I also find this interesting. I am going out on a limb and assuming the rate of copper dissolution is exacerbated, the "force" is actually leaching, not pressure, and the root cause may have been excessive touch-up or rework. Any opinions or observations otherwise? -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner Sent: Monday, January 03, 2011 3:39 PM To: [log in to unmask] Subject: Re: [TN] solder joint height values (h) for leadless SMT components? Dale, That is interesting. I have a possible dumb question: You mention copper at 1um or less the copper become deformed such that if follows the component termination contour. What is the source of these forces and does this imply the copper is being torn from the FR4? Thanks, Bob K. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Hart, Dale L CTR USAF AFMC AFRL/RXSA Sent: Monday, January 03, 2011 3:41 PM To: [log in to unmask] Subject: Re: [TN] solder joint height values (h) for leadless SMT components? John, I've been doing failure analysis for the Air Force for nearly 30 years. I've cross sectioned countless chip resistors and capacitors during this time. If I find the standoff height of solder under a component termination greater than 50um it is extremely rare. The average height is between 25 to 40um. Approximately 5 percent of these components have 1um or less. Of these 1um or less the copper pad underneath the termination is deformed to the contour of the component termination. This has also occurred with both iron/nickel and copper "L" shaped gull wing leads. The manufacturers that populate and reflow these boards tell me that their stencil was, for example, 7 mils and their paste height was within their acceptable limits. I have asked if they performed line proofing with a DPA of representative components at various locations on their panel the answer is usually no or that is was done so long ago that none of the present operators remember it being done. Have I seen failures of solder joints of this pedigree, the answer is yes but not as many you might expect. It depends on the operational environment (thermal cycling, shock and vibration). Also these type of solder joints are very susceptible to cracking if they have been encapsulated. Dale Hart UTC Failure Analysis Engineer dlh -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Nieznanski, John A - GS Sent: Monday, January 03, 2011 1:24 PM To: [log in to unmask] Subject: [TN] solder joint height values (h) for leadless SMT components? Happy 2011 TechNetters! The solder joint height parameter "h" is a very sensitive parameter in the E-W solder fatigue equations per IPC-D-279 and IPC-SM-785, particularly for leadless components. For leadless components, it is common to use half the solder stencil height for "h", in lieu of measured values. So for example, given a 6 mil thick solder stencil, "h" is often assumed to be 3 mils with some acceptably small variations, to be quantified and confirmed. However there are many leadless SMT components that have solder joints that can not be characterized by a single "h" value. For example, SMT resistors often have a solder fillet that is between 50% and 100% the thickness of the component along the outside edge of the terminals and much less under the terminals. Similary for leadless MELF diodes and castellated LCC packages. Ideally it would be great to build test boards with all these configurations to get failure data and correlate these results with DPA measurements of solder joint cross sections. Are you aware of any published papers that discuss this type of analysis and/or attempt to derive and correlate an "effective h" for leadless component types and terminals based on measurements? In lieu of any published efforts, does it seems plausible or reasonable to develop the concept of an "effective h" or "average h" based on several design and assembly parameters such as solder pad dimensions, wetted terminal dimensions, solder stencil thickness, component-to-pad misregistration, etc? Is this just wishful thinking? Alternatively, I suppose one could make the "weakest link" argument and say that the "minimum h" is going to dominate the behavior of the solder joint, not the "average h" or "effective h", since the "minimum h" is where the shear stresses and strains will concentrate and initiate wearout. Thanks in advance for any results, comments, thoughts or suggestions that you can provide............ Regards, John Nieznanski ________________________________ This e-mail and any files transmitted with it may be proprietary and are intended solely for the use of the individual or entity to whom they are addressed. If you have received this e-mail in error please notify the sender. Please note that any views or opinions presented in this e-mail are solely those of the author and do not necessarily represent those of ITT Corporation. The recipient should check this e-mail and any attachments for the presence of viruses. ITT accepts no liability for any damage caused by any virus transmitted by this e-mail. ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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