There are many reasons for insufficient fill of a PTH (Plated Through Hole) and the occurrence of voids in barrels.  Board thickness, finished hole size, board plating material, flux type and application method and type of solder all affect this possible defect.  Let’s look at these two problems and a few possible reasons for them.

Regarding Insufficient Fill:
Check to review the application of flux primarily its penetration through the board.

Verify the topside board temperatures going into the wave or coming off the preheaters.  Is it high enough to allow for the molten solder to rise through the board?

Review the depth of the board in the wave.  Try running the board at least ½ of the board thickness in the wave to use the hydrostatic pressure of the wave to push the solder up into the plated through holes.

Regarding Voids in the Solder Joints:
This can also be caused by a variety of reasons and the primary one, which comes to mind, is bad plated through hole barrels. Check the plating in the barrels to make sure there are no voids. See IPC-600 for the criteria. Plating voids will have a tendency to create out gassing and therefore create voids in the solder joints.

The other issue which could be causing this problem is if VOC free fluxes are being used and the application is putting on too much flux and it is not drying off during the preheat stage of the wave solder system. As the flux product hits the wave solder system, the liquid will volatize very quickly and can create voids and blow holes in the solder joints.

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