Introduction:
Miniaturization and multi-function of products are the main driving force for the development of electronic industry. Printed circuit board is the most common electronic assembly platform. These include rigid or flexible polymer substrates and ceramic substrates for metal wiring. Typically, various components and subsystems are mounted on the board surface. However, in order to reduce the number of components on the surface of the printed circuit board, researchers are constantly studying how to make better use of the spatial layout of the PCB, so as to embed the components inside the printed circuit board.
Advantages of Embedding Components into PCBs:
1) Short line length and simplified layout.
2) Due to the reduction of soldering points, the system reliability is improved.
3) Due to the reduction of parasitic effects, the electrical properties are more stable, especially in the high frequency field.
4) Embedded components cost less than separated components.
At present, various technologies for embedded components in PCB are developing rapidly, but no one technology occupies an absolute leading position.
Embedding of Passive Components into Flex-PCBs:
Embedded passive components mainly include embedded capacitance and embedded resistance. At present, many schemes have been developed to embed capacitors and resistors. For capacitors, the upper and lower electrodes can be etched on the dielectric diaphragm with copper covering on both sides, and embedded capacitors can be obtained by layering on it. The embedded capacitance can also be obtained by using paste or sputtering film to form dielectric film and electrode.
For resistance, polymer resistance paste can be printed on copper foil electrode. It can also electroplate the resistance film between the copper foil and the substrate, and then etch the copper foil and the resistance film respectively to form the line and resistance. In addition, the copper foil is first selected on the location, printing ceramic thick film slurry, high temperature sintering to form capacitance and resistance, and then through the lamination to form embedded elements.
The appeal technology is mainly for rigid PCB, for flexible PCB, its special is the bending of embedded components put forward new requirements. The following article will introduce a new method of embedding film resistance in flexible PCB.
Embedding Resistance of Electroless Plating Ni:
Electroless plating of Ni films with a thickness of less than 100nm on the printed circuit board to form a resistive configuration is commercially feasible.
In this way, we are able to produce resistors with a resistance value of 25-50Ω. Due to the manufacturing process of flexible substrates, after the copper on the surface is etched away, the underlying PI shows a spongy surface. Since the thickness of Ni film is less than 100nm, the surface morphology of other areas before and after nickel plating is relatively similar, except for some areas where Ni overgrows to form clusters.
Film Resistance of the Performance Test:
For the convenience of testing, we designed a special test circuit diagram, each circuit board contains 400 test resistors. The process was also changed by depositing a nickel `film on the entire surface of the circuit board and then cutting the resistance pattern with a laser.
Conclusion:
The results show that the electroless nickel plating process can be used for flexible circuit boards. Although the bending capacity of nickel film is limited, it can completely meet the requirements of reel-to-reel process.
