The difference between wave soldering and reflow soldering (on)
Wave soldering and reflow soldering are two more common welding methods. Let's talk about the difference between wave soldering and reflow soldering.
Surface stickers. Surface mounting technology, abbreviated as SMT, has penetrated into various fields as a new generation of electronic assembly technology. SMT products have the advantages of compact structure, small size, vibration resistance, impact resistance, good high frequency characteristics and high production efficiency. SMT has occupied a leading position in the circuit board assembly process.
The typical surface mount process is divided into three steps: applying solder paste-mounting components-reflow soldering
First step: applying solder paste.
The purpose is to apply an appropriate amount of solder paste evenly on the pads of the PCB to ensure that the pads corresponding to the chip components and the PCB achieve good electrical connection and sufficient mechanical strength during reflow soldering.
Solder paste is a paste made of alloy powder, paste flux and some additives, which has a certain viscosity and good touch characteristics. At normal temperature, because the solder paste has a certain viscosity, electronic components can be pasted on the PCB pads. The general components will not move when the inclination angle is not too large, and there is no external force collision. When the paste is heated to a certain temperature, the alloy powder in the solder paste melts and flows, and the liquid solder infiltrates the solder end of the component and the PCB pad. After cooling, the solder end of the component and the pad are interconnected by the solder to form electrical and mechanical Connected solder joints.
Solder paste is applied on the pad by special equipment. The equipment includes:Automatic printing machine, semi-automatic printing machine, manual printing station, semi-automatic solder paste dispenser, etc.
Application method / applicable situation / advantage / disadvantage
The machine printing batch is large, the supply cycle is tight, the fund is sufficient for mass production, the production efficiency is high, the use process is complicated, and the investment is large.
Manual printing Small and medium-sized batch production, product development, easy operation, low cost, manual positioning is required, and mass production cannot be performed
Manual R & D of ordinary circuit board, repair pad solder paste, no need for auxiliary equipment, can be developed and produced, only suitable for components with pad spacing above 0.6mm
Step 2: Place components
In this process, the chip components are accurately mounted to the corresponding position on the PCB surface where the solder paste or patch glue is printed, using a mounting machine or manually.
There are two mounting methods, and the comparison is as follows:
Application method / applicable situation / advantage / disadvantage
The machine mounting batches are large, and the supply cycle is tightly suitable for large-scale production. The use of complex procedures and large investment.
Manual placement of small and medium-sized batch production, product development, easy operation, low cost, production efficiency depends on the proficiency of the operator
The main tools for manual manual placement: vacuum suction pen, tweezers, IC suction and placement aligner, low power stereo microscope or magnifying glass, etc.
Step 3: Reflow soldering
Reflow soldering is a literal translation of the English ReflowSoldring, which is to re-melt the solder paste solder pre-distributed on the printed board pads to achieve mechanical and electrical connection between the soldered ends or pins of the surface-mounted components and the printed board pads Soldering for connection.
Analyze the principle of reflow soldering from the SMT temperature characteristic curve (see figure). First, when the PCB enters the preheating temperature range of 140 ° C to 160 ° C, the solvent and gas in the solder paste evaporate, and at the same time, the flux in the solder paste wets the pads, component solder ends and pins, and the solder paste softens Collapsing, covering the pad, isolating the pad and component pins from oxygen; and allowing the surface-mounted components to be fully preheated, and then entering the soldering area, the temperature is rapidly raised at the international standard temperature of 2-3 ℃ per second The rise causes the solder paste to melt, and the liquid solder will wet, diffuse, diffuse, and reflow on the PCB pads, component solder ends and pins to form a metal compound on the soldering interface to form a solder joint; finally the PCB enters the cooling zone Solidify the solder joints.
The difference between wave soldering and reflow soldering (below)
Introduction of reflow soldering method:
Machine type / heating method / advantage / disadvantage
Infrared reflow soldering has high radiation conduction thermal efficiency, large temperature steepness, easy to control the temperature curve, and easy to control the upper and lower PCB temperature during double-sided soldering. There is a shadow effect, the temperature is uneven, and it is easy to cause local burnout of components or PCB.
The convection conduction temperature of hot air reflow soldering is uniform and the welding quality is good. The temperature gradient is not easy to control.
Forced hot air reflow soldering Infrared hot air mixed heating Combining the advantages of infrared and hot air furnace, you can get excellent welding results when welding products.
