Wave soldering and reflow soldering are two common ways of welding electronic products in the production process of electronic products. The difference between them is mainly: wave soldering is used for welding plug-in circuit boards, reflow soldering is used to weld SMT patch circuit boards. The rising de come and talk about the details of big wave soldering and reflow difference.
SMT products have the advantages of compact structure, small size, vibration resistance, impact resistance, high frequency characteristics and high production efficiency. SMT has occupied a position in the circuit board assembly process.
The typical surface mounting process is divided into three steps: applying solder paste - - - -- reflow soldering
Step 1: apply solder paste
The purpose is to apply the appropriate solder paste to the pad of PCB to ensure that the pad corresponding to the PCB corresponds to a good electrical connection and sufficient mechanical strength when reflow soldering.
Solder paste is a paste composed of alloy powder, paste welding agent and some additives, which has definite viscosity and good contact characteristics. Under normal temperature, the paste has certain viscosity, can be pasted on the pad electronic components PCB, the tilt angle is not too large, there is no external force collision, general component is not moving, when heated to a certain temperature of solder paste, solder paste alloy powder melt and flow, the liquid weld end the components of the solder infiltration and PCB pad, and the pad cooling after welding end components are interconnected in solder, the solder joints formed in electrical and mechanical connection.
Solder paste is applied to the pad by special equipment. Its equipment includes: automatic printing machine, semi-automatic printing machine, manual printing table, semi-automatic solder paste dispenser, solder paste mixer, auxiliary equipment, etc.
Advantages and disadvantages of machine printing: semi-automatic solder paste printing machine, large batch or high accuracy, high flexibility, tight delivery cycle, batch production and production efficiency fully automatic: printing within a precision 0.2mm range, large volume, but high investment cost.
Manual printing of small batch production, high precision product development, low cost, simple positioning method for mass production, R & D is only applicable to welding spacing components in the above 0.5mm printed circuit board of the ordinary manual coating, repair pad paste must auxiliary equipment, R & D and production can be applied only to pad pitch components in 0.6mm the above coating.
Second step: mount components
This process is the appropriate position of the PCB surface on the surface of the solder paste or sticker glue by attaching the device or hand. There are two kinds of packing methods, which are compared as follows:
The first is the advantages and disadvantages of the application of the placement machine: machine printing, large batch, tight supply cycle, enough funds, mass production, high production efficiency, complex process and large investment.
Second: manual printing, small batch production, product development, simple operation and low cost, the production efficiency shall be in accordance with the proficiency of operating personnel, manually mount main tools: vacuum suction pen, tweezers, IC suction aligner and low power microscope or magnifying glass.
Third step: reflow Soldering
PCB to 140 to 160 DEG C temperature preheating zone, solder paste in solvent and gas evaporated, at the same time, solder paste flux wetting pads, and solder paste components welding end pin, softening, collapsing, covering the pad, the pad and component pins and oxygen isolation; and the surface mount components are fully the preheating, then enter the welding zone, the temperature to 2-3 DEG C per second when standard temperature rises rapidly to reach the melting rate of solder paste, solder pads, liquid components of PCB welding end and pin wetting, diffusion, and overflow back mixing in the welding interface on the formation of metal compounds, the formation of solder joints; after PCB enters the cooling zone so that the solder solidification.
Reflow welding method: different reflow soldering has different advantages, and process flow is certainly different.
Infrared reflow: radiation heat transfer efficiency is high, the temperature gradient is large, easy to control the temperature curve, double welding PCB temperature control. There is a shadow effect, the temperature is not uniform, and it is easy to cause part or PCB to burn out.
Hot air reflow welding: the convection temperature is uniform and the quality of the welding is good. Temperature gradient is not easy to control
Forced hot air reflow soldering: the advantages of infrared hot air mixing heating combined with infrared and hot blast stove can achieve excellent welding effect when products are welded. Forced hot air reflow soldering can be divided into two kinds according to their production capacity.
1. temperature zone equipment: mass production is suitable for mass production, PCB board is placed on the walking belt, it must go through a number of fixed temperature areas sequentially, and there will be temperature jump when there is too little temperature zone, which is not suitable for welding of high-density assembly panels. It is also large in volume and high in power consumption.
2. temperature area small platform equipment: small and medium volume production in a fast development in a fixed space, the temperature according to the setting conditions change with time, the operation is simple. The rework of defective surface mounting elements (especially large components) is not suitable for mass production.
Because the reflow process has the characteristics of "reflow" and "self positioning effect", it makes the reflow soldering process more flexible for assembly accuracy, and it is easy to achieve high automation and high speed of welding. At the same time, due to the characteristics of reflow and self localization effect, reflow soldering process is more stringent for the design of pads, standardization of components, components and quality of PCB, solder quality and process parameters.
Cleaning is the process of removing contaminants and impurities on the surface of the cleaned material by physical and chemical reactions. In terms of solvent cleaning or water cleaning, surface wetting, dissolution, emulsification and saponification should be carried out. By applying mechanical force of different ways, we will get rid of dirt.