|Ask the Experts|
January 20, 2021
Toe Fillet Requirements on Gull Wing Components
Is a toe fillet required on gull wing components? If it is not a requirement why is that so? We have some gull wing components that have no toe fillet, but all the other requirements for the solder connection are met.
|Expert Panel Responses|
Per IPC-610, section 8.3.5 Flat Gull Wing Leads and section 8.3.6 Round or Flattened (coined) Gull Wing Leads, there is no requirement for toe fillets. The only requirement is that if the lead overhangs the pad or land area it does not violate the minimum electrical spacing requirements of the product.
The reasoning for not having a requirement for toe solder fillets is that the method of manufacturing for the gull wing lead frames creates a condition on the end of the lead which may or may not be solderable. Therefore we did not create a criteria for a solder fillet at the end of the leads, if it happens it is okay and if it does not happen that is still acceptable.
The strength of the solder joint for gull wing components is in the heel fillet of the component and the length of the component lead, so the solder joint at the toe of the component does not add anything to the strength and reliability of the solder joint.
If all the other requirements are met except the toe fillet is missing as mentioned the condition is acceptable.
I hope this answers your question.
Vice President, Technical Director
Per J-STD-001, there is no requirement:
In practice, nearly all failures of gull-wing joints begin at the heel and propagate toward the toe. In addition, the fact that most gull-wing leads are formed and cut after plating leaves the toe relatively unprotected from oxidation, and so getting a toe fillet may be difficult.
The NASA requirement is the width of the end solder joint (toe) must be greater than 75% of the lead width.
Regional Sales Manager
OK International Inc.
There is a requirement to have the pad extend past the toe of the lead... however, the solder presence cannot be left to a toe fillet only. The design of the pads, coupled with the dynamics of the reflow will pull the solder from heel to toe and vice/versa.
For this reason, the toe fill maximum requirement is often noted, but the minimum, due to control is not defined as a defect.
Capital Equipment Operations Manager
Specialty Coating Systems
Structurally the most significant elements of a gull-wing solder joint are the solder between the pad and the bottom of the gull wing lead and the solder at the heel of the lead (heel fillet). Toe fillet can't be counted on nor does it add any appreciable strength. Many gull-wing devices are sheared from their lead-frames after lead-frame plating. This results in exposed basis material at the point of plating buss shearing.
So if the basis isunsolderable or tough to solder, as most are, it may not wet during the soldering process. A good example of this would be a copper alloy lead-frame. Copper (sans OSP-coating) is notably difficult to flux with typical No-Clean flux formulations. Plating is added to such lead-frames to allow reasonable solderability during SMT processing.
Alloy 42 (Kovar) is another lead-frame alloy this is not solderable without the addition of a plating. Therefore, there can be no expectation for toe fillet formation during soldering. Bottom Termination components (BTCs) are another class of component with sheared lead and no reasonable prospect for toe fillet formation. Lastly, recall that IPC 610 allows for some gull wing toe overhang beyond the PWB's solder land.
Certainly on an overhanging lead one could not expect to have a toe fillet. Concentrate on getting the rest of the solder joint right, especially an in-spec (per -610) heel fillet.
Per IPC-610, there is no requirement for toe fillets. If all the other requirements are met except the toe fillet is missing as mentioned the condition is acceptable.
My recommendation is to try to stay away from a common bad practice: finding an issue where there is none.A solder joint is conforming or non-conforming there is nothing in between.
Engineering and Operations Management
The reasons given for lack of a fillet and its contribution to solder joint strength make good sense. How do you reconcile the statements about no requirement for a toe fillet with Figure 8-89 in IPC-A-610F and the definitions for a defect being "minimum end joint width (C) is less than 50% lead width (W)"? If my customer points to this figure for a condition where there is no toe fillet, I don't see how I can defend this position.Jeff McCoy, ARQ Electronics
The answer to Reader Comment from Jeff McCoy is, the height of the end joint is Dimension G--not any part of the toe height. If wetting is evident for minimum 50% of the lead width, the requirement is met.John De Leeuw, TE Subcom
The most stress bearing area of all Gull wing devices are the heels. Further, the foot will share the load across its length. Hence if one fix the heel, along with a reasonable percentage of the foot length it will suffice. This also arrives from the structure of the Gull wing device. From my years of experience, what is required is a concave filleting at the heels as well as along the foot.Dr. KUTTIYIL THOMAS OOMMEN THARAKAN, VSSC
As per IPC-610, the length of filleting along the foot shall be equal to or greater than three lead widths or 75% of the of the foot length. However, if the foot length is less than three widths, the entire length shall be filleted.
Toe filleting is thus not mandatory as far as the strength of solder joint is considered. However if one can achieve it, it is well and good. To have filleting at the toe, one has to plate the area, since by virtue of its manufacturing the ends of the leads are obviated from plating, which calls for another process.
Toe fillets created a number of lively discussions in the past 2 decades of solder paste formulation development at Alpha. Covering the bare copper with solder is better than having exposed copper for harsh atmosphere applications. It is easy to do if you do not have any SIR requirements for the paste. Using a paste with high wetting properties, while passing J-STD-004B SIR could be the resolution of this issue
Director of Reclaim Business
Alpha Assembly Solutions
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