Printed circuit boards are a structural base, without which not a single complex radio or electronic device in microprocessor technology can do today. The manufacture of this base involves the use of special raw materials, as well as technologies for forming the design of the carrier plate. Wave soldering is one of the most effective structural molding methods for printed circuit boards.
Preparations
The initial stage, during which two tasks are solved - the choice of the component base and the list of necessary consumables for the operation, as well as the equipment setup. As part of the first task, in particular, the base for the board is prepared, its dimensions are fixed, and the contours of the solderedconnections. Of the consumables, wave soldering requires the addition of special agents to reduce future oxide formation. In addition, modifiers of the technical properties of the structure can also be used if it is planned to be used in aggressive environments.
The equipment for this operation is usually a compact but multifunctional machine. The capabilities of a typical wave soldering machine are designed to serve single-layer or multi-layer boards with an operating range of about 200 mm in width. As for the tuning of this unit, first of all, the dynamic characteristics and waveform are set. The main part of these parameters is regulated through the wave supply nozzle, in particular, allowing you to set the flow of Z- and T-shaped forms. Depending on the requirements for the printed node, speed indicators with the direction of the wave are also assigned.
Workpiece fluxing
As in welding processes, when performing soldering, the flux plays the role of a cleaner and a stimulator for the formation of a quality joint. Powder and liquid fluxes are used, but in both cases their main function is to prevent metal oxidation processes before the soldering reaction begins, otherwise the solder will not bond the joint surfaces. The liquid flux is applied using a sprayer or a foaming agent. At the time of laying, the mixture should be diluted with the necessary activators, rosin and mild acids, which will improve the reactions. Foam solutions are applied withusing tubular filters that form a fine bubble foam. In the metalized wave soldering process, such coatings improve wetting and stimulate the action of modifiers. Typically, both liquid and solid fluxes involve separate flushing or stripping of excess material. But there is also a category of indelible active substances that are completely included in the structure of the desoldering material and do not require any stripping in the future.
Preheat
At this stage, the printed circuit board is preparing for direct contact with the solder. Heating tasks are reduced to reduce thermal shock and remove solvent residues and other unnecessary substances that remain after fluxing. The equipment for this operation is included in the infrastructure of the wave soldering installation and is a convection, infrared or quartz heater. The operator only needs to correctly set the temperature. So, if the work is carried out with a single-layer board, then the heating temperature can vary within 80 - 90 ° C, and if we are talking about multi-layer (from four levels) blanks, then the thermal effect can be within 110-130 ° C. With a large number of plated through holes, especially when working with multilayer boards, thorough intermittent heating should be ensured at a temperature rise rate of up to 2 °C / s.
Performing soldering
Temperature mode during soldering is set in the range from 240up to 260 °C on average. It is important to observe the optimal level of thermal exposure for a particular workpiece, since lowering the degree can lead to the formation of non-solders, and exceeding it can lead to structural deformation of the functional coating of the board. The time of the contact operation itself lasts from 2 to 4 seconds, and the height of the solder during wave soldering is calculated individually, taking into account the thickness of the board. For example, for single layer structures, the solder should cover approximately 1/3 of the thickness of the structure. In the case of multilayer workpieces, the immersion depth is 3/4 of the board thickness. The process is implemented as follows: with the help of a soldering machine compressor, a wave flow is formed in a bath with molten solder, along which the board with the elements placed on it moves. At the moment of contact of the bottom of the board with solder, solder joints are formed. Some modifications of the installations provide the ability to change the inclination of the carrier conveyor within 5-9 °, which allows you to choose the optimal angle for the flow of solder.
Refrigeration conditions
It is not at all necessary to use special means for intensive cooling. Moreover, natural cooling is more useful in terms of obtaining a normal structural state of the workpiece. Another thing is that after completion of wave soldering, thermomechanical stress should be avoided, which can be caused by the difference in linear expansion of the material of the processed heated nodes and the main components of the board.
Conclusion
Wave methodthermal soldering is characterized by many advantages from reducing the risk of deformation processes to low cost of operation. By the way, to perform the procedure in a full cycle, minimal organizational labor costs are required compared to alternative methods. At the same time, progress does not stand still and various modifications of the technology are emerging today. In particular, double wave soldering allows the segmentation of the flow functions, improving the quality of the joints on the contact surface. The second wave is endowed with an exclusively cleaning function, within which excess flux and solder bridges are more effectively eliminated. Of course, in this case, the complexity of the equipment is not complete. The units are completed with pumps, nozzles and control units for each wave separately.
