Ask the Experts
December 10, 2018
Requirement for Toe Fillets
I am a junior inspector at an electronics manufacturing service. I ran into a little bit of confusion regarding what is the requirement for a toe fillet.
Figure 8-89 of the IPC gives a requirement for the End Joint Width of the Gull Wing but then on pages following that figure the IPC illustrates an allowance for Toe Overhang and it goes on to state that there is no minimum toe fillet requirement other than "evidence of a fillet" but the placement of this statement seems to talk about the heel fillet height as well as solder thickness.
Expert Panel Responses
When one talks about toe fillets, one has to discuss solderability. Often in the manufacture of ICs lead-frames are separated/die-cut from their plating buss AFTER plating. The exposed basis metal is often a copper alloy. Without solderability coating or solderable plating, the exposed copper at the tip of the lead-frame will oxidize quickly. Oxidized copper is very difficult to solder and would require a strong fluxing agent. If not solderable, toe fillet will be absent or poor.
Gull-wing type solder joints get nearly all their strength from two areas: 1) heel fillet and 2) the solder bond between the length of the lead which lies atop the length of the PCB's bonding pad. Sometimes side fillets will add strength but these days lead width is often on the order of pad width so discussion of side fillets may be moot. Peel strength of a gull-wing lead is always dominated by the heel fillet first and then the longitudinal lead-to-pad solder bond. Never has a solder joint been compromised by a poor or missing toe fillet.
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
I would suggest contacting IPC directly for clarification.
Technical Support Engineer
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.
It's hard to tell, but it seems as if you may be using an older revision of the IPC-A-610. To start, I'd refer to Table 8-5. The minimum End Joint Width for Class 1 & 2 is 50%, Class 3 is 75%. This is regardless of any "Toe overhang". You still need to have the minimum width of the solder joint required by the specific Class your assembly is supposed to meet, regardless of whether or not the toe is hanging over the edge of the pad or not.
If you are inspecting a specific feature of a solder joint, such as Minimum End Joint Width of a Gullwing device, only use the criteria for that combination. Move on to Toe Overhang, Side Joint Length or Heel Fillet Height (or whatever else you are looking at) and apply only the criteria for that specific feature. It seems as if you may have mixed multiple criteria and features.
As a side note, another thing to remember when using the IPC-A-610 is that the photos and pictures are for reference only. In section 1.7 Examples and Illustrations, it states that many of the examples and illustrations are gross exaggerations to better depict the condition discussed and that it is necessary that the user pay attention to the subject of each section to avoid misinterpretation. So don't judge anything in the illustration beyond the specific subject it's attached to. I would suggest, assuming you haven't, taking a CIS course to become more familiar with the IPC-A-610, it really helps to clear some of these types of things up.
Esterline Interface Technologies
Mr. Hughes has been in the electronics manufacturing field for 20 years. Operating the processes and as a manufacturing engineer for the last 14 years. He is also a CIT as well as an SMTA Certified Process Engineer.
This section of the IPC-A-610 document does discuss the wetting of the lead (L) and (D), the width of the solder joint (C) and the importance of the heel fillet (F) and solder thickness (G).
I've also extrapolated this statement from 8.3.13 Plastic Quad Flat Pack - No Leads (PQFN)
There are some package configurations that have no toe exposed or do not have a continuous solderable surface on the exposed toe on the exterior of the package. A toe fillet will not form.
The reasoning for making this statementin section 8, is to discuss the inability of end of the leads to be soldered due to manufacturing process of cutting off the leads and leaving the base material exposed and oxidized. Although we are looking at wetting section we need to have this statement in this section to let people know that in certain cases, non wetting of the end of the leads or the breakaway tabs along the component edge which are exposed are not necessarily solderable.
Vice President, Technical Director
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.