Differences and Functions of PCB Solder Mask and Solder Pad
Differences and Functions of PCB Solder Mask and Solder Pad
Solder Mask Introduction The solder mask pad is the area on the board where the green solder mask is applied. Actually, this solder mask layer uses negative film output, so after the shape of the solder mask layer is mapped onto the board, it doesn't actually show green solder mask; instead, it exposes the copper. Usually, to increase the thickness of the copper trace, lines are drawn on the solder mask layer to remove the green solder mask, and then solder is added to achieve the effect of increasing the copper trace thickness. Solder Mask Requirements Process Requirements: The solder mask layer plays an important role in controlling soldering defects during the reflow soldering process. PCB designers should minimize the spacing or air gaps around pad features. While many process engineers prefer the solder mask layer to separate all pad features on the board, the lead spacing and pad size of close-pitch components will require special consideration. While unpartitioned solder mask openings or windows on four-sided QFPs may be acceptable, controlling solder bridges between component leads can be more difficult. For BGA solder mask, many companies offer a type that doesn't contact the pads but covers any features between them to prevent solder bridging. Most surface mount PCBs are covered with solder mask, but if the thickness exceeds 0.04 mm (″), it can affect solder paste application. Surface mount PCBs, especially those using close-pitch components, require a low-profile photosensitive solder mask. Workpiece Preparation: Solder mask must be applied using either a liquid wet process or a dry film overlay. Dry film solder mask is supplied in thicknesses of 0.07–0.1 mm (0.03–0.04″) and is suitable for some surface mount products, but it is not recommended for close-pitch applications. Few companies offer dry films thin enough to meet close-pitch standards, but several offer liquid photosensitive solder mask. Typically, the solder mask opening should be 0.15 mm (0.006″) larger than the pad. This allows for a 0.07 mm (0.003″) gap on all sides of the pad. Low-profile liquid photosensitive solder resist is economical and typically specified for surface mount applications, providing precise feature dimensions and clearances. Solder resist layer introduction: Used during machine placement. It corresponds to the pads of all surface mount components. In SMT processing, a stencil is usually used. Holes are drilled in the PCB corresponding to the component pads. Solder paste is then applied to the stencil. When the PCB is under the stencil, the solder paste drips down, ensuring that each pad is properly coated with solder. Therefore, the solder resist layer should generally not exceed the actual pad size. Using "<=" is most appropriate. The required layers are almost identical to those for surface mount components, mainly including the following elements: 1. BeginLayer: Thermal Relief and AntiPad should be 0.5mm larger than the actual size of the regular pads. 2. EndLayer: Thermal Relief and AntiPad should be 0.5mm larger than the actual size of the regular pads. 3. DEFAULTINTERNAL: Intermediate layer The Role of Solder Mask and Solder Flux Layers The solder mask layer primarily prevents the PCB copper foil from being directly exposed to the air, providing protection. The solder flux layer is used by stencil manufacturers to create stencils, which allow for accurate placement of solder paste onto the surface mount pads during soldering. Difference Between PCB Solder Flux and Solder Mask Layers Both layers are used for soldering; it doesn't mean one is for soldering and the other for applying solder mask. Rather: 1. The solder mask layer means creating openings in the entire solder mask layer to allow for soldering. 2. By default, areas without a solder mask layer should be covered with solder mask. 3. The solder flux layer is used for surface mount packages.
