Study on Welding of PCB-FPC Composite Plates
1 Introduction
Printed circuit boards (PCBs) and flexible printed circuits (FPCs) have been widely used in many electronic products due to their light weight, small size, low cost, good electrical properties and wide compatibility. In order to simplify and improve the design, the surface mount technology (SMT) technique is used to increase device density and minimize circuit size, then the FPC-PCB composite plate technology have been applied.
The above FPC-PCB composite board is composed of flexible circuit board and rigid board, which has both the flexibility of FPC and the rigidity of PCB. Such layers are usually stacked together. Before welding process, FPC-PCB welding methods are used to connect the flexible circuits and maximize the utilization of the surface mount technology.
The loss of yield rate will increase if the welding method is improper. This article will consult some design guidelines and analyze the welding process parameters and technology of FPC-PCB composite plates, so as to provide reference for the implementation of FPC-PCB welding and improve the welding process of electronic equipment.
2 FPC-PCB Edge Connecting Technology
FPC-PCB edge connecting technology includes the following three steps:
(1) The FPC must go through surface gluing process. Firstly, select the appropriate electrical adhesive for the surface of FPC to ensure a good connection with the body.
(2) Use a male-female connector to put the FPC firmly in the PCB slot and connect the edge of the FPC to the top of the connector at the same time, to ensure a good connection between the FPC and the PCB.
(3) Use a taping machine to press the mechanical joints of the FPC-PCB assembly, so that the FPC and the PCB can be stably connected with each other.
3 Welding Process Analysis
The welding of FPC-PCB composite boards is generally divided into two steps: firstly, the welding of flexible circuits with PCBs through metal-to-metal contact or direct plating; secondly, the welding of metalized or vacuum-deposited thin film to the PCB by manual or machine.
3.1 Metal - to -Metal Contact Welding
The metal-to-metal contact welding is to contact the metal region of the FPC with the metal region of the PCB and then weld them together. The metal region of the flexible circuit board can be processed into the form of terminal pins by using the punching process, and corresponded to the metal region of the PCB. The metal-to-metal contact welding suits for secure welding and is used for connecting the composite SoC, module, chip and other high-density sheet packages to the PCB.
3.2 Direct Plating Welding
The direct plating welding is to plating the metal region of the FPC directly on the metal region of the PCB and then weld them together. This method is suitable for the surface-mount composed device which has large area or rough surface. The metalization of the flexible circuit board before welding and the directly plating of the metal layer onto the PCB use the thin film technology, and the resulting interface is strong and the micro-cracks are few.
3.3 Manual Welding
During the manual welding process, it is necessary to manually type the solder paste onto the welding points of the FPC-PCB combination, and then heat it with a soldering iron until the solder melts. The key problem of manual welding process is the correct alignment of the FPC to the PCB land and good deposition of theSolderingpaste.
4 Welding Process Parameter Analysis
4.1 FPC Layer Thickness
The surface mount technology requires that the FPC layer thickness must be from 0.08mm to 0.3mm. The thicker the FPC thickness is, the harder it is to be inside the plug-in box of the board, so it is not suitable for use in the surface mounting of some relatively small electronic components with dense pads. The thinner the FPC the easier its to be inserted into the plug-in box of the board, so the thinner FPCs are more suitable for the connector box of the board.
4.2 FPC Copper Thickness
FPC copper thickness is an important parameter for the welding of FPC-PCB composite plates, and it directly affects the strength of the joint between the FPC and the PCB, as well as its electrical performance. Generally speaking, the current carrying parts of the FPC should have the maximum copper thickness, which should reach and basically not exceed 1oz. If it exceeds 1oz, thermal cracking is likely to occur during the reflow process.
5 Conclusion
This paper consults some design guidelines, and analyzes the welding process parameters and technologies of FPC-PCB composite boards. Through this article, some relevant information can be provided to the implementation of FPC-PCB welding, so that the design and production of electronic devices can be improved, and the yield rate can be optimized.
