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 the rules will vary depending on the board design, and the manufacturer's capabilities.
It is highly recommended to check with the PCB manufacturer for their precision and limit on the width of the solder mask Dam. Solder mask dam between surface mount pads will prevent solder bridging during assembly. Below is one of the PCB suppliers' capabilities.
In summary, the width of the solder mask dam depends on the copper thickness and solder mask color. Generally, 3 mils of solder mask dam can be achieved using 1oz of copper but that will be 6 mils for 2 oz of copper. Similarly, the minimum solder mask dam for the Green solder mask is 3 mil and others will be 5 mil.
To get a solder mask dam between pads (if individual relief is required) spacing between copper should be 8 mils or more. Otherwise, the finished PCB will get block relief without a solder mask dam.
PCB under microscope with and without solder mask dam can be seen in the photo below.
