Discussion on SMT Process of Flexible Printed Board

As electronic products develop in a short, thin and light direction, electronic components are required to be integrated and miniaturized. Traditional through-hole mounting technology (THT) is no longer sufficient, and a new generation of placement technology, surface mount technology (SMT), has emerged.

  In a broad sense, SMT includes Surface Mount Component (SMC), Surface Mount Device (SMD), and Surface Mount Printed Circuit Board (SMB). A comprehensive set of complete process technology processes for common mixed printed circuit boards (Printed Circuit Board), dispensing, pasting, surface mount equipment, component pick-and-place systems, soldering and in-circuit testing. .

  Because SMC and SMD reduce the influence of lead distribution characteristics, and the surface of the printed circuit board is firmly soldered, the parasitic capacitance and the parasitic inductance between the leads are greatly reduced. The electromagnetic interference and radio frequency interference are greatly reduced, and the high frequency characteristics are improved.

  These components have been widely used in satellite communication products: for example, high-frequency products such as low-noise down-conversion amplifiers (LNBs) used for receiving satellite signals on the ground, and printed circuit boards for high-frequency applications. Parameter: Dielectric constant X, also required. For example, when the operating frequency of the circuit is <109HZ, X<2.5 of the printed circuit board substrate is usually required. Experiments show that the X of the printed circuit board substrate is in addition to the substrate. In addition to the characteristics, it is also related to the content of the reinforcing material. The higher the content of the reinforcing material of the substrate, the larger the X value, so the reinforcing material content of the printed circuit board substrate of the high-frequency circuit cannot be too high, which makes the mechanical performance of the printed circuit board for the high-frequency circuit not strong enough, and even some High-frequency products also require printed circuit boards that are very thin and have a thickness of only 1/3 of the thickness of a typical printed circuit board. Such printed circuit boards are more susceptible to breakage. This feature can cause difficulties in the production of such products.

  In the following, we will talk about some of our experiences in production practice and the methods used to overcome the weakness of the thin printed circuit boards used in high-frequency products in the surface mount production. Mass production can proceed smoothly.

  The surface mount process mainly consists of three basic steps: coating solder paste, patch and soldering. Below we focus on the first two basic links.

  In the case of mass production, we usually use a fully automatic printing machine to print (ie, apply solder paste). Before the printed circuit board enters the printing machine and is coated with solder paste, it must be fixed in the printing machine. There are usually two ways to fix the printed circuit board: the first is the transfer guide and positioned; the second is the vacuum adsorption fixed and positioned under the transfer guide.

  For thinner and fragile printed circuit boards, if solder paste is applied to the first fixed printed circuit board type of press, we will see the printed circuit board placed in the transfer guide of the press. After entering the appropriate position, the two transfer rails will clamp the printed circuit board opposite each other, which will cause the middle portion of the printed circuit board to be slightly raised. On the one hand, this clamping force easily breaks the printed circuit board; on the other hand, due to the convex portion in the middle of the printed circuit board, the entire printed circuit board needs to be coated unevenly. This will affect the coating quality of the solder paste.

  This can be avoided if the solder paste is applied to the second fixed printed circuit board type printing press, because in this fixed printed circuit board mode, the transfer guides are not facing each other, so it will not Applying a opposing force to both sides of the printed circuit board, the central portion of the printed circuit board will not be convex. The printing machine uses the vacuum suction device under the transfer guide to adsorb the printed circuit board on the transfer rail and prints. The board is not broken by the external force of the clamp.

   In addition, we will see that the middle bottom of the printed circuit board is floating. In order to ensure that the thin printed circuit board is flat and does not bend when it is coated, we will add a self-made platform to the vacuum adsorption device to support the printed circuit board in actual operation. The area of the platform can be matched with the printed circuit board, thus avoiding the problem of affecting the quality of the coating due to the unevenness of the printed circuit board surface.

  In order to ensure the yield and product quality, the above second printing with vacuum adsorption fixing function is used for production. After applying the solder paste, it enters the patch. Before the printed circuit board enters the placement machine for patching, it must first be fixed in the placement machine.

  There are usually two ways for the placement machine to hold the printed circuit board. The first is to clamp the printed circuit board toward the opposite side of the transfer guide on the patch platform. Thereby fixing the printed circuit board and positioning; the second type is that the conveying rail is provided with a pressing strip, and when the printed circuit board is advanced to the corresponding position on the conveying rail, the pressing strip on the guiding rail is automatically pressed and printed The two sides of the circuit board are fixed on the rail and positioned.

  Regardless of which of the above-mentioned printed circuit board fixing methods is adopted, there is no support at the bottom of the printed circuit board to form a dangling, and if a thin and easily broken printed circuit board is used for patching, with the patch platform The movement and the action of the patch off does not completely guarantee that the printed circuit board surface is not bent. This affects the accuracy of the placement of the chip. In addition, the first printed circuit board method allows the printed circuit board to be subjected to a facing clamping force on both sides of the printed circuit board. The central portion of the printed circuit board is raised. If the printed circuit board is a jigsaw, the joint portion may be broken.

  For this reason, in practice, we will fix the printed circuit board in a customized tray, then feed the tray into the transfer rail and enter the placement machine for patching, so that the printed circuit board will not directly receive the guide rail. The external force is applied to cause breakage, and the tray plays a supporting role for the printed circuit board, and the problem of the accuracy of the position of the patch due to the unsupported deformation of the printed circuit board is avoided in the patch.

  In this way, we require a good consistency between the trays (including the shape, frame, size, and dimensions involved in the positioning of the printed circuit board). The consistency of the tray directly affects the accuracy of the placement of the patch. Of course, the above method is not perfect, and it also has some defects. Because the method is relatively high in the production of the tray, in addition to the high consistency requirements, there is still a problem to be solved, that is, the fixing problem of the printed circuit board. Therefore, in the production of the tray, attention should also be paid to the way of fixing the printed circuit board, which not only ensures that the printed circuit board cannot be shaken in the tray, but also makes the printed circuit board easy to pick and place, which increases the difficulty and cost of manufacturing the tray.

  In view of this, we have proposed another solution, which is to make the tray into a simple frame, and the printed circuit board is fixed in the tray by vacuum adsorption, which makes the tray easy to manufacture and consistent. Sex is also easier to ensure and the pick-and-place of printed circuit boards is easy, but this method requires a slight modification of the existing placement machine. Since the current placement machine does not have the function of vacuum-adhering and fixing the printed circuit board, a small vacuum adsorption device needs to be additionally added, and the operation of the vacuum pump of the device needs to be synchronized with the movement of the transfer rail to fix the printed circuit board.