CO2 Laser Direct Copper Foil Processing Technology (I)

The high density of the PCB substrate depends on the micro-holes and lines connecting between the layers, and directly combines the performance of the electronic products. Therefore, the drilling technology of the PCB substrate becomes one of the key technologies for manufacturing the PC8 substrate. Portable products such as Notebook computers and mobile phones are the first high-tech consumer housing products based on BUILD UP substrates. With the increasing demand for low cost, high finished product yield, and high energy demand, direct copper foil processing can compensate for the shortcomings of current laser processing methods and has begun to be valued.

1. The status and topics of laser drilling

Low cost, high finished product yield, high productivity

Most of the laser processing methods used in the initial stage are the common conformal mask methods currently used for everyone (on the surface copper foils of RCC substrates, the required processing aperture (open copper window) is chemically etched, and then The laser beam is larger than the 50- to 100-μm laser beam opening of the copper window, and it is also more suitable for the existing process; however, the burst (continue pulse) process can easily become an arc-shaped wall, so the cycle is generally used ( Cycle Pulse) is more suitable for processing.

Because of such factors, the processing speed becomes slower. The large window method (chemical etching of copper windows larger than the processing aperture of 100 μm on the surface copper foil of RCC substrate) was developed to solve the problem of the high speed of the conformal mask method.

Because it is applicable to high-speed burst processing, it has now become the mainstream of production and is widely used. Since the packaging substrate is processed directly on a pure resin material by high-speed burst processing, advanced techniques such as surface treatment before plating are required and management is also limited to some unique substrate manufacturers.

BH (blind hole) drilling of almost all buildup substrates is made using CO2 lasers. The general purpose build-up substrate of RCC material has a pore size of about 100 to 125 μm, and the package-related substrate The hole diameter is about 100 μm or less, and recently it is required to have a level of about 60 μm. The suitability of the recent FR-4 material has also continued to increase. New application requirements such as through-hole TH (through-hole) processing have also increased.

According to the CO2 laser test level, the hole diameter can be drilled to 30μm, and the practical limit is between 4O and 50um. During the build up of the substrate, the inherent deformation and shrinkage of the substrate will increase, and the offset between the etching window and the inner Pad will increase. Therefore, as long as the diameter of the inner Pad becomes smaller, the yield will decrease, so it will be limited by a very small line width/line spacing.

The above-mentioned window etching is the same process as the circuit that requires photoresist coating, exposure, development, and etching to form a circuit, so the process cost is another subject.

2. Characteristics of Cu Direct Processing

The Cu direct processing method is to improve the laser absorption rate after surface treatment with a thin copper foil of 5 to 9 μm, and to directly form BH using a CO laser.

First, the surface copper foil was removed by CO laser to expose the alignment point on the inner layer, and the position and automatic correction were confirmed by CCD CAMERA. Because drilling is done with the inner alignment reference, the deviation of the hole position is smaller and can have higher accuracy. Therefore smaller Pad and fine Pitch are possible. It is also not necessary to make the crack of the copper window, and the etching process can be omitted, so the cost of the process can be reduced. Electroplating and line formation also apply to the processes of the Conformal MASK process and the LARGE WlNDOW process.

And because 2 PULSE processing is possible, the processing speed can be very fast. In addition, Hitachi has also developed the high PEAK power, the short PULSE MODULATION STEPPULSE and the TOP HAT BEAM, and has the same hole quality as the current machining method.

3.Cu direct topics such as engineering method

The copper surface of the shiny surface has a low CO laser absorption (2% to 3%), so that it is difficult to form a practical blind hole in the build-up of a surface copper foil conductor layer of 12 to 18 m. This is also the case, it is necessary to use about 10 times the energy of the insulating layer in order to decompose and remove the surface copper foil. After the surface copper foil penetrates, excess energy is absorbed by the insulating layer and is formed under the surface of the copper foil at the entrance of the hole to form an undercut. In addition, because the strength of Peeling of the inner layer copper foil conductor layer is increased, the surface of the inner layer copper foil is roughened, and the laser absorption rate becomes higher and the temperature greatly increases, so that the inner layer copper foil conductor layer is penetrated.

Thinned surface copper foil and surface treatment to improve the laser absorption rate, use about the decomposition of the energy of the insulating layer, the use of CO2 laser can also be blind holes in the surface of the copper foil and insulation layer.

In addition, to ensure the reliability of processing, Hitachi has developed the applicable TOP HAT BEAM and MODULATION STEP PULSE functions.

(to be continued)