The solder mask plays a crucial role in managing solder defects during the reflow soldering process, and PCB designers should aim to minimize the spacing or air gaps around pad features.
While many process engineers prefer to separate all pad features on the board with solder mask, the pin spacing and pad size of fine-pitch components necessitate special considerations. Although solder mask openings or windows that are not fully enclosed on all four sides of the QFP may be acceptable, controlling solder bridges between component pins can become more challenging. For BGA solder masks, many companies offer designs that do not contact the pads but cover the areas between them to prevent solder bridging.
Most surface mount PCBs are coated with a solder mask; however, if the solder mask thickness exceeds 0.04 mm (“), it may impact the application of solder paste. Surface mount PCBs, particularly those with fine-pitch components, require a low-profile photosensitive solder mask.
**1. Solder mask:** The solder mask refers to the area of the PCB that is coated with green epoxy. As it is a negative output, the actual effect of the solder mask area is that it is not covered with green epoxy but is instead tinned, appearing silver-white.
**2. Soldering layer:** The paste mask is utilized for machine assembly and corresponds to the pads of all surface mount components. Its dimensions are identical to those of the top and bottom layers and are designed to allow solder to flow through the stencil.
**3. Key points:** Both layers are essential for soldering, but this does not imply that one is for soldering and the other for green epoxy. The presence of a layer indicates the green epoxy area; if a specific region has this layer, it means that area is insulated with green epoxy. So far, I haven’t encountered such a layer! The PCB we designed has a solder layer on the pads by default, meaning the pads are made of silver-white solder. It’s not unusual for them to lack green epoxy. However, the wiring sections on the PCB we designed only include the top or bottom layer without a solder layer; yet, the wiring on the finished PCB is coated with a layer of green epoxy.
**4. This can be understood as follows:**
1. The solder mask layer opens a “window” in the overall solder mask, allowing for soldering.
2. By default, areas without a solder mask should be coated with green epoxy.
3. The paste mask layer is intended for surface mount packaging! The SMT package uses: top layer, top solder layer, and top paste layer; the top layer and top paste are the same size, while the top solder layer is slightly larger. The DIP package only utilizes the top solder and multi-layer layers (after some analysis, I found that the multi-layer layer actually overlaps in size with the top solder, bottom layer, top solder, and bottom solder layers). The top solder and bottom layers are slightly larger than the top and bottom layers.
**5. Question:** Is the statement “the copper skin layer corresponding to the solder layer is tinned or gold-plated only if there is copper” accurate?
This was mentioned by someone working in a PCB factory. Their point was that for the area designated on the solder layer to be tinned, the corresponding solder layer must have copper (i.e., the area aligned with the solder layer must include part of the top or bottom layer). Now, I conclude that “the copper skin layer corresponding to the solder layer must contain copper to be tinned or gold-plated” is indeed correct! The solder layer indicates the area that is not covered by green epoxy.
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While many process engineers prefer to separate all pad features on the board with solder mask, the pin spacing and pad size of fine-pitch components necessitate special considerations. Although solder mask openings or windows that are not fully enclosed on all four sides of the QFP may be acceptable, controlling solder bridges between component pins can become more challenging. For BGA solder masks, many companies offer designs that do not contact the pads but cover the areas between them to prevent solder bridging.
Most surface mount PCBs are coated with a solder mask; however, if the solder mask thickness exceeds 0.04 mm (“), it may impact the application of solder paste. Surface mount PCBs, particularly those with fine-pitch components, require a low-profile photosensitive solder mask.
**1. Solder mask:** The solder mask refers to the area of the PCB that is coated with green epoxy. As it is a negative output, the actual effect of the solder mask area is that it is not covered with green epoxy but is instead tinned, appearing silver-white.
**2. Soldering layer:** The paste mask is utilized for machine assembly and corresponds to the pads of all surface mount components. Its dimensions are identical to those of the top and bottom layers and are designed to allow solder to flow through the stencil.
**3. Key points:** Both layers are essential for soldering, but this does not imply that one is for soldering and the other for green epoxy. The presence of a layer indicates the green epoxy area; if a specific region has this layer, it means that area is insulated with green epoxy. So far, I haven’t encountered such a layer! The PCB we designed has a solder layer on the pads by default, meaning the pads are made of silver-white solder. It’s not unusual for them to lack green epoxy. However, the wiring sections on the PCB we designed only include the top or bottom layer without a solder layer; yet, the wiring on the finished PCB is coated with a layer of green epoxy.
**4. This can be understood as follows:**
1. The solder mask layer opens a “window” in the overall solder mask, allowing for soldering.
2. By default, areas without a solder mask should be coated with green epoxy.
3. The paste mask layer is intended for surface mount packaging! The SMT package uses: top layer, top solder layer, and top paste layer; the top layer and top paste are the same size, while the top solder layer is slightly larger. The DIP package only utilizes the top solder and multi-layer layers (after some analysis, I found that the multi-layer layer actually overlaps in size with the top solder, bottom layer, top solder, and bottom solder layers). The top solder and bottom layers are slightly larger than the top and bottom layers.
**5. Question:** Is the statement “the copper skin layer corresponding to the solder layer is tinned or gold-plated only if there is copper” accurate?
This was mentioned by someone working in a PCB factory. Their point was that for the area designated on the solder layer to be tinned, the corresponding solder layer must have copper (i.e., the area aligned with the solder layer must include part of the top or bottom layer). Now, I conclude that “the copper skin layer corresponding to the solder layer must contain copper to be tinned or gold-plated” is indeed correct! The solder layer indicates the area that is not covered by green epoxy.
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me
