Flex-Rigid PCB Technology
What is Flex-Rigid PCB?
Flex Printed circuit and Printed circuit board gave the birth of new products Flex-Rigid PCB. It contains one or more rigid zones and one or more flexible zones on a printed circuit board, they through pressing and other processes formed circuit board with FPC characteristics and rigid PCB characteristics.
Flex-Rigid PCB fabricate process chart
1. Material selection
The DuPont copper clad material flexible plate (AP adhesive series) flexible polyimide substrate, polyimide is a kind of very good flexibility, excellent electrical and thermal properties of the material, but it is hygroscopic and larger not alkali resistant. For rigid plates, the rigid material of the PI resin series can be pressed together with P95 base material, and the warping deformation of the composite resin system can be avoided when the rigid-flexible plate is pressed together.
The adhesive between the deflection plate and the hard plate is best to use the No flow (low flow) Prepreg for pressing, because of its small mobility of soft and hard plastic transition regions are of great help, will not cause the transition zone to be reprocessed or cause the function of affected due to overflow. The outer protective material pattern, also is the solder layer, there are three kinds of choices, the first is the traditional film (Coverlay), is a selection of polyimide material plus adhesive directly with the circuit board etching after protection to laminated pressed, the covering film for pressing performing a welding part, therefore, can not meet the assembly of more detailed requirements, second kinds of photosensitive dry film developing type cover, with film machine after pressing, the leakage of sensitive developing welding part, to solve the problem of fine assembly, the third category is the liquid screen printing type covering material commonly used are thermosetting polyimide materials, such as the sun and the PSR-4000 type flexible circuit board for developing photosensitive solder resist ink, this kind of material can satisfy the flexible plate of fine pitch and high-density assembly requirements.
Pattern transfer occupies a very important position in the printed circuit board with high density, fine lines, flexible lines, especially. Because the flexible single chip is thin and soft, it brings great difficulties to the surface treatment, and the cleaning condition and roughness of the copper foil surface directly affect the adhesion of the dry film and the manufacture of fine lines. The mechanical cleaning plate has high requirements for equipment, and not suitable for pressure may cause the substrate deformation, folding and telescopic size, the operation is not easy to control, so we can choose to use the electrolytic cleaning method.
This method can not only ensure the surface cleanness but also use the micro-etching method to ensure the roughness of the copper surface, which is beneficial to the fabrication of 0.1mm~0.15mm line width/spacing graphics. In addition to paying attention to the control of acid etching the etch rate to ensure that the design requirements of the width, spacing, more attention should be paid to prevent the single curl, fold, is the best guide plate and auxiliary and close the exhaust system on the device.
Poor dimensional stability and flexible substrate materials, this is because of the polyimide material moisture absorption strong, after wet processing or shrinkage in different temperature and humidity in the environment of severe deformation, caused by the difficulties of multilayer laminated alignment.
In order to overcome the difficulties, can adopt the following measures: to be considered in the design of design and drawing the spot target alignment spots, in order to guarantee the punching hole or the rivet hole alignment accuracy, without deviation caused by interlayer graphics in the pack the lead scrap. The positioning hole after punching by OPE can eliminate the error caused by material stretching and deformation during wet treatment. After lamination, drill holes with X-ray to determine offset and make drilling more accurate.
In view of the material characteristics and environmental characteristics of polyimide, drawing the outer film with reference to the offset of drilling hole, so as to improve the coincidence degree between the outer plate and the grilling plate. In this way, we can satisfy the requirement of interlayer alignment to guarantee the 0.1mm~0.15mm ring width and guarantee the accuracy of outer pattern transfer.
Even punching location by OPE, before the treatment of laminated monolithic interlayer contraposition, also has a great impact. First, because of the polyimide material, not alkali resistant, swelling in an alkali solution, so in the process of treatment of black, brown, in strong alkaline processes such as oil, black, brown and other appropriate to reduce the temperature and reduce the time.
The method is feasible because the adhesive layer is used without considering the change of the bonding layer in the alkali solution. Secondly, the monolithic baking after oxidation should be avoided vertically, and the horizontal baking method should be adopted to reduce the bending deformation and keep it as smooth as possible. After baking as far as possible to shorten the molding time, moisture absorption and prevent the single again.
Because the flexible monolithic laminate before deformation, poor planeness, coupled with the adhesive sheet of the resin flow is much lower than that of the prepreg, laminated with rigid plate so as to make the adhesive sheet and a good combination and embedding fine line spacing, we chose to use the cover form a good material as laminated gasket materials, such as polypropylene film, polytetrafluoroethylene (PTFE), silicon rubber sheet, can improve the quality of laminated flexible plate. After the test, it is considered that the ideal gasket material is silicone rubber, which can ensure the shape of the gasket and reduce the shrinkage deformation of the pressing part.
The structure of the hard and soft composite plate is complex, so it is very important to determine the optimum technological parameters of drilling hole to obtain good hole wall. In order to prevent the inner ring of copper and the nail head of the flexible base material, we should choose the sharp bit first. If the number of processed printed boards is large or the number of holes in the processing plate is large, the drill bit must be changed after drilling a certain number of holes.
Bit speed and feed are the most important process parameters. When the feed is too slow, the temperature rises sharply, causing a lot of drilling. The feed is too fast, it is easy to break the drill bit, bond sheet, as well as the dielectric layer of the tear and nailhead phenomenon.
It should be pointed out that the ductility of the copper plating layer is higher than that of rigid-flexible bonding and flexible multilayer printed board and has higher tensile strength. When heating shock, the total expansion rate of rigid-flexible composite printed board substrate is 1.65% larger than that of the copper layer in the hole, and this index is only 0.03% in rigid multilayer plate.
Thus it can be seen that the tensile stress of the metal hole in the rigid-flexible combined PCB is much larger than that of the rigid multilayer plate. At the same time, the thickness of the copper plating layer also has a certain influence on the reliability of the rigid-flexible printed circuit board. Most rigid and flexible laminates increase the reliability of the metalized hole by increasing the thickness of the copper layer on the whole wall.