For surface mount boards, especially for BGA and IC mounting, the via hole must be flat, positive and negative 1 mil,
no red tin on the edge of the via hole; tin beads in the via hole, in order to reach the customer The requirements of the through hole plug hole process can be described as various, the process is particularly long, the process control is difficult,
often in the hot air leveling and green oil solder resistance test oil drop; the problem of oil explosion after curing occurs.
According to the actual conditions of production, the various plugging processes of PCB are summarized,
and some comparisons and explanations are made in the process, advantages and disadvantages:
Note: The working principle of hot air leveling is to remove excess solder from the surface of the printed circuit board and the holes by hot air. The remaining solder is evenly covered on the pads and the unresisted solder lines and surface package points,
which is the way of processing the surface of the printed circuit board. one.
- Hot air leveling and post-hole hole process
The process flow is plate surface solder mask → HAL → plug hole → solidification. The production is carried out by
a non-plugging process. After the hot air is leveled, the aluminum mesh screen or the ink-blocking net is used to complete
the via hole of all the plugs required by the customer. The plug ink can be made of photosensitive ink or thermosetting ink.
In the case of ensuring the same color of the wet film, the plugging ink preferably uses the same ink as the board surface.
This process can ensure that the through hole does not drop oil after the hot air is leveled, but it is easy to cause the plug hole ink to pollute the surface and uneven. Customers are prone to soldering during placement (especially within the BGA).
So many customers do not accept this method.
Second, the hot air leveling front plug hole process
2.1 Using the aluminum sheet to plug holes, solidify and grind the board for pattern transfer.
This process uses a CNC drilling machine to drill the aluminum sheet of the plug hole to make a screen,
and to make the plug hole, to ensure that the plug hole is full, plug hole The ink plug hole ink can also be used as a thermosetting ink, and its characteristics must be high in hardness, small in shrinkage of the resin, and good in adhesion to the whole wall.
The process flow is: pre-treatment → plug hole → grinding plate → pattern transfer → etching → plate surface solder mask.
This method can ensure that the through hole of the through hole is flat, the hot air leveling will not have quality problems such
as oil explosion and oil drop at the hole, but this process requires thickening copper at one time so
that the copper thickness of the hole wall reaches the customer’s standard. Therefore, the whole plate copper plating is very demanding, and the performance of the plate grinding machine is also very high, ensuring that the resin on the copper surface is completely removed, the copper surface is clean and not polluted. Many PCB factories do not have a one-time thick copper process,
and the performance of the equipment does not meet the requirements, resulting in the use of this process in the PCB factory is not much.
2.2 Direct screen printing surface soldering after plugging with aluminum
This process uses a CNC drilling machine to drill the aluminum sheet of the plug hole to make a screen.
It is installed on the screen printing machine to carry the plug hole. After the plug hole is completed,
it should not be parked for more than 30 minutes. The screen is directly screen printed with a 36T screen.
The process is: pre-treatment – plug hole – silk screen – pre-bake – exposure – development – curing
This process can ensure that the through-hole cover oil is good, the plug hole is flat, the wet film color is consistent,
the hot air leveling can ensure that the conduction hole is not tinned, the tin beads are not hidden in the hole,
but it is easy to cause the ink in the hole after curing The pad causes poor solderability; after the hot air is leveled,
the edge of the via hole is foamed and oil is lost. It is difficult to use this process for production control,
and the process engineer must use special procedures and parameters to ensure plug quality.
2.3 Aluminum plate plug hole, development, pre-curing, after the plate is polished.
Using a CNC drilling machine, the aluminum piece required to be plugged is drilled to make a screen,
which is installed on the shift screen printing machine to make the plug hole, the plug hole must be full,
the two sides protrude better, and then the solidified plate is processed for the surface treatment. The process is: pre-treatment – plug hole pre-bake – development – pre-cure – surface solder mask
Since the process uses plug hole curing to ensure that the HAL does not drop oil and oil after the HAL, after the HAL, the tin in the via hole and the tin on the via hole are difficult to completely solve, so many customers do not receive it.
2.4 Plate surface soldering and plugging are completed at the same time.
This method uses a 36T (43T) screen, installed on the screen printing machine, using a pad or a nail bed, and plugs all the through holes while completing the board surface. The process flow is: pre-treatment – silk screen – -Prebaking–exposure–development–curing
The process time is short, the utilization rate of the equipment is high, the hole can be ensured that the hot air is not drained, and the via hole is not tinned. However, since the screen hole is used for plugging, a large amount of air is stored in the via hole, and when solidified, The air expands and breaks through the solder mask, causing voids and unevenness. The hot air leveling will have a small number of conductive vias. At present, after a large number of experiments, our company chooses different types of inks and viscosity, adjusts the pressure of silkscreen, etc., basically solves the voids and irregularities of the via holes, and has been mass-produced by this process.
Alice Lu,
Email: sales16@andwinpcb.com