Forced hot air reflow soldering is divided into two types according to its production capacity:
Machine type / applicable situation / advantage / disadvantage
The mass production of temperature zone equipment is suitable for mass production of PCB boards placed on the transport belt. It must pass through a number of fixed temperature zones in sequence. If there are too few temperature zones, there will be temperature jumps, which is not suitable for welding high-density assembled boards. Moreover, it is bulky and consumes a lot of power.
The small and medium-sized batch production of small desktop equipment without temperature zone is quickly developed in a fixed space. The temperature changes with time according to the set conditions, and the operation is simple. It is especially suitable for BGA,QFP,PLCC. Rework of defective surface-mount components (especially large components) is not suitable for mass production.
Due to the characteristics of "reflow" and "self-positioning effect" in the reflow soldering process, the reflow soldering process requires looser placement accuracy, and it is easier to realize a high degree of automation and high speed of welding. At the same time, because of the characteristics of reflow and self-positioning, the reflow soldering process has stricter requirements on pad design, component standardization, component end and printed board quality, solder quality, and process parameter settings.
Cleaning is a process that uses physical action and chemical reaction to remove contaminants and impurities on the surface of the object being cleaned. Whether it is solvent cleaning or water cleaning, it must undergo surface wetting, dissolving, emulsification, saponification, etc., and the mechanical force is applied to peel off the dirt from the surface of the surface assembly board, and then rinse or rinse, Finally blow dry, dry or dry naturally.
Reflow soldering is a key process in SMT production. Reasonable temperature curve setting is the key to ensure the quality of reflow soldering. Inappropriate temperature curve will cause soldering defects such as incomplete soldering, virtual soldering, component overhanging, excessive solder balls, etc. on the PCB board, affecting product quality.
SMT is a comprehensive system engineering technology, which covers the substrate, design, equipment, components, assembly process, production accessories and management. The SMT equipment and SMT process require stable voltage on the operation site, to prevent electromagnetic interference, to prevent static electricity, to have good lighting and exhaust emission facilities, and have special requirements for the temperature, humidity, and air cleanliness of the operating environment. Personnel should also undergo professional technical training.
Wave soldering refers to the flow of molten solder (lead-tin alloy) through an electric pump or electromagnetic pump into a solder wave that is required by design. It can also be formed by injecting nitrogen into the solder pool to make the pre-installed components printed The soldering of the board through the solder wave realizes the mechanical and electrical connection between the soldering ends or pins of the components and the pad of the printed board. There are many types of wave soldering systems according to the wave shapes of different geometric shapes used by the machine.
Wave soldering process: insert the component into the corresponding component hole → pre-coat flux → pre-bake (temperature 90-1000C, length 1-1.2m) → wave soldering (220-2400C) → cut off the excess insert foot → check.
The reflow soldering process is to re-melt the paste solder that has been pre-distributed on the pads of the printed board to realize the mechanical and electrical connection of the soldering ends or pins of the surface-mounted components and the pads of the printed board .
Wave soldering has a new welding process with the enhancement of people's awareness of environmental protection. In the past, tin-lead alloys were used, but lead is a heavy metal that is very harmful to the human body. So now there is a lead-free process. It uses * tin-silver-copper alloy * and special flux and requires higher soldering temperature and higher preheating temperature. It is also necessary to set up a cooling zone workstation after the PCB board passes the soldering zone. This aspect is In order to prevent thermal shock, on the other hand, if there is ICT, it will affect the detection.
Wave soldering can be basically solved as it is for soldering slightly larger and relatively small components.He is different from reflow soldering here, and reflow soldering heats the board and components, in fact, it liquefies the solder paste that was originally brushed In order to achieve the purpose of connecting the component to the board. 1. Wave soldering working mode: the board enters the machine port-after the sensor is sensed-spray FLUX (flux)-preheating zone begins to preheat-spraying begins at the tin Tin-cooling. 2. Reflow soldering work mode: heating in several temperature zones-tin liquefaction-cooling.
Wave soldering: melting solder forms wave soldering to components;
Reflow soldering: High temperature hot air forms reflow soldering to components.
Reflow soldering is to have solder in front of the furnace, just melt the solder paste in the furnace to form solder joints,Wave soldering is soldering in the furnace without solder in front of the furnace.
Reflow soldering is to solder chip components, wave soldering pin components,At present, a lot of boards are used by both. Generally, they are first mounted (no feet, surface mount) after reflow soldering, then plug-in (with feet), and then pass the wave machine.