Reasons of Solder Mask Bridge Breaking Off
The solder resist between the SMD pads is called solder mask bridge (solder mask dam), which serves to prevent bridging and short circuit when welding. Sometimes the solder mask bridge breaking off happens in many printed circuit board companies, greatly affecting the product quality and production efficiency. With the rapid development of PCB industry, the requirements of customers are getting higher and higher. It is time to improve the processing capacity of the solder resist bridge. What are the factors affecting the solder mask bridge breaking off?
The main reason of solder mask bridge breaking off is that the intensity of UV decreases rapidly with transmission depth of solder mask ink, and when the ink is more than a certain thickness, it will not be fully photo-polymerized, so undercut and solder mask bridge breaking off happen when developing. Meantime, the exposure energy, developing time, precure time and so on will have an influence on breaking off solder mask bridge.
Soldermask Bridge Breaking Off
Exposure Energy
With the increase of the thickness of the solder resist under the same exposure degree, the undercut of the solder resist bridge increases. Solder mask bridge breaking off also appears, and the soldermask bridge production capacity decreases. When exposed to ultraviolet light, part of the UV is absorbed by the liquid photosensitive solder resist ink. Due to the absorption the intensity of UV decreases gradually, so when the UV light reaches the bottom of the ink, its energy is very weak. Therefore, when the ink reaches a certain thickness, the bottom of the resin will not be in complete photo-polymerization status, resulting in undercut while developing.
When the bottom ink is not fully photo-polymerized, increasing the exposure energy can result in better photo-polymerization of the bottom ink, so that undercut reduces. But it is difficult to eliminate. And in the actual production of the PCB circuit boards the exposure energy cannot be unlimited increased.
Developing Time
When the soldermask is not thick, such as 25-50um, the developing time has less impact on undercut of solder mask bridge; When the soldermask is thick, such as 70-75um, the developing time has more impact on undercut of solder mask bridge. With the decrease of developing time, the undercut of solder resist decreases obviously.
Precure Time
When the soldermask is not thick, such as 25-50um, the precure time has less impact on undercut of solder mask bridge; When the soldermask is thick, such as 80-85um, the precure time has more impact on undercut of solder mask bridge. With the increase of precure time, the width of solder resists increases, so the undercut decreases.
Other Factors
Other factors, such as precure temperature, developing pressure, developer potency and developing temperature will also influence the undercut and solder mask bridge breaking off.
Conclusions:
1. When the ink reaches a certain thickness, it will not be fully photo-polymerized, because the intensity of UV decreases rapidly with transmission depth of solder mask ink, so undercut and solder mask bridge breaking off happen when developing.
2. Increase exposure energy can reduce undercut.
3. Developing Time, precure time, precure temperature, developing pressure, developer potency and developing temperature will also influence the undercut and solder mask bridge breaking off.
