For SMT board, especially for BGA and IC pasted on the conduction requirements must level off, the plug hole convex concave plus or minus 1 mil, can not have guide hole edge on the red tin, guide hole is Tibetan beads, in order to meet customer requirements, conducting plug hole process is multifarious, exceptionally long process flow, process control difficult, often in hot air leveling and green oil resistance to soldering experiment off; After curing, oil explosion and other problems occur. According to the actual production conditions, various PCB plug hole processes are summarized, and the process and advantages and disadvantages are compared and elaborated:
Note: The working principle of hot air leveling is to use hot air to remove excess solder on the surface and hole of printed circuit board, and the remaining solder is evenly covered on the pad and open solder line and surface seal decoration, which is one of the surface treatment methods of printed circuit board.
I. Plug hole process after hot air leveling
The process flow: plate surface blocking welding →HAL→ plug hole → curing. Non-plug hole process is adopted for production. After hot air leveling, aluminum screen plate or ink screen is used to complete the plug holes of all fortress as required by customers. Plug hole ink can be sensitive ink or thermosetting ink, in order to ensure the consistency of wet film color, plug hole ink is best used with the same ink board. This process can ensure that hot air leveling after the through hole does not drop oil, but easy to cause plug hole ink pollution board surface, uneven. The customer is prone to virtual welding when using SMT machine to mount (especially in BGA). So many customers don’t accept this approach.
II. Hot air leveling before plug hole process
2.1 Graphic transfer is carried out after plug hole, solidification and grinding of aluminum sheet
This process process with CNC drilling machine, drilling out to plug hole aluminum sheet, made of screen, plug hole, ensure that the plug hole full plug hole, plug hole ink plug hole ink, also available thermosetting ink, its characteristics must be hardness, resin shrinkage change is small, and the hole wall binding force is good. The process flow is as follows: pretreatment → plug hole → grinding plate → graphic transfer → etching → plate surface resistance welding
By this method can guarantee the smooth conduction of plug hole, hot air leveling there would be no oil, the side of hole blasting off the quality problems such as oil, but the process requirement of disposable thickening copper, make this hole wall copper thickness meet customer standard, so the whole board high demand for copper plating, and also has a high requirement on the performance of the grinding machine, to ensure that the resin on the surface of the copper thoroughly remove, such as copper surface is clean and not contaminated. Many PCB plants do not have the one-time thickening copper process, and the performance of the equipment is not up to the requirements, resulting in this process is not used in PCB plants.
2.2 Block welding of screen printing board surface directly after plug hole of aluminum sheet
This process process uses NUMERICAL control drilling machine, drill out the aluminum sheet to plug hole, made into a screen version, installed on the screen printing machine plug hole, after the completion of the plug hole parking shall not exceed 30 minutes, with 36T screen screen direct screen printing board surface resistance welding, the process process is: pretreatment – plug hole – screen printing – pre baking – exposure – development – curing
With this process can ensure that the conduction hole cover oil is good, plug hole flat, wet film color is consistent, hot air leveling can ensure that the conduction hole is not tin, tin beads are not hidden in the hole, but easy to cause the ink pad in the hole after curing, resulting in poor solderability; After hot air leveling, the edge of the through-hole bubbles and drops oil. It is difficult to use this process for production control, and it is necessary for process engineers to adopt special processes and parameters to ensure the quality of the plug hole.
2.3 Aluminum plug hole, development, precuring, grinding plate surface welding.
With CNC drilling machine, drilling the aluminum sheet required plug hole, made into a screen, installed on the shift screen printing machine plug hole, plug hole must be full, both sides of the protruding is better, and then after curing, grinding plate surface treatment, the process is: pretreatment – plug hole a pre-drying – developing – pre-curing – surface welding
Since plug hole curing in this process can ensure that oil does not drop or burst through the hole after HAL, but tin beads hidden in the hole and tin on the through hole after HAL cannot be completely solved, so many customers do not accept it.
2.4 Block welding and plug hole are completed at the same time.
This method uses 36T (43T) screen, installed on the screen printing machine, the use of pad or nail bed, in the completion of the board at the same time, all the through hole plug, the process is: pretreatment – screen printing – pre – drying – exposure – development – curing.
The process time is short, high equipment utilization, can guarantee after hole out oil, hot air leveling guide hole is not on the tin, but because of using screen printing to plug hole, in the hole memory with a large number of air, when curing, air inflation, to break through resistance welding membrane, have holes, uneven, hot air leveling will guide hole is a small amount of tin.
Post time: Nov-16-2021