2.2 Screen Printing Method after Plugging Via Holes with Aluminum Sheets
In this process, a CNC drilling machine is used to drill the aluminum sheets that need to be plugged, creating a screen plate that is then installed on a screen printing machine for the plugging process. It is important not to leave the plugging for more than 30 minutes after its completion. The process flow includes pretreatment, hole plugging, silk-screening, pre-baking, exposure, developing, and curing. This method ensures thorough coverage of the via hole with oil, creates a flat plug hole, and maintains consistent color of the wet film. However, it may result in poor solderability on the pad and foam and oil residue at the edge of the via hole after hot air leveling. Special processes and parameters are necessary to ensure the quality of the plug hole using this method.
2.3 Soldering Method after Plugging Via Holes with Aluminum Sheets
After plugging the holes with aluminum sheets, the board undergoes development, pre-curing, and grinding, followed by surface soldering. A CNC drilling machine is used to drill the aluminum sheets needed for plug holes, and a screen plate is made and installed on a shift screen printing machine for the plug holes process. It is important that the plug holes are fully filled with both sides preferably protruding. The process flow includes pre-treatment, hole plugging, pre-baking, development, pre-curing, and application of the board surface solder mask. The advantage of this method is that it ensures the via hole retains oil and does not expel oil after hot air leveling. However, it may not completely solve the issue of tin bead and tin on the via hole after HAL, which makes it unacceptable for some customers.
2.4 Simultaneous Completion of Solder Mask on the Board Surface and Plug Hole
This method utilizes a 36T (43T) screen mesh, which is installed on a screen printing machine along with a backing plate or a nail bed. It allows for the plugging of all via holes while completing the solder mask on the PCB board surface. The process flow includes pretreatment, screen printing, pre-baking, exposure, development, and curing. This method is time-efficient and maximizes equipment utilization. It ensures that the via holes do not lose oil and avoids tin on the via holes after hot air leveling. However, it may result in air expansion during the process, causing voids and unevenness in the solder mask. Additionally, there might be a small number of via holes with hidden tin during hot air leveling. Our company has successfully tackled these issues through extensive experimentation, including the use of different ink types and viscosities, and adjustments to silk screen printing pressure. This method is now implemented for mass production.