|Ask the Experts|
October 2, 2020
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.
I would suggest contacting IPC directly for clarification.
Technical Support Engineer
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.
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
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 statement in 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
Mr. Hughes is correct; the end joint width must be judged separate from the toe overhang. You can have toe overhang and still make a judgement on the end joint width at the point of the end of the PCB pad. The intent of the end joint width, from my experience, is to ensure the part is positioned well enough to give strength to the solder joint. The strength does not come from the end joint or the toe fillet; it comes from the heel and side joint.Mark Whitaker, Fischer Connectors, Inc.
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