug. To accomplish the plug, the LPI solder mask is screened onto the board, typically from one side only. Depending on the hole size and aspect ratio this single-sided plugging can fill up to about 70% of the hole. Lesser plug levels are possible, as is plugging from both sides. However in either case with a partial plug there is a risk of solution entrapment in the hole behind the plug which can result in damage to the copper in the hole or increased ionic contamination levels.
Kathy Nargi-Toth
Director Quality and Technology
Kathy has 30 years PCB expertise including R&D, engineering and technical marketing. Knowledge of PCB fabrication processes, laminates, plating finishes, and solder masks. Lean, Six-Sigma Black Belt Certified. IPC standards committee member, past chairmen Suppliers Council Leadership Subcommittee. Certified IPC Trainer.
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