Layout Tipps & Tricks
Bonding SMT components
Bonding SMT components
There are three restrictions for wave soldering of previously glued SMT components:
Components with a length ≤ 1.2 mm (e.g. component size 0402) cannot be wave soldered. They can no longer be bonded with sufficient safety. Component size 0603 should be avoided in the wave soldering technique, since the process reliability is lower than with larger sizes.
Components with a height > 2.5 mm can easily be washed away by the pressure of the wave or, if they are even taller, they will stick to fixed parts of the wave soldering system. In the case of film capacitors, their heat sensitivity must be taken into account.
Ceramic capacitors above size 0805 should not be soldered to the shaft because of thermal stress.
Common practice is to bond SMT components in a separate process step and solder them together with the THT components on the solder side above the solder wave. Care must be taken to ensure that the SMT components are aligned in the direction of the wave, otherwise solder shadows will lead to unsoldered components. (see illustration)
In addition, attention must be paid to possible short circuits with transverse ICs. ICs with 0.8 mm pitch can still be soldered without solder bridges if the layout is correct for the waves (existing solder traps and correctly positioned components). Solder traps must be twice as long as the pad and rounded or pointed at the end. (see SO14 in the figure)
If no or rectangular solder traps are used, spilling back solder can lead to solder beads or short circuits between the pins.
With the 0.8 mm pitch, there is a free space of about 0.4 mm between adjacent pads. The practical achievability of the wave soldering minimum depends, among other things, on the local topology of the assembly and the associated flow conditions on the assembly. Tests should be carried out before series production is started. The practical soldering limit for a wave soldering result free of solder bridges with standard equipment is a uniform minimum distance of about 0.4 mm to 0.5 mm.
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