Surface mount technology, referred to as SMT, as a new generation of electronic assembly technology, has penetrated into various fields. SMT products have the advantages of compact structure, small volume, vibration resistance, impact resistance, good high-frequency characteristics and high production efficiency. SMT occupies a dominant position in the process of circuit board assembly.
The typical surface mounting process is divided into three steps: applying solder paste-mounting components-reflow soldering.
Step 1: Apply solder paste.
Its purpose is to evenly coat a proper amount of solder paste on the pads of PCB, so as to ensure that SMT components and the corresponding pads of PCB can achieve good electrical connection and have sufficient mechanical strength during reflow soldering.
Solder paste is a paste with certain viscosity and good touch characteristics, which is made of alloy powder, solder paste flux and some additives. At room temperature, due to the viscosity of solder paste, electronic components can be pasted on the pads of PCB. Under the condition that the inclination angle is not too large and there is no external force collision, the general parts will not move. When the solder paste is heated to a certain temperature, the alloy powder in the solder paste melts and flows again, and the liquid solder penetrates into the welding terminals and PCB pads of components. After cooling, the solder terminals and pads of the components are connected with each other by solder, forming solder joints for electrical connection and mechanical connection.
The solder paste is coated on the pad by special equipment, including:
Fully automatic printing machine, semi-automatic printing machine, manual printing table, semi-automatic solder paste distributor, etc.
Advantages and disadvantages of application methods
Large batch of machine printing, tight supply cycle, sufficient funds for mass production, high production efficiency, complex use process and large investment.
Manual printing is used in small and medium-sized batch production, which is simple in product development and operation, low in cost, and needs manual positioning, so it is impossible to carry out mass production.
The research and development of manual drop coating of ordinary circuit boards can repair solder paste without auxiliary equipment, and is only suitable for drop coating of components with pad spacing above 0.6 mm
Step 2: Install the components.
This process is to accurately attach the chip components to the corresponding positions on the PCB surface printed with solder paste or patch adhesive by mounter or hand.
There are two installation methods, which are compared as follows:
Advantages and disadvantages of application methods
The machine has large installation batch and tight supply cycle, which is suitable for mass production. The process is complicated and the investment is large.
Hand-mounted small and medium-sized batch production, simple product development and operation, low cost. The production efficiency depends on the proficiency of the operators.
The main tools for manual mounting: vacuum suction pen, tweezers, IC suction and placement aligner, low-power stereo microscope or magnifying glass, etc.
Step 3: Reflow welding
Reflow soldering is a literal translation of English reflow soldering ring, which realizes the mechanical and electrical connection between the soldering terminals or pins of surface-mounted components and the printed board pads by remelting the solder paste pre-distributed on the printed board pads.
Analyze the principle of reflow soldering from SMT temperature characteristic curve (see figure). First, when the PCB enters the preheating temperature range 140℃ ~ 160℃, the solvent and gas in the solder paste evaporate. At the same time, the soldering flux in the solder paste wets the pads, soldering terminals and pins, and the solder paste softens and collapses, covering the pads and isolating the pads and pins from oxygen. The surface mount components are fully preheated, and then enter the welding zone, and the temperature is rapidly raised at the international standard heating rate of 2-3℃ per second, so that the solder paste reaches a molten state, and the liquid solder wets, diffuses, overflows and reflows on the bonding pads, component welding ends and pins of the PCB, generating metal compounds at the welding interface and forming solder joints; Finally, the PCB enters the cooling zone to solidify the solder joint.
Introduction of reflow soldering method;
Advantages and disadvantages of mechanical heating mode
Infrared reflow soldering has high radiation conduction thermal efficiency, large temperature gradient, easy control of temperature curve, and easy control of PCB upper and lower temperatures during double-sided soldering. Shadowing effect, uneven temperature, easy to cause local burning of components or PCB.
Hot air reflow welding has uniform convection conduction temperature and good welding quality. The temperature gradient is not easy to control.
Forced hot air reflow welding infrared hot air mixed heating combines the advantages of infrared and hot air stove, and can obtain excellent welding effect when welding products.
Forced hot air reflow soldering can be divided into two types according to its production capacity:
Advantages and disadvantages of machine type applicability
Mass production of temperature zone equipment is suitable for mass production of PCB boards placed on the walking belt, and PCB boards must pass through several fixed temperature zones in turn. If there are too few temperature zones, there will be a temperature jump phenomenon, which is not suitable for welding high-density assembly boards. But also has huge volume and high power consumption.
Small batch production of small desktop equipment in temperature-free zone develops rapidly in a fixed space, and the temperature changes with time according to the set conditions, which is simple to operate and especially suitable for BGA QFP PLCC. It can repair defective surface mount components (especially large components) and is not suitable for mass production.
Because the reflow soldering process has the characteristics of "reflow soldering" and "self-positioning effect", the reflow soldering process requires loose mounting accuracy, and it is easier to realize high automation and high speed of welding. At the same time, due to the characteristics and self-positioning effect of reflow soldering, reflow soldering process has stricter requirements on pad design, component standardization, component end and PCB quality, solder quality and process parameter setting.
Cleaning is a process of removing pollutants and impurities on the surface of the cleaned object through physical action and chemical reaction. Whether using solvent cleaning or water cleaning, the surface must be wetted, dissolved, emulsified, saponified, etc. By applying different mechanical forces, the dirt is stripped from the surface of the surface assembly board, then rinsed or rinsed, and finally dried, dried or naturally dried.
Reflow soldering is a key process in SMT production, and reasonable temperature curve setting is the key to ensure the quality of reflow soldering. Improper temperature curve will lead to PCB welding defects, such as incomplete penetration, virtual welding, component tilting, too many solder balls, etc., which will affect product quality.
SMT is a comprehensive system engineering technology, involving substrate, design, equipment, components, assembly technology, production parts and management. SMT equipment and SMT process require stable voltage, anti-electromagnetic interference, anti-static, good lighting and exhaust facilities, and have special requirements for the temperature, humidity and air cleanliness of the operating environment. Operators should also receive professional technical training.