Issue link: https://iconnect007.uberflip.com/i/1276973
AUGUST 2020 I DESIGN007 MAGAZINE 69 may be applied in which the land's toe exten- sion should nominally extend half the com- ponent chip or lead height. This concept can serve as a starting point for helping to achieve a perfect solder joint. Johnson: Would this mean that differing height 0201 chips would need different land geom- etries? Dack: You got it! It is very common for small chip resistors to maintain a common Z height or thickness. But chip capacitors, on the other hand, have various heights that—without sol- der deposition compensation—affect the forma- tion of the solder fillet. This is where the IPC- 7351 land pattern calculator comes in handy. With this calculator, a designer can input all of the geometric attributes of a component or lead—including Z height—and the calculator will adjust the land sizing appropriately. After calculation, some of these geometries could be considered standard or modified, depending on whether they're used in new designs, up-rev'd designs, or legacy designs. We get a lot of customers with legacy designs that were done 5–10 years ago. The fact remains that all of these designs use different land sizes and spacing for their 0201 footprints. But the reason the assembly supplier is still able to solder the parts on is only through adjustment of the other process elements. This is the magic behind the myth of the "perfect" footprint. Regardless of the perfecting of footprint geometry, the chal- lenge has always been put upon the assembly supplier to adjust all of the manufacturing vari- ables to create a perfect solder joint. Johnson: Are there any more factors that come into play in order for EMS providers to yield perfect solder joints? Dack: Yes. For instance, in the case of the 0201 package, we see that many customers' design- ers use the same footprint geometries to attach their 0201 packages onto a PCB design on both top and bottom. Many times, the component density is different on each side. It is very com- mon to see only small chip components on the bottom side of a PCB. This consistency makes selecting a stencil thickness less of a challenge. But if small chip components are installed in the same areas of large power components with massive lands, it sets up a challenge for solder deposition. Larger lands need more sol- der calculated using different ratios. Therefore, thicker stencils need to be used to get enough solder onto the lands. But a thick stencil will overwhelm a small 0201 component footprint with solder paste. What to do? Again, the assembly manufacturing engi- neer's only option after the design is complete is to go back to adjusting manufacturing variables. They have many tricks up their sleeve regarding variables. One trick is to use a thick stencil to satisfy the requirements of the larger parts. But then the manufacturing engineer can specify that the stencil is etched thinner in areas, which must address the smaller components. We can get several stencil thicknesses in one by "step- ping" the stencil in a customized way. Johnson: Unless designers want to redesign after talking to you, they need to take into account the height of the components that they're using. That points them to that IPC calculator. Does an EMS provider's DFM team check the calculations for every component height on a PCB assembly? Dack: Every EMS provider should be checking this. They all have design review teams. Ide- ally, an EMS provider would run a base design through a DFM process. The DFM process is two-fold. At the EMS level, we look out for our bare board manufacturers because we're often tasked with procuring the bare board in panel Example of a skewed chip.