0402 obviously is harder to hand assemble. If you need to get them assembled by machine, please notice the following tips.
- The assembler’s pick & place machine can even handle 0402 at a good rate & yield.
- Double-sided load. Some boards just need this assembly step, like BGA often puts decoupling caps on the bottom of the board.
- Electrical properties. The obvious one is, of course, the power rating of resistors. Voltage rating is not only likely worse due to the physical smaller dimensions, but also consider the voltage bias degradation effect of ceramic class 2 capacitors. For smaller packages in same voltage/capacitance, this effect may be worse.
On the other hand, some properties, like lead inductance, is lower. In addition, sometimes you can put capacitors right at the chip pins for QFP style packages, while routing signals underneath the chip. In effect, the loop area for the decoupling capacitor is much much smaller than a larger size capacitor, which may be placed further away to comply with signal routing.
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