Process control of Castellated/Plated Half Holes PCB

Abstract: With the continuous development of electronic products, customer with more diverse requirements on PCB edge plated half-hole. While the quality of edge PCB plated half hole affects assembly and use directly. This article introduce processing techniques and control methods through second profiling, second drilling to make plated half-hole on the PCB edge.

Key words: Plated Half Hole pcb,second profile; second drilling; plated half-hole PCB, circuit boards edge half PTH,Edge-plated PCB,Edge PCB board with half through holes

1. Introduction:
It has been already mature technology to do PCB boards with PTH half-Moon Through Hole
in the edge, but it is still a challenge at how to control the quality for PCB edge plated half-hole after profiling, such as hole wall copper thorn cocked, residual. Those kinds of PCB with a whole row of plated half-hole on the edge usually characterized by relatively small individuals, used for PCB onboard, as sub-PCB for mother board. ThoseThrough Plated Half-Hole used to connect with mother board and components pins. If these semi-plated hole with residual copper, there may cause unsteady soldering, insufficient solder or even short between two foots when plug-in in PCB assembly manufacturing. This paper focuses on the problem encounter on production of PCB edge half PTH hole and the way to control.

2. Processing principle:
Either drilling or milling process, the rotation direction of SPINDLE is clockwise. When the machining tool reach point A, the hole wall copper layer of point A are closely linked with base layer, which avoid hole wall separated from the copper layer when extension, and the same time make assure no copper crooked and residue. While machining tool reach point B, because of copper attached to the hole wall, with no support of A schematic diagram, the tool forward running, the copper layer will curl with the cutter rotating under the external force to produce copper stab tilt, residues.

3. The types of half-hole PCB circuits

4.Processing problems & improving methods:
Gernally, there are 4 types half hole printed circuits, which suitable for two ways:
4.1 Type one and type two are large plated half-hole spacing electronic circuits, second milling on the positive and negative:
Due to half hole spacing of these two types castellated circuits board is bigger than 3mm, first milling process could complete CS layer half hole plating, and then the second milling process will complete SS side plating. In the positive and negative processing, PCB board edge with two different separate stress points. The finished half-plated-hole is smooth and tidy by this way. This way is also relatively simple and easy to guarantee the quality.
4.2 Type three and type four Edge Plated PCB are with the diameter of half hole less than 0.8mm, and spacing of the center between two holes is about 1mm , and the space of two adjacent rows no bigger than 2.5mm. For the processing of these types printed circuit pcb, we need to consider space of each half-hole and part of two sides. If it encounters following problems, there may appear micro connection problem.
4.2.1 Problem analysis and improvement measures:
① Design :  In the original design, half-palted hole space is only 0.56mm, and pad clearance of each half hole is only 0.15mm
In the producing process, the cutter cuts hole edge directly, when the cutter wearing, the pads may flanging. At the same time, the pad space is too small, which will cause failure micro-connection of pads.
Improved design: change the two rows cooper connection as hole ring and increase 0.05mm to space of half-hole pad pitch. And in order to ensure the width of half plated hole the same time, we do the following improvement: reduce pads width 0.025mm only for Half-Moon Through Hole pad parts close to (B position), keep the width of rest of the pad, which could ensure the pad pitch increase to 0.20mm (C position).
② Support PCB : As the half hole is drilled on the PTH edge, the half hole of the bottom circuit board may with hole flanging if there is no enough support when the drill bit drilling. So the bottom PCB board using in the second drilling couldn’t be ignored. It must be changed after each second drilling to make sure enough support for drilling and reduce the copper side flips.
③ Choose the drill: Normal drill bit may appear biased drilling, break when drilling the PTH hole, slot drill is more suitable for processing such circuit pcb.
④ If the second drilling made as the whole PCB board processing, there may be irregular harmomegathus, which will lead to inconsistent for each array.

Improvement measures:

In order to avoid irregular harmomegathus in the production lead to the inconsistency of each array after second drilling, the producing process flow nee to adjust:
Original process: Sawing →inner layer →lamination→drilling →PTH→plating→outer layer →soldermask → silkscreen→ENIG→second drilling→profile
Improved process: Sawing→inner layer →lamination→drilling →PTH→ plating→outter layer→solder mask →silkscreen→ENIG→first machining (outer profile)→second drilling→second machining(inner tank )
If using a single Plated Through Half Hole PCB board processing secondary drilling and secondary machining after process changes, the operation time for putting up and down PCB board will increase. In order to reduce operation time, we use second drilling combination panel, that is design it as single PCB compensation, repanelizing according to the requirement ,every time to put up PCB is still the original imposition PCB board. Because it is single PCB board compensated output data , it could avoid inconsistency shown in Figure 7 due to printed pcb material irregular harmomegathus.

4.2.2 Result of implementation:

The quality of half plating hole achieve desired level through improvement in design principle, manufacturing processes and optimization of processing methods , which assure the final quality of half PTH hole circuit PCB. And the four corners and the middle of whole board circuit keep the same.

5. Conclusion

(1) Through engineering design optimization and spacing pads increasing, pads micro flange connection between the pads was eliminated.

(2) Though PCB the frequency of bottom PCB usage, copper edge flips is prevented.

(3) Through improving the process, panel way, PCB board inconsistent due to PCB material irregular harmomegathus is solved

In actual manufacturing, we need to make overall analysis and improvement from the 4M1E perspective to make sure to improve plated half-hole processing technology  and promote overall quality finally .