Issue link: https://iconnect007.uberflip.com/i/1513827
42 SMT007 MAGAZINE I JANUARY 2024 Challenges with rework of underfilled com- ponents include but are not limited to the fol- lowing: 1. Solder displacement: Soened underfill can displace solder at elevated temperatures which can result in defects. 2. Can be expensive: Low yields and prohib- itive cost of long processing times for experi- enced "high touch" rework are typical. 3. PCB damage: e underfilled rework process may cause laminate damage, mask destruction, and lied pads, impacting reli- ability, and resulting in additional repairs. 4. Difficult rework: Due to the inherent instability of underfilled components, tradi- tional rework methods using thermal or ther- mal/mechanical options become less feasible. e process begins with thoroughly examin- ing the board to identify any components that might interfere with milling. Using a univer- sal fixture which accommodates most PCBs, we mount the board onto the mill platform. Subsequently, the mill is programmed to pre- cisely locate the component, measure board coplanarity, and determine its depth relative to the board. Shaving off layers as thin as a few microns per pass, the mill continually assesses coplanarity and depth while grad- ually approaching an appropriate depth above the pads. e remnant solder becomes the pre-tinned pad for the replacement component. Cold milling becomes a viable alter- native for component removal under specific conditions: • High risk of heat damage to the PCB or rework location • Components can be sacrificed during the removal process • Previous issues arising from traditional heat-based removal methods • Underfilled components requiring rework Benefits of cold milling: • No additional thermal stress and associated risks • e mill contains an integrated active ionizing source reducing the risk of ESD-related issues • A vacuum chuck surrounding the bit which removes debris as material is milled • Laser-based measurement system for precision cuts within 0.001" from the board surface • Cost-effective • Expedited lead-times (depending on quantity) Risks with cold milling: • Pad damage especially when no underfill is present • Milling too deep exposing the copper layer • Physical damage to adjacent components if not removed prior to milling SMT007 Bob Wettermann is the former president of BEST, Inc.