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January 2011

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From:
"Nieznanski, John A - GS" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Nieznanski, John A - GS
Date:
Mon, 3 Jan 2011 13:24:04 -0500
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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


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