Specific defects that are related to lead-free soldering are due to the higher process temperature that is demanded. The process temperature is related to the alloy melting point and so to the alloy composition. Most related defects can also be found when soldering with tin-lead solder, but they are then often less prominent.
Process related defects in wave soldering are fillet lifting and (mini)cracks in joints.
Also the risk of re-reflow increases for reflow soldered components on the topside of the PCB, due to the higher solder temperatures that are demanded for wave soldering.
Other related effects can be (tin) whiskering on lead-free galvanic plated parts, like on leads.
In case the galvanic plating contains bismuth to prevent whisker formation, then there is the risk of the formation of alloy parts with a low melting point in such joints. This may affect the long time joint reliability, depending on the application of the equipment.
In principle the quality inspection demands for lead-free solderjoints are the same as for tin-lead solders. However due to the metallurgical composition, the joint surfaces will look less shiny and they even can have a somewhat rough appearance.
This is not a defect.
It might be even so that microcracks are found on the joint surface. The origin of these cracks are the volume reduction of about 4% that takes place during the solidification of the solder. If the joint leaves the solderwave, then the last part that solidifies will create the shrinking hole(s). Often these holes are formed inside the joint, but they can also be found on or near the joint surface, depending on the joint construction. In that case one can find small cracks or dents at the joint surface. These are unavoidable normal effects that become more prominent due to the shorter solidification traject as in lead-free soldering the solder temperature is just above the alloy liquides temperature.
Fillet lifting is caused due to the big Z-expansion of the board material at soldering temperature. Due to that, the solderpad gets a wedge shaped form, because the copper barrel to which the solderpad is connected has a much lesser expansion than the base material under the copper pad. In this position the solder starts to solidify while at the same time the board material returns to its original shape and dimensions.
The solidified solder keeps however the solderpad in the wedge shaped position and this is what creates the fillet lifting effect.
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