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68 DESIGN007 MAGAZINE I AUGUST 2020 Let's talk about the perfect 0201 footprint for a moment. I actually searched for "the per- fect 0201 footprint" online. I wanted to find out what designers are considering when they mean their component footprint is perfect. As you might imagine, I found dozens of various land sizes, shapes, and land spacing for a gen- erally specified 0201 component package. It seemed that each designer tweaked the num- bers to their own idea of "perfection" without defining any of the manufacturing variables. I then analyzed one of our customers' designs, featuring 0201 parts that had been running suc- cessfully on our lines for many years. Guess what? The footprint didn't match any of the perfect footprints defined on the internet, but the footprint seemed to work perfectly for us. Johnson: What was going on here? Dack: The reason the solder joints were form- ing perfectly for us on our customer's board assembly was not due to a magical concept of a perfect footprint. Success was achieved because the designer had built in enough allowance for the footprint to be combined with all of the other elements of manufacturing to cre- ate the perfect solder joint. The designer's cognizance of compo- nent Z height and how it affects the solder fillet is what made this footprint successful (Table 1). The relationships between the metalized ends of a chip compo- nent and the PCB land geometry are not to be underestimated. However, designers must under- stand that the purpose of stake- holder collaboration is not to yield the perfect footprint. This is a myth. We collaborate to optimize the footprint to work with all of the other manufacturing requirements on the PCB. This allows the assembly stakeholder the best chance of forming perfect solder joints for all the components by manipula- tion of any of the many other variables. If we look into the IPC-A-610 acceptability specification, there are clearly documented photographs showing a perfect solder joint. Looking at the perfect solder joint, you'll see that there is the wetting of the solder material after reflow. Evidence of wetting is a key factor, which has allowed the solder to climb up the component height on the metalized end about halfway, creating the perfect solder fillet. There is a relationship between the exten- sion of the land beyond the component's met- alized end and how far the solder fillet will climb up the component. Too short of a toe extension will reduce the fillet size. Too long of a toe extension may lead to too much solder deposition. The solder may climb too far up the component end leading to defects includ- ing tombstoning, skewing, or solder ball con- tamination. As it turns out, a rule of thumb Some examples of tombstoning. Table 1.

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