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Knowledge For PCB What is solder mask?

What is solder mask?

August 25, 2022     Knowledge For PCB

Solder Mask

The part of the printed circuit board to be greased
The solder mask is the part of the printed circuit board that is to be greased. In fact, the solder mask uses a negative output, so after the shape of the solder mask is mapped onto the board, it is not a green solder mask, but rather a copper skin is exposed.

Solder Resist Layer Concept


Printed circuit boards are basically composed of pads, vias, solder resist layers, silkscreen layers, copper traces, various components, and other parts. Usually, in order to increase the thickness of the copper skin, the soldermask layer is used to scribe on the green oil, and then add tin to achieve the effect of increasing the thickness of the copper line.
Flux layer and solder resist layer difference
Two layers are on the tin soldering with, does not mean a tin, a green oil; but:
1, the solder resist layer means that the whole piece of solder resist green oil on the window, the purpose is to allow soldering;
2, by default, there is no solder resist layer of the area are to be on the green oil;
3, flux layer for SMD package;
Solder resist layer requirements edit Podcast

Process requirements

The role of the solder resist layer in controlling solder defects during the reflow process is important, PCB designers should minimize the spacing or air gap around the pad features.
While many process engineers would rather the solder resist layer separate all pad features on the board, the pin spacing and pad size of closely spaced components will require special consideration. While a non-partitioned soldermask opening or window on the qfp on all four sides may be acceptable, controlling the solder bridges between component pins may be more difficult. For bga soldermask, many companies offer a soldermask layer that does not touch the pads, but covers any features between the pads to prevent solder bridging. Most surface mount PCBs are covered with a solder resist layer, but the application of a solder resist layer, if thicker than 0.04mm (″), may affect the application of solder paste. Surface mount PCBs, especially those using closely spaced components, require a low-profile light-sensitive solder resist layer.
Process Fabrication

Solder resist materials must be applied through either a liquid wet process or a dry film lamination. Dry film solder resists are available in 0.07-0.1mm (0.03-0.04″) thicknesses and can be suitable for some surface mount products, but this material is not recommended for dense pitch applications. Few companies offer dry films thin enough to meet dense pitch standards, but several companies can offer liquid photoresist materials. Typically, the solder resist opening should be 0.15mm (0.006″) larger than the pad. This allows for a 0.07mm (0.003″) gap on all sides of the pad. Low profile liquid photoresist materials are economical and are often specified for surface mount applications, providing accurate feature sizes and gaps.


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