|Ask the Experts|
April 9, 2021
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?
|Expert Panel Responses|
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.
S T and S Testing and Analysis
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.
Tenting on the top side only will minimize the risk of voiding caused by outgassing as the solder flows down the via hole.
Senior Manufacturing Engineer
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).
Circuit Connect, Inc.
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.
Technical Support Engineer
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 photo imageable (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.
Director Quality and Technology
NCAB Group USA
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