If you are using individual washers in your process, there are several things to keep in mind:
The ID (inner diameter) of the washer will be the most critical dimension. It needs to fit over the pin. If your washers are flux-coated, be sure to allow for the added flux in your calculation.
Some IDs are too small for flux coating as the flux coating can easily obscure the ID and make it difficult to smoothly place a washer over a pin.
When you are flux coating your washers, less is more. In other words, 0.5% to 1.5% flux by weight is sufficient to reduce the surface oxides and prepare the surface for wetting.
Solder volume can be adjusted by adjusting the thickness. Contact us to help determine the best inner and outer dimensions for your application. Next we'll need to know the board thickness, hole size, and pin size. Then we can recommend the proper thickness for your washer.