Ask the Experts
March 27, 2023 - Updated
July 10, 2012 - Originally Posted

Test for Flux Penetration

What is good test for flux penetration through the plated hole barrel?


Expert Panel Responses

Flux penetration is important to the flow of solder up into the plated through hole.

The elements which need to be inspected are Flux coverage on the solder source side and flux penetration through the plated through holes.

You can use a glass plate to check for flux coverage on the bottom side of the board. For the plated through hole penetration, use a raw board and apply a thin sheet of absorbent paper on the top side of the board and process it through the fluxer system.You should be looking for flux residues on the absorbent paper sheet. Also look for flux around the pads on the top side of the board.

A specific area to be cautious of is the edges, make sure the flux penetrates the hole near the edges of the board to make sure you get good solder flow in those holes.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

The tried and true method is to lay a flux sensitive paper over an unpopulated board and pass it over your machine's fluxer. Good penetration will show on the sensitive paper as a discoloration of the paper over (top side) each hole.

The sensitive paper may be: pH paper, fax paper, or even a thin brown cardboard stock to see if wetting occurred over the desired holes. The fax and pH papers are nice because they hold the color change while the simple card stock paper will dry fast and thus NOT be a permanent record. The type of paper depends on the flux used and its characteristics: acidity (pH), IPA content, etc.

There are general tools available for testing flux penetration such as the Fluxometer(r), which uses the method described above, with a "mesh" board to check for uniform flux penetration over the entire fluxed area.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

What we used to do was to use a plate with perforations of various sizes arranged in a pattern that repeated on a grid. Over the plate is placed a sheet of paper(1) which is sensitive to the flux and provides a visual indication of contact with the flux. Over that sheet is placed a solid sheet to keep the paper in contact with the perforated plate. The entire "sandwich" is sent through the fluxer, but not the rest of the process.

The paper is examined for penetration. The variation in penetration with respect to hole size(2) and in across-conveyor and along-conveyor directions, can be analyzed.Devices similar to the above may be available as a package; we developed and built or own, based on our specific needs.
  1. For alcohol-based fluxes, FAX paper(thermal paper) was used, as the alcohol triggered a blue-black color transition that was clearly visible. This paper is not as common as it used to be, but should still be available. For water-based fluxes, typically a pH-reactive paper is used.
  2. The range of hole sizes used must include holes that are small enough to simulate the behavior ofa larger hole with a lead inserted. While the behavior of a smaller, open hole is not identical, it is an acceptable proxy.

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.

You can use a paper on top of the board that is reactive to your specific flux. This is the same method as we (ECD) uses on our Fluxometer product,and paper can be sourced from us directly.

Mark Waterman
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.

Yes there is a simple visual test, unfortunately the test is only completed on bear PCB boards so it will not produce 100% the side effects of having components in a hole.But the test is useful and helpful to users.

The test uses either thermal fax paper for alcohol based fluxes or pH litmus paper for water based fluxes. The user would then place and secure the test paper down to the topside of a bear assembly and process the vehicle through the fluxer process. The flux atomized spray that penetrated the holes in the PCB would leave a visual imprint on the appropriate test paper that was used.

Also there is a test vehicle offered today in the market place by ECD called a Flux-O-Meter. This unit consists of a test vehicle PCB, a pallet with a hold down unit and the proper test papers. I hope this information is helpful to you.

John Norton
Eastern Manager
Vitronics Soltec
John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.
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