PCB Technical - Overview of the distribution of SMT flexible printed boards

With the development of electronic products in the direction of short, small, light and thin, correspondingly, the integration and miniaturization of electronic components are required. Traditional through-hole mounting technology (THT) can no longer meet the requirements, and a new generation of SMT mounting technology, namely surface mount technology (SMT), has emerged.

In a broad sense, SMT includes surface mount components (SMC: Surface Mount Component), surface mount devices (SMD: Surface Mount Device), surface mount printed circuit boards (SMB: Surface Mount Printed Circuit Board), The general term for a complete set of PCB technology content such as ordinary mixed printed circuit board (pcb: Printed Circuit Board), dispensing, paste coating, surface mount equipment, component pick-and-place system, soldering and online testing.

Because SMC and SMD reduce the influence of lead distribution characteristics, and the PCB surface is firmly soldered, the parasitic capacitance and parasitic inductance between leads are greatly reduced. To a large extent, electromagnetic interference and radio frequency interference are reduced, and high-frequency characteristics are improved. Such components have been widely used in satellite communications products: for example, low-noise down-frequency amplifiers (LNB) and other high-frequency products that need to be used when receiving satellite signals on the ground, and PCBs used in high-frequency, its characteristic parameters: There are also requirements for the electrical constant X. For example, when the working frequency of the circuit is <109HZ, the X of the PCB substrate is usually required to be <2.5. Experiments show that the X of the PCB substrate is not only related to the characteristics of the substrate, but also related to the reinforcement material Content related. The higher the content of the reinforcing material of the substrate, the greater the X value, so the content of the reinforcing material of the PCB substrate for high-frequency circuits cannot be too high, which makes the mechanical properties of the PCB for high-frequency circuits not strong enough, even some high-frequency products It is also required that the PCB to be collected is very thin, and its thickness is only 1/3 of the usual PCB thickness, so that the PCB is more prone to breakage. This feature will bring difficulties to the production of such products. In this regard, let’s talk about some of our experience in production practice and the above methods used to overcome the weakness of thin PCBs used in high-frequency products that are easy to break in the surface mount production, which makes these products mass-produced. Can proceed smoothly.

The surface mount process mainly includes three basic links: applying solder paste, patching and soldering. Below we will focus on the first two basic links.

In mass production, we usually use a fully automatic printing machine for printing (that is, coating solder paste). When the PCB enters the printing machine and is coated with solder paste, it must be fixed in the printing machine first. The printing machine fixes the PCB method There are usually two types: conveying guide rails and positioning; the second is to use vacuum suction to fix and position under the conveying guide rails.

For the thin and fragile PCB, if the PCB solder paste is coated in a printing machine with a fixed PCB method, we will see that the PCB is put into the conveying rail of the printing machine and enters the appropriate position. The guide rails will clamp the PCB towards each other, which will cause the middle part of the PCB board to bulge slightly. On the one hand, this clamping force is easy to cause the PCB to break; on the other hand, because the middle of the PCB is raised, the entire surface of the PCB to be coated is uneven. This will affect the coating quality of the solder paste.