What is the reflow soldering for PCB assembly?

Table of Contents

1. What is reflow soldering for PCB assembly

2. The principle of reflow soldering for PCB assembly

3. Classification of reflow soldering for PCB assembly

4. The process of reflow soldering for PCB assembly

 

1. What is reflow soldering for PCB assembly

SMT the reflow soldering process is the most commonly used method for surface mount technology (SMT) to solder electronic components to PCB boards. Reflow soldering refers to the use of solder paste (a mixture of solder and flux) to solder electronic components on the pads of PCB board, the solder is melted to achieve permanent bonding by controlling the heating. Different heating methods such as reflow oven, infrared heating lamp or hot air gun can be used for soldering. The reason why it is called “reflow soldering” is because the gas circulates in the welding machine to generate high temperature to achieve the purpose of soldering.

 

2. The principle of reflow soldering for PCB assembly

The principle of reflow soldering is divided into several processes:

A.Heating zone: When the PCB enters the heating zone, the solvent and gas in the solder paste evaporate, the flux in the solder paste wets the pads, component ends and pins at the same time. And the solder paste softens, collapses, and covers the pads and isolate the pads and component pins from oxygen.

B.Insulation zone: When the PCB enters the heat preservation area, make the PCB and components fully preheated to prevent the PCB from suddenly entering the high temperature area of welding and damaging the PCB and components.

C.Welding zone: When the PCB enters the soldering area, the temperature rises rapidly to make the solder paste reach a molten state, and the liquid solder wets, diffuses, flows, or reflows to the PCB pads, component ends and pins to form solder joints.

D.Cooling zone: When PCB enters the cooling zone so that the solder joints are solidified; the reflow soldering for PCB assembly is completed.

 

 

3. Classification of reflow soldering for PCB assembly

According to the reflow heating area, it can be divided into two categories: one is to reflow the PCB as a whole, and the other is to reflow the PCB by partial heating. Heating and reflow soldering of the PCB as a whole can be divided into hot plate reflow soldering, infrared reflow soldering, hot air reflow soldering, hot air plus infrared reflow soldering, and gas-phase reflow soldering. The partial heating and reflow soldering of PCB can be divided into laser reflow soldering, focused infrared reflow soldering, beam reflow soldering, and hot air flow reflow soldering.

A). Heat plate conduction reflow soldering for PCB assembly: relying on the heating by the heat source under the conveyor belt or the push plate, and heating the components on the substrate by means of heat conduction, the structure is simple and the price is cheap. Generally used for single-sided assembly of thick film circuits using ceramic (Al2O3) substrates.

B). Infrared (IR) reflow oven: most of this type of reflow oven is conveyor belt type, but the conveyor belt only supports and transfers the substrate. The heating method is mainly based on the infrared heat source to heat by radiation and the temperature in the furnace Compared with the previous method, it is uniform and has a larger mesh, which is suitable for SMT reflow soldering process and heating of double-sided assembled substrates. This type of reflow oven is the basic type of reflow oven. Many of them are used in China and the price is relatively cheap.

C). Vapour phase reflow soldering: Vapour phase reflow soldering is also known as vapor phase soldering (VaporPhaseSoldering, VPS), also known as condensation soldering (condensation soldering). Heating fluorocarbons (FC-70 chlorofluoroalkane solvents were used in the early days), melting point is about 215°C, boiling produces saturated steam, there are condensers on the top and left and right of the furnace to limit the steam in the furnace, and meet the low temperature to be welded When PCB components release the latent heat of vaporization, the components and pads are welded after the solder paste is melted. The gas-phase of VPS is saturated vapor with low oxygen content and high thermal conversion rate, but the solvent cost is high, and it is typical ozone-depleting substance. Therefore, its application is greatly restricted. The international community basically no longer uses this kind of damage. Environmental approach.

D). Hot-air reflow soldering: The hot-air reflow soldering furnace transfers heat energy through the laminar movement of hot air. The heater and fan are used to keep the air in the furnace heated and circulated. The parts to be welded are heated by the hot gas in the furnace to achieve welding. Increase the temperature zone to 8 or 10, so that it can further accurately control the temperature distribution of each part of the furnace, which is more convenient for the ideal adjustment of the temperature curve. The reflow soldering for PCB assembly furnace with full hot air forced convection has been continuously improved and perfected and has become the mainstream equipment for SMT soldering.

E). Infrared + hot air SMT reflow soldering process: The infrared hot air reflow oven effectively combines the advantages of infrared reflow soldering and forced convection hot air reflow soldering, and is an ideal heating method in the 21st century. It takes full advantage of the strong penetrating power of infrared radiation, high thermal efficiency, and energy-saving. At the same time, it effectively overcomes the temperature difference and shielding effect of infrared SMT reflow soldering process, and makes up for the effect of hot air reflow soldering on the excessively fast gas flow rate.

F). Hotwire reflow soldering: Hotwire reflow soldering is a welding technology that uses heating metal or ceramics to directly contact the weldment. It is usually used in the cable connection technology of flexible substrates and rigid substrates. This heating method generally does not use solder paste, Mainly use tin plating or anisotropic conductive glue, and need special welding tips, so the welding speed is very slow, and the production efficiency is relatively low.

G). Hot-air reflow soldering: Hot-air reflow soldering refers to a method of soldering using hot air through air or nitrogen in a special heating head. This method needs to process nozzles of different sizes for different sizes of solder joints and the speed is relatively slow. Under repair or development.

H). Laser reflow soldering, beam reflow soldering: Laser heating reflow soldering uses the characteristics of the laser beam’s good directivity and high power density. The laser beam is concentrated in a small area through an optical system in a short time. Make the heated part form a local heating zone. Commonly used lasers include CO2 and YAG, which is a schematic diagram of the working principle of laser heating reflow soldering. The heating of laser heating SMT reflow soldering process has the characteristics of high localization, no thermal stress, small thermal shock, and heat-sensitive components are not easily damaged. However, the equipment investment is large and the maintenance cost is high.

I).Induction reflow soldering: The induction reflow soldering equipment uses a the transformer in the heating head, and uses the principle of inductive eddy current to weld the weldment. This welding method has no mechanical contact and has a fast heating speed; the disadvantage is that it is sensitive to position and difficult to control the temperature. There is a risk of overheating, and electrostatic sensitive devices should not be used.

J). Poly-infrared reflow soldering: Focused infrared reflow soldering is suitable for rework workstations for rework or partial soldering.

 

 

4. Introduction to reflow soldering for PCB assembly

The the reflow soldering process is a surface-mounted technology manufacturing process and its process is more complicated, which can be divided into two types: single-sided mounting and double-sided mounting.

A. Single-sided mounting: pre-coated solder paste → patch (divided into manual placement and machine automatic placement) → reflow soldering → inspection and electrical testing.

B.Double-sided placement: A side pre-coated solder paste → patch (divided into manual placement and machine automatic placement) → reflow soldering → B side pre-coated solder paste → placement (divided into manual placement and machine automatic placement) Mounting) → reflow soldering → inspection and electrical test.

The simplest process of reflow soldering for PCB assembly is “screen printing solder paste-patch-reflow soldering. Its core is the accuracy of the screen printing. For the patch, the yield rate is determined by the machine’s PPM. The reflow soldering is to control the temperature rise and the highest Temperature and falling temperature curve.