According to The Gas Review, dry etching is being used increasingly in manufacturing processes for mounting semiconductors and printed-circuit boards (PCBs) used in electronic devices.

When creating multilayer flexible PCBs using polyimides, the previous wet etching process adversely affects the materials, causing faults, such as insufficient continuity in layer connections. Recent demands on these multilayer PCBs include reducing size and weight for usage in cellular phones and other consumer products. Usage in the popular advanced driver-assistance systems (ADAS’s) and other on-board applications is also being considered, and manufacturing is expected to continue increasing.

PCBs found in all sorts of electric devices

PCBs provide electronic circuit to a wide range of fields from consumer products, such as cellular phones and TVs , to precision machinery, such as automobiles and rockets. For the mounted semiconductor packages, they perform an important function by making electrical connections between components.

PCBs come in three types: rigid circuit boards, flexible circuit boards, and flex-rigid circuit boards (which combine the first two types). Applications depend on board characteristics. For example, rigid circuit boards are used where reliability is required, such as in automobile engine controls, and flexible or flex-rigid circuit boards are used where downsizing and lighter weights are required, such as in consumer products like cellular phones. Of these, mixed carbon tetrafluoride (CF4) and oxygen (O2) is used in manufacturing flexible or flex-rigid circuit boards.

Dry etching required for polyimide multilayer construction

CMK uses CF4 and other gases in manufacturing in what they call “rigid-flex circuit boards”. They are build-up circuit boards which are produced by building up rigid circuit boards and flexible circuit boards as conductor and insulation layers. CMK has a maximum monthly production capacity of 6,000m2. Niigata Factory is the main centre for production. CF4 is required because of the polyimide material that is used in multilayer flexible circuit boards.

A polyimide material forms the thin, soft base film that provides insulation. Using it in a PCB enables much thinner boards than is possible with rigid boards. They can be bent freely and contribute to making electronic devices both smaller and lighter.

Koichi Niisato, Chief Engineer of the Exhibition Planning & Management section in the Technical Development division at CMK’s G Station Factory, said for production of multilayer circuit boards, the introduction of dry etching with CF4 is progressing in manufacturing. He explained the reason for this, “When boring holes in rigid circuit boards with drills, carbon dioxide gas lasers, or UV lasers, the resin smear that remains can be completely washed away with the alkaline permanganate reagent used in wet etching. With polyimide materials, however, use of the same etching method results in deterioration and interferes with smear removal. If the next process is performed in that state, there is a risk of insufficient continuity between the layers of plating not adhering properly and of other problems.”

Therefore, when manufacturing multilayer flexible PCBs, a mixture of CF4 and O2 is used to remove the smear residue with dry etching.

The use of dry etching to move smear was established more than 20 years ago. The residual resin is removed by placing the circuit boards into a vacuum chamber and using the mixture of CF4 and O2 to generate plasma.

CMK has several dry etching machines. They use up to 400 litres of CF4 and up to 6,000 litres of O2 annually. The gases are procured as packaged gases from major gas manufacturers and other sources. Exhaust CF4 gas treatment is performed with the wet process because o the limited amount that is used.

Use of flexible PCBs in automotive electrification drives demand

CMK is placing manufacturing emphasis on automotive applications. The automobile industry is one of the largest users of PCBs, and now automated driving has driven this market even further. Of the PCBs manufactured by CMK, last year’s figures show that 70% of PCB applications were in automobiles.

However, most of that was in engine control units (ECUs) and brake controls, where the highly reliable rigid circuit boards are used. CMK’s Manager Katsutumo Nikaido explained, “The use of PCBs in automotive engines is ‘old technology’ that is already established, and from the perspective of automotive applications, its reliability is not questioned. From that perspective, flexible PCBs still are in need of building-up a track record of reliability in consumer products and other devices.”

On the other hand, the spread of ADAS’s will accelerate increasing manufacturing of multilayer flexible PCBs, including the rigid-flex PCBs made by CMK. ADAS’s use cameras to analyse the surrounding environment, determine the distance to the car in front with millimetre wave radars and implement other methods to aid drivers and thereby increase driving safety. This technology is indispensable to the practical application of automated driving and it will greatly increase the number of sensors installed on automobiles.

CMK already takes orders from several large automobile manufacturers for rigid-flex PCBs for automotive camera modules, and they expanded production last year. Nikaido said, “Flexible PCBs are the only product in the industry that continues to grow. Based on that, I believe that usage of gas will continue to grow proportionately as well.”

Increased demand for exhaust gas treatment systems raises expectations of related equipment manufacturers

Increased application of dry etching will bring benefits to gas equipment as well. Of these, both dry and wet methods are expected to become the main methods for exhaust gas treatment systems, and the introduction of equipment for CF4 seems to progressing.

One of the main manufacturers of dry processing systems said that they felt that there is a real need for equipment. “Business has been increasing from two to three years ago and introduction is advancing in production centres overseas as well. Each factory introduced about five systems.” In terms of processing trends, they expect increased demand in the future. “More than just properly processing fluorocarbon-based gases, the feeling is strong that hazardous gases like hydrogen fluoride just cannot be released into the environment.”