What does it mean to have a window in the solder mask layer of a PCB? Reasons for having a window in the solder mask layer of a PCB

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What does it mean to have a window in the solder mask layer of a PCB? Reasons for having a window in the solder mask layer of a PCB [Copy link]

The concept of solder mask

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

Process requirements for solder mask

The role of solder mask in controlling soldering defects during the reflow soldering process is important, and PCB designers should try to minimize the spacing or air gaps around pad features.

While many process engineers would prefer that the solder mask separate all pad features on the board, the pin spacing and pad size of fine pitch components will require special consideration. While unpartitioned solder mask openings or windows on four-sided QFPs may be acceptable, controlling solder bridging between component pins may be more difficult. For solder mask on BGAs, many companies provide a solder mask that does not touch the pads but covers any features between pads to prevent solder bridging. Most surface mount PCBs are covered with solder mask, but the application of solder mask, if thicker than 0.04mm ("), may affect the application of solder paste. Surface mount PCBs, especially those using fine pitch components, require a low-profile photosensitive solder mask.

Processing of solder mask

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

Understanding of PCB solder mask window

The solder mask opening refers to the size of the copper exposed at the location where soldering is required, that is, the size of the part not covered by ink. The cover line refers to the size and amount of the circuit part covered by solder mask oil. If the cover line distance is too small, it will cause wire exposure during the production process.

Reasons for opening windows in PCB solder mask

1. Aperture window: This is because many customers do not need ink to plug the hole. If the window is not opened, the ink will enter the hole. (This is for small holes) If the large hole is plugged with ink, the customer cannot key. In addition, if it is a gold plate, a window must also be opened

2.PAD (copper) window opening: customers need welding and surface treatment (gold plating/tin spraying, etc.).