FIGURE 2 shows the same type of reflow wicking defect on a gullwing lead with the solder
reflowing preferentially to the lead, rather than equally between the pad and lead.
One involves separately
reflowing one component on top of the primary one at the rework station or in the reflow oven.
This was part of the general increase in understanding of the total process of
reflowing solder paste: evaporating solvents, activating flux chemistries and permitting time for the reduction of oxides on soldering surfaces, controlling heating rates for critical materials within the assembly, etc.
This results in one end of the component
reflowing before the other end, causing the package to stand up on one end to resemble a tombstone.
The method is feasible for
reflowing 3D power modules that use various types of solders.
FIGURE 1 SHOWS the misplacement of a package-on-package after dipping the balls in dip paste and
reflowing the assembly.
This is usually the result of a non-optimized reflow profile, in which solder finishes
reflowing and flux completely volatizes before reflow occurs at the solder ball.
However, if
reflowing under air, soaking at around 200[degrees]C for lead-free reflow will result in much more severe oxidation on parts and paste than soaking at the typical 160[degrees]C for SnPb reflow.
Oven setup and process optimization software may improve both productivity and quality by permitting a more scientific and accurate approach to
reflowing assemblies under such difficult circumstances.
Also, if
reflowing a Pb-free BGA using a conventional leaded temperature of 215[degrees]C, ensure the balls on the BGA do not slump and a reliable solder joint is formed.