Design for Manufacturability rules and guidelines are a collection of PCB design best practices to help our design be as easily manufacturable as possible. The easier it is to manufacture a PCB, the more likely it will be to reduce both manufacturing errors and costs. DFM rules have been developed by PCB manufacturing experts and they will vary depending on the design technology of the board, and the capabilities of the manufacturer.
It is highly recommended to check with PCB manufacturer for their precision and limit on the width of solder mask Dam.Solder mask dam between surface mount pads will prevent solder bridging during assembly. Below is one of the PCB suppliers' capability.
In summary, solder mask dam width depends on the copper thickness and solder mask colour. Generally, 3 mil of solder mask dam can be achieved using 1oz of copper but that will be 6 mil for 2 oz of copper.Similarly, minimum solder mask dam for Green solder mask is 3 mil and others will be 5 mil.
In order to get solder mask dam between pads (if individual relief is required) spacing between copper should be 8 mil or more. Otherwise, finish PCB will get block relief without solder mask dam.
PCB under microscope with and without solder mask dam can be seen in the photo below.
