Printed Circuit Design Tutorial (E. Solder mask)
E. Solder mask
Solder mask or solder resist is a lacquer- like layer of polymer that provides a permanent protective coating for the copper traces of a printed circuit board (PCB) and prevents solder from bridging between conductors, thereby preventing short circuits. Solder mask was created primarily to facilitate wave soldering used in mass assembly. Solder mask is traditionally green but is now available in many colors. You can check our website for the different color we can offer, and gold phoenix also has the capacity to mix different solder mask color on the same board, if you have this kind requirement you can email us the gerber files and we can send you a quote.
Fig 11 Solder Mask Resolution
Fig 12 Solder Mask layers of Fig 11
For the end user what you really should care is the color of the solder mask, and then the resolution of the solder mask. In Fig 11 shows a fine pitch IC, to assembly this kind IC we would like to have solder mask between of pads so they will not be shorted during the assembly. In order to print solder mask between the pads gold phoenix will require Solder mask dam between pads: minimum: 0.004", preferred: 0.006". if the clearance between pads are less than 4mil we will not be able to print the solder mask. Fig 12 Solder Mask layers of Fig 11, in Fig 12 the black area means solder mask, and we need to have 4mil clearance between the pads.
Fig 13 Solder Mask Expansion
When you design the PCB, in the PCB files if the clearance between pads is 4mil that really do not mean that you will have a solder mask dam in the boards you received. Most of the PCB design software has a feature to control the solder mask, called “solder mask expansion”. Due to the limitation of the equipment, the pads on the solder mask layers larger than the pads on the copper, it is impossible have a solder mask opening the same size as the copper pads due to the limitations of the equipment. If we have the mask opening the same size as the copper layer then the solder mask will cover the pads if the mask layer shifts a little during the manufacture process. Most of the PCB house will need solder mask expend 0.1mm=4mil larger than the copper pads, which is shown in Fig 13, in the expansion area there will be no solder mask. For example if on the copper layer the clearance between pads is 12mil, solder mask expansion is 4mil, which means that the clearance on the solder mask layer is 4mil (12mil-4mil*2, 4mil added on two pads), there will be no solder mask between the pads if we use black solder mask, and green solder mask will be hit the most of the manufacture’s limit. With gold phoenix with green solder mask we need at least 4.5mil clearance between pads, and 6mil clearance for black solder mask in order to print solder mask between pads.