SuZhou Tonkida Electromechanical Technology Co., Ltd Professional SMT whole line supplier

A typical surface mount process is divided into three steps: application of solder paste ---- mount components ----- reflow welding

Step: Apply solder paste

The purpose is to apply an appropriate amount of solder paste evenly on the solder pad of the PCB to ensure that the patch components and the corresponding solder pad of the PCB achieve good electrical connection and have sufficient mechanical strength during reflow welding.

Solder paste is a paste made of alloy powder, paste flux and some additives, which has fixed viscosity and good touching properties. At room temperature, because the solder paste has a certain viscosity, electronic components can be pasted on the PCB pad, in the case of tilt Angle is not too large, and there is no external force collision, the general component 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 wets the welding end of the component and the PCB pad. After cooling, the welding end of the component and the pad are interconnected by solder to form an electrical and mechanical connection solder joint.

Solder paste is applied on the pad by special equipment, including: GSD automatic printing machine,GSD semi-automatic printing machine, manual printing table, semi-automatic solder paste dispenser,GSD solder paste mixer auxiliary equipment.

Machine printing :GSD semi-automatic solder paste printing machine batch large or high precision, high flexibility, tight supply cycle, mass production, production efficiency automatic: precision 0.2mm printing, large quantities, but the investment cost is high!

Manual printing small batch production, low precision product development, low cost, simple positioning, can not be mass production, only suitable for the welding pad spacing above 0.5mm component printing manual drop coating ordinary circuit board research and development, repair pad solder paste auxiliary equipment, can be developed and produced only suitable for the welding pad spacing above 0.6mm component drop coating.

Step 2: Mount components

This process is to accurately mount the chip components to the corresponding position on the PCB surface of the printed solder paste or patch adhesive with the chip mounter or manual. There are two mounting methods, and the comparison is as follows:

Advantages and disadvantages: machine printing, large batch, tight supply cycle, sufficient funds, mass production, high production efficiency, complex use process, large investment!

Second: manual printing, small and medium-sized batch production, product development, easy operation, low cost, production efficiency depends on the proficiency of the operator, manual manual installation of the main tools: vacuum pen, tweezers, IC suction and release alectors, low ploidy microscope or magnifying glass.

Step 3: reflow welding

The principle of reflow welding is analyzed from SMT temperature characteristic curve (see figure).

When the PCB enters the preheating temperature zone of 140 ° C ~ 160 ° C, the solvent and gas in the solder paste evaporate. At the same time, the flux in the solder paste moistens the solder pad, the welding end of the components and the pins, and the solder paste softens and collapses, covering the solder pad and the pin of the components, isolating the solder pad and the pin of the components from oxygen. When entering the welding zone, the temperature rises rapidly at the standard temperature rate of 2-3℃ per second to make the solder paste reach a molten state. The liquid solder on the PCB pad, component welding end and pin wetting, diffusion, diffusion and reflux mix on the welding interface to generate metal compounds and form solder joints. After the PCB enters the cooling zone to solidify the solder joint.

Introduction to reflow welding methods: Different reflow welding has different advantages, and the process is of course different.

Infrared reflow welding: high radiation conduction heat efficiency, large temperature gradient, easy to control the temperature curve, and easy to control the upper and lower temperature of PCB when welding on both sides. There is a shadow effect, the temperature is not uniform, and it is easy to cause local burning of components or PCB

Hot air reflow welding: convection conduction temperature uniform, good welding quality. Temperature gradients are not easy to control

Forced hot air reflow welding: infrared hot air mixed heating combined with the advantages of infrared and hot blast furnace, in the product welding, can get excellent welding results, forced hot air reflow welding, according to its production capacity is divided into two kinds:

1. Temperature zone equipment: mass production is suitable for mass production of PCB board placed on the walkway, to pass through a number of fixed temperature zones in sequence, temperature zone is too small there will be temperature jump phenomenon, not suitable for high-density assembly board welding. And bulky, high power consumption.

2. Temperature zone small desktop equipment: small and medium-sized batch production is rapidly developed in a fixed space, the temperature changes with time according to the set conditions, easy to operate. Repairable defective surface mount components (especially large components) are not suitable for mass production.

Because the reflow process has the characteristics of "reflow flow" and "self-positioning effect", the reflow process requires relatively loose mounting accuracy, and it is easier to realize the high automation and high speed of welding. At the same time, because of the characteristics of re-flow and self-positioning effect, reflow welding process has more stringent requirements for pad design, component standardization, component end and printed board quality, solder quality and process parameter setting.

Cleaning is the use of physical action, chemical reaction to remove the surface of the cleaning of pollutants, impurities process. On the use of solvent cleaning or water cleaning, it is necessary to go through the surface wetting, dissolution, emulsification, saponification, etc., and by applying different ways of mechanical force to peel the dirt off the surface of the surface assembly board, and then rinsing or rinsing clean, after blowing dry, drying or natural drying.

Reflow welding is a key process in SMT production, and reasonable temperature curve setting is the key to guarantee the quality of reflow welding. Improper temperature curves will cause PCB welding defects such as incomplete welding, virtual welding, component warping, and excessive solder balls, which will affect product quality.

SMT is a comprehensive systems engineering technology that covers substrates, design, equipment, components, assembly processes, production accessories and management. SMT equipment and SMT process requirements for the operation site voltage to be stable, to prevent electromagnetic interference, to prevent static electricity, to have good lighting and exhaust gas emission facilities, temperature, humidity, air cleanliness of the operating environment have special requirements, operators should also go through professional technical training.