Issue link: https://iconnect007.uberflip.com/i/1513827
JANUARY 2024 I SMT007 MAGAZINE 59 only process" instead of solder paste, they need to validate the material is covering all the PCB pads affected. If they are printing solder paste on the package solder balls, the stencils must be designed accordingly to ensure a good per- centage of solder paste transfer during the package placement, and similarly for alterna- tive methods such as flux dipping or others. at could be risky depending on the pack- age design. If you don't have enough mate- rial there, that could induce some defects like opens or shorts. If you have too much material, that could induce solder bridging defects. is is why inspection is very important. Depending on the case, y o u c o u l d s a c r i f i c e inspection and only use a microscope or an SPI inspection for the sol- der paste volumes. You would have more data collected and available, especially when we are talking about new pack- a g e r e w o r k r e c o m - mendations. In conclu- sion, there are differ- ent rework processes depending on the type of repair and the type of component and board design; on all of them it is important to do the measurements and process validations to facil- itate good results. Matties: What are the typical types of errors that you have to rework? What do you see the most? We dedicate a lot of time to development. Regarding defects typical from the SMT lines on fully populated boards, time aer time, we see defects as solder bridging, opens, insuffi- cient solder, and polarity issues. Defects could be on BGAs and on non-BGAs, it's the polar components—capacitors and resistors—in dif- ferent locations where they are very populated on both sides. impractical? As we start getting these increasingly finer features, at some point it just becomes scrap. Likewise for the very large ones with multiple thousands of ball grid solder joints underneath that massive package. Have you seen the limit yet? I'm not seeing a limit yet, but the challenges are there and are more aggressive year by year to have successful results in rework. But we need to continue working closely with the industry and the tool suppliers, so they continue to develop tools we will need to support these necessities. e other component is to work with the board designs, so they understand the limits— proper use of keep-out z o n e s , f o r e x a m p l e , around the BGA. Matties: Do I hear you saying it's the role of the designer now to under- stand what the limits are for rework? Yes, but it's important to give the feedback. For example, if you have a v e r y p o p u la te d b o a r d with a lot of components around the BGA, it takes a skilled technician to know how to prop- erly place your mini stencil and apply the paste without introducing defects in the compo- nents around it; you may be contaminating the other components with solder paste. Matties: I've heard that if there's one process that needs better understanding, it's the solder paste process. We're using too much solder paste—or not enough, or too much flux, or not enough, and so on. How do we optimize that process? Good point. Technicians commonly use the microscope to inspect the solder printing with mini stencils before placing and reflowing the new part on the board. When they use a "flux If you have too much material, that could induce solder bridging defects.