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The most common solderable coating on leaded components is tin/lead coatings between 5-10µm. If the lead is made from some materials, there will also be a copper layer between the base material and the tin/lead. In Figure 1, the tin/lead plating is so thin that the solder could not wet the leads during testing.

During assembly with palladium plated components the visual appearance of the soldering may look poor due to the non fusible coating but the joints may be perfectly sound.

Figure 1: The tin/lead plating on these leads is so thin that the solder could not wet the leads.

The solderability of a component deteriorates during storage. Use a testing service to confirm the quality of parts when they are first received, then reconfirm the solderability if the components are stored in excess of one year.

The most common cause of failure is the quality of the parts when received, or aging during storage due to thin solderable coatings. The method of storage can also affect parts.

Figure 2: Poor solderability caused by the age of the part.

Non wetting of the toe of leaded surface mount devices is common and will continue to occur with the greater use of no clean low residue pastes. The lead tip when formed and cut during component manufacture is left exposed with no protective coating. It oxidises during storage and is very difficult to solder. The toe of the lead is not covered in the component manufactures specification or in the solderability standards. The inspection standard should not reject joints where no wetting is visible at this point.

Poor solderability of the lead as supplied from the manufacturer. In Figure 3 the solder paste has reflowed and wet the pad but failed to solder the J lead. The lead shows very little evidence of any solderable coating.

Figure 3: This J-lead shows little evidence of any solderable coating.

Poor solderability of the leads has caused the poor wetting. The solder paste during reflow has wetted the pads but has been unable to flow up the leads. In the case of Figure 4 it is a component problem which needs to be discussed with the supplier. As a minimum requirement there should be 5 microns of tin/lead coating the pins to provide a long solderable life.

Figure 4: Poor solderabilty causes poor wetting.

Chip Leaching	Component Cracking	Cracked Joint	Crystallized Joints	Failure to Reflow
Joint Failure	Joint Voiding	Lifted Component	Open Joints	PCB Measelling
Plastic Coplanarity	Plastic Softening	Poor Pad Wetting	Poor Solderability & Wetting	Solder Beading
Solder Short	Solder Wicking