Rigid-Flex PCB Manufacture Process

The drilling dirt level and thickness of Rigid-Flex PCB increase with the increase of temperature during drilling, and increase faster above the glass transition temperature of the resin. Therefore, some Rigid-Flex PCB manufacturers have tried freezing drilling to reduce the effect of drilling contamination by reducing the temperature of the processing board. The specific method is: first freeze the rigid-flexible printed board at low temperature (put it in the cold storage or refrigerator) for several hours, and then drill the hole under the condition of air-conditioning and heat preservation after taking it out. The hole drilled by this method has only a little bit of dirt, and the effect is very obvious.

It should also be noted that although we have done a lot of work in wet processing, punching OPE holes, and lamination alignment to ensure the accuracy of interlayer alignment, the polyimide material itself is exposed to moisture and heat. The impact is large, and uncertain interlayer deviation and inter-board deviation will inevitably occur. Therefore, before drilling holes, X-ray should be used to drill small holes to determine the different expansion and contraction amounts of different boards, and to calibrate the data with reference to the expansion and contraction amounts to ensure that the drilling is accurate and effective. At the same time, the offset is handed over to the engineering department to process the scaling of the film, and refer to the drawing of the outer film to ensure the accurate alignment of the outer graphics transfer.

Rigid-Flex PCB Manufacture Process

Rigid-Flex PCB de-drilling and etchback

The contamination in the holes of Rigid-Flex PCB is mainly polyimide resin, epoxy glass fiber, and epoxy resin. Flexible polyimide resin is inert to concentrated sulfuric acid solution, and will swell in strong alkaline potassium permanganate solution. Therefore, conventional wet decontamination is difficult to work. We have also tried to use concentrated sulfuric acid or alkaline potassium permanganate solution to decontamination, changing the concentration, temperature, treatment time and other parameters. Many tests have not received satisfactory results, so we gave up the traditional Wet chemical decontamination, using plasma method instead.

Plasma chemical processing system-plasma decontamination and etchback system, generally consists of five parts: vacuum chamber, vacuum pump, RF generator, microcomputer controller, and original gas. Different types of plasma processing equipment are only slightly different in the structure of the electrode in the vacuum chamber and the gas input position and manner. Plasma refers to the ionized gas, which is the state when atoms completely or partially lose their electronic layer under the action of the radio frequency energy generator. It is composed of ions, electrons, free radicals, free radicals, and ultraviolet radiation particles. It is electrically neutral and has high chemical activity. The biggest advantage of plasma decontamination is that it has no selectivity, that is, regardless of the resin type of the board being processed, it can be processed as long as the parameters are adjusted. For example, the high-activity plasma stream can quickly and uniformly remove the contamination caused by epoxy resin, polyimide, acrylic, glass fiber, etc. from the hole wall, and can form a certain amount of etchback. Effectively realize three-dimensional connection and improve the reliability of metallized holes.

     Plasma decontamination is generally divided into three steps:

    (1) After the equipment cavity reaches a certain degree of vacuum, inject high-purity nitrogen and high-purity oxygen into it in proportion. The main function is to clean the hole wall and preheat the printed board to make the polymer material have a certain activity, which is beneficial to Follow-up processing. Generally, it is 80°C for 10 minutes.

    (2) Use CF4, O2 and N2 as the original gas to react with the resin to achieve the purpose of decontamination and etchback, generally 85°C, 35 minutes.

    (3) Use O2 as the original gas to remove the residues or "dust" formed during the first two treatment steps and clean the hole walls.

Process parameters of plasma decontamination and etchback used by Rigid-Flex PCB

Process parameters (system pressure 280mTorr) The first stage The second stage The third stage

Percentage of CF4 gas (%) 0 25 0

O2 gas percentage (%) 0 80 100

Percentage of N2 gas (%) 100 0 0

Vacuum degree (mTorr) 110 110 110

RF power (Kw) 1.5 2.5 2.5

Processing time (min) 10 35 5

However, it is worth noting that when plasma is used to remove multi-layer flexible and Rigid-Flex PCB holes, the etch rate of various materials is different. The order from large to small is: acrylic film, epoxy resin, poly Imide, fiberglass and copper. It can be clearly seen from the microscope that there are protruding glass fiber heads and copper rings on the wall of the hole. In order to ensure that the electroless copper plating solution can fully contact the hole wall, so that the copper layer does not produce voids and cavities, the residue of the plasma reaction, the protruding glass fiber and the polyimide film on the hole wall must be removed. The treatment methods include chemical Method and mechanical method or a combination of both. The chemical method is to soak Rigid-Flex PCB with hydrogen fluoride amine solution, and then use ionic surfactant (KOH solution) to adjust the chargeability of the hole wall. Mechanical methods include high-pressure wet sandblasting and high-pressure water washing. The combination of chemical and mechanical methods has the best effect. The metallographic report shows that the state of the metallized pore walls after plasma decontamination is satisfactory.

Rigid-Flex PCB chemical copper plating, copper electroplating

It should be pointed out that the elongation rate of the electroplated copper layer is required to be greater than the thermal expansion rate of the rigid-flexible combination and Rigid-Flex PCB and has a higher tensile strength. When subjected to thermal shock, the total expansion rate of the Rigid-Flex multilayer PCB substrate is 1.65% larger than that of the copper-plated layer in the hole, while this index is only 0.03% in a rigid multilayer PCB. It can be seen that the tensile stress of the metalized holes in the Rigid-Flex PCB is much greater than that of the rigid multilayer board. At the same time, the thickness of the copper plating layer also has a certain impact on the reliability of Rigid-Flex PCB. Most Rigid-Flex multilayer PCB manufacturers rely on increasing the thickness of the copper layer of the hole wall to improve the reliability of the metallized hole.

Rigid-Flex PCB surface solder mask and solderability protective layer

Since the flexible board has a requirement for flexing during use, the polyimide protective film is usually used in the flexible window or the flexible part to protect the circuit by crimping, but for the precision circuit, the polyimide is in the cover type. It is difficult to meet the requirements on the window and the window, but it can be coated with solder mask ink. Ordinary solder mask ink is easy to be brittle and has no flexibility and cannot meet the requirements, so we can choose a screen printing flexible liquid photosensitive Development type solder mask ink, both can play the role of solder mask, moisture resistance, pollution prevention, mechanical deflection resistance, etc. Another method is to paste the development type flexible cover dry film, but the raw material price is higher and it needs Only the vacuum film machine can complete the coating well. The solderability protective layer uses an organic anti-oxidation protective film to ensure that the surface of the pad is flat and solderable.

Rigid-Flex PCB shape processing

Rigid-Flex PCB needs to be milled on the milling machine. The main attention should be paid to the flexible PCB part, because the flexible PCB part PCB is easy to be twisted and the milled shape is uneven and rough. Pads of the same thickness as the outer layer of the rigid PCB can be placed on the upper and lower sides of the flexible PCB window, and pressed tightly when milling the shape, to ensure a smooth and uniform shape edge. If the flexible window is not opened in advance, and the laser is used to remove the waste of the flexible PCB window at the end, the shape of the milled flexible PCB part will be more ideal, but not every stack can be laser-based .