Hi Amol, Not having a heel fillet can be a problem—but not likely for a fine-pitch QFP. Your causes of not having a heel fillet could be (1) there is no soldering pad at the heel bend (you saying you have near-vertical leads could be a clue); (2) your peak reflow temperature is too low (the component die is a heat sink and the leads are quite a bit cooler than the PCB pad); (3) not enough solder paste. Werner -----Original Message----- From: Kane, Amol (349) <[log in to unmask]> To: [log in to unmask] Sent: Wed, Mar 31, 2010 3:24 pm Subject: [TN] Lead forming angle and solder joint formation I have had a very similar as Bev issue come up here, only in this case it has to do with the absence of a heel fillet on a completely plated part. My question is this: Does the lead forming angle play a role in whether or not a heel fillet forms when a part is soldered to the pad? We have a fine pitch QFP device (19 mil pitch) that we are soldering at SMT. Our quality department is rejecting the boards because of a absence of a visible heel fillet per IPC 610D class 3. When I look at an unsoldered part under the scope, the angle between the foot and the leg is damn close to 90 degrees! We are using a 5 mil thick stencil with 1 mil reduction on the width and 1:1 in the length, resulting in a 2-3 mil actual solder height (after the flux has blown off). Has anyone seen a scenario that prevents the solder from wicking up the foot to the leg forming a heel fillet because of the forming angle? I am also looking to see if a possible contamination issue on the part can result in this phenomenon. Thanks, Amol -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian Sent: Tuesday, March 30, 2010 9:24 PM To: [log in to unmask] Subject: Re: [TN] Need some opinions Dave, A distinction has to be made between failing the SMT process and failing the product. Obviously the issue has little to do with screen printing, placement or reflow - all other parts have the proper amount of solder paste, are placed properly, have good looking joints and intermetallic of an appropriate thickness is there, even on the leaded component in question (where there is an actual joint!). So the process is in control. With regards to the two other component types, I have some comments. SOIC sheared toes have been a pain for quite some time. However, we already know that toe fillets only provide 5% of joint strength at most and industry available reliability data certainly shows that absence of toe fillets on leaded components is not something to be overly concerned about. OK, let's discuss LGAs and QFNs with sawn vertical surfaces. 1) The easiest way to avoid bulbous joints with these components is tight control of pad size and the amount of solder paste deposited. 2) If negative wetting angles are produced, they are not pointed in the direction where it can do the most damage. They point straight down into the bulk of the solder. Negative wetting angles at the heel fillet of a leaded device are pointing right down or close to the intermetallic demarcation line. 3) We have never seen a bulbous joint of a LGA or QFN on a rigid board cause a failure during thermal cycling, drop or tumble. 4) However, we do not allow bulbous joints on these devices on flex circuits. I'll let everyone think about that one. Maybe we are over cautious, but. Bev RIM _____ From: [log in to unmask] [mailto:[log in to unmask]] Sent: Tuesday, March 30, 2010 5:30 PM To: TechNet E-Mail Forum; Bev Christian Cc: [log in to unmask] Subject: Re: [TN] Need some opinions Hi Bev - I wouldn't "fail" the soldering process or the solder joint if I can show the reason is the lack of plating "by design" as you described. However, as other folks have pointed out, there are other functional issues having a SMT component lead sufficiently plated which need to be considered. The industry is already starting to ask the same questions in terms of have sheared toes (SOIC type components) or sawed surfaces (QFN type components) but those cases are not as severe as your case. Dave Hillman Rockwell Collins [log in to unmask] Bev Christian <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 03/25/2010 08:08 PM Please respond to TechNet E-Mail Forum <[log in to unmask]>; Please respond to Bev Christian <[log in to unmask]> To [log in to unmask] cc Subject [TN] Need some opinions Technetters, It seems that more and more component suppliers are not plating their leads all the way up to the component body. In fact some are getting downright skimpy and only plating to the bottom bend, never mind the upper one. If you put down a lot of solder paste you end up with a bulbous joint, a negative wetting angle and a stress concentration point. If you put down less paste and have a co-planarity issue, then you have the potential for opens. The other option of course is getting the supplier to plate higher up the leads. Question: would you consider the condition of the formation of the negative wetting angles a failure during process qualification? Bev RIM ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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