Hi Bev! I have seen problems in this area, where I did not have enough data to get satisfactory statistical evidence and all other indicators--X-RAY, adjacent parts, visual--indicated things were OK. I believe my problem was running these parts with tin plated leads and the associated oxidation through an RMA flux based SnPb line. I am concerned that the flux/temp combination of this line is not aggressive enough to strip the oxide from these parts. Now I am receiving parts from other vendors to be used in a higher level assembly I'm responsible for--so I am asking for "evidence of wetting" in order to pass IPC criteria. So we are arguing about what this evidence might be. Wayne -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian Sent: Friday, April 11, 2008 9:19 AM To: [log in to unmask] Subject: Re: [TN] FN Parts--Evidence of Wetting Wayne, It doesn't prove it, but if the x-ray images show solder everywhere across the pads or everywhere minus a few void areas then that is a good sign. If all the solder was jammed up to one side of the pads or off the pads then the alarm bells should be going off. Also, you could do solderability testing of components before they are used. However, there are problems with this too. Unless the problem is endemic, you will never statistically find it testing 5 to 10 parts. And for a lot of these suckers are so small that I am essentially using my wetting balance as a fancy dip and look tester, rather then a piece of equipment that gives me numerical data. Bev RIM -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne Sent: Friday, April 11, 2008 9:12 AM To: [log in to unmask] Subject: Re: [TN] FN Parts--Evidence of Wetting Hi Dave! Saw you at IPC/Apex and was going to say hi, but you looked way too busy. So, from your response below, the FN parts are treated very differently from all other SMT parts: Whereas most parts require a visible, measurable fillet to confirm wetting, acceptable evidence of wetting for the FN parts a combination of the following: -an adjacent part with fillets looks like it flowed real good and meets IPC specs. -the solder on any visible portion of the pads looks like it got plenty hot and flowed on the pad -you have documented destructive SPC data on "sister" boards and have demonstrated/documented good Cpk on all contributing factors OK, maybe that's acceptable to IPC, but it makes me darn uncomfortable if I'm dealing with a short run of units I got built at a contract manufacturer. I guess I'm supposed ask for thorough documentation of all of these contributing factors so I can confirm I have good joints. If this is really the case, it seems different enough from the rest of the IPC specs that perhaps some additional clarification is called for to explain the note "evidence of wetting". Not only that, but if IPC really thought that statistics is the right way to get the evidence, they are smart enough to enter a numerical number for the statistical criteria to meet. Maybe different thresholds for the three classes. The idea that the kind of evidence you have described is acceptable for Class III (medical/military/aero, etc) gives me the creeps because many of these apps have short production runs (likely not enough statistical data to make me comfortable). Furthermore, while the original FN packages always had instructions from the manufacturer to place a circuit board pad for and solder attach to the die paddle, many manufacturers are neglecting this today--in fact I have found one QFN where the data sheet says specifically to NOT attach to this. So the total stress is just on the pads where the only requirement is "evidence of wetting". Thanks, Wayne Thayer -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman Sent: Friday, April 11, 2008 8:15 AM To: [log in to unmask] Subject: Re: [TN] FN Parts--Evidence of Wetting Hi Wayne! Ah, if it were only that easy! A QFN component essentially is a "blind solder joint" so you must rely on your process to produce an acceptable solder joint and confirm that process consistency through testing. We are faced with a variety of component styles now, such a bottom only terminated devices, where there is no visible solder joint available for inspection. Using a combination of process validation, Xray inspection techniques and product integrity testing, you should be able to confirm the solder joint integrity. A very common industry myth is that we "inspect" quality into our electronic assemblies - the reality is that the inspection step is only a confirmation that your design and assembly processes are controlled/consistent. The JSTD-001 and IPC-610 specifications attempt to provide as much assistance as possible. Good Luck. Dave Hillman Rockwell Collins [log in to unmask] Wayne Thayer <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 04/10/2008 09:35 PM Please respond to TechNet E-Mail Forum <[log in to unmask]>; Please respond to Wayne Thayer <[log in to unmask]> To [log in to unmask] cc Subject [TN] FN Parts--Evidence of Wetting Relevant IPC specs claim that no toe fillet is required for DFN/QFN parts and there is no minimum solder joint height. The only requirement for the joint other than alignment/pad size is "evidence of wetting". How do I get this evidence without a toe fillet? I suppose if I have enough joint height I can see under the part like a BGA, but I've never seen a FN part mounted that high. Would be nice if the spec included some pix/examples of this "evidence". Any recommendations would be very much appreciated. 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