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What Are Reflow Soldering & Wave Soldering?

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Published by Pinsheng Electronics Co., Ltd April 07,2020

Reflow Soldering & Wave Soldering

We have already known SMT (Surface Mount Technology) is a surface mount soldering method that is different from THT (Through-hole Technology) which is need components inserting through the holes.

Reflow soldering is applied in the SMT assembly, while wave soldering got used for THT assembly, the two soldering methods are performed in different equipment. In actual production, choosing reflow soldering or wave soldering depends on component packages and specific requirements. 

Inside the Reflow Soldering Oven 

In modern electronics manufacturing where components sizes are increasingly miniaturized, it is common to use only reflow soldering or both reflow soldering and wave soldering for PCB assembly.

Let's go through the different processes between reflow soldering and wave soldering below.

 

1. Wave Soldering

In the oven used for wave soldering, the molten alloy which mainly consists of Sn & Cu is preliminarily poured. And the oven maintains a relatively high temperature, the upwelling of molten solder is produced by a pump to generate a standing wave. When the PCB in which the component is inserted passes through over the surface of the solder wave with a bevel conveyor, a firm mechanical and electrical connection will form between the exposed metal pins and the pads on PCB.

The brief flow of wave soldering: 

Plugin the components >> Pre-coat flux >> Preheat >> Wave soldering >> Cooling >> Shear pin >> Inspection

Wave soldering

Wave soldering is usually divided into 4 steps in the oven,

1.Apply flux, which is classified as corrosive and non-corrosive. Its main function is to clean and remove metal surface oxide, while preventing secondary oxidation;

2.Preheat, dehumidify, accelerate welding, and prevent thermal shock to deform the components;

3.Wave soldering, the temperature rises, the molten alloy adheres to the pins and pads to form an electrical connection.

4.Cooling, the temperature drops reasonably and reaches room temperature, the liquid alloy condenses to form a mechanical connection.

In some cases, the PCBA will also be cleaned to remove the slag.

Wave soldering ovenInside flow in wave soldering oven

Pros of Wave Soldering

  • Shorten the contact time between PCB and high-temperature molten solder, reducing the warpage of the board

  • The molten solder isolates air, reducing the timing of oxidation

  • The molten solder in the whole oven recirculates to make the solder composition to be averaged

  • The active solder in the wave soldering oven improves the solder joint quality

 

2. Reflow Soldering

In the reflow soldering process, the solder paste which consists of solder and flux is applied on the pads, and place the components upon the solder paste to form a temporary connection. Then, the PCB is soldered in the oven by means of hot air, infrared rays or other heat radiation, the temperatures in the oven will be strictly controlled, the solder paste will melt and solidify to form stable solder joints.

The typical flow (Single-side & Double-side):

Single-sided Mounting: Solder paste printing >> Pick-n-Place >> Reflow soldering  >> Inspection

Double-sided mounting: A-side solder paste printing >> Pick-n-Place >> Reflow soldering >> B-side solder paste printing >> Pick-n-Place >> Reflow soldering  >> Inspection

 

There are generally four working areas in the reflow soldering oven,

1. Temperature rises, the water is evaporated, the solder paste softens to cover the pads and pins and isolates them from the oxygen;

2. Preheat, PCB and components are fully preheated to prevent thermal shock damaging PCB and components;

3. Reflow soldering, the temperature rises again rapidly and the solder paste melts, the liquid solder reflows to form the solder joints;

4.Cooling zone, solder joint solidify.

 

Temperature curve in Reflow soldering oven
 

Pros of Reflow Soldering

  • In reflow soldering, the components do not need to be directly immersed in the molten solder, so that the thermal shock is smaller

  • The solder is only be applied to the required parts in reflow soldering, saving solder

  • The solder can be limited in reflow soldering to avoid the defects such as bridging

  • The solder generally does not contain so many impurities

 

3.Selective Wave Soldering 

In general, reflow soldering is more time-saving and cost-effective than wave soldering, but so far wave soldering cannot be replaced in some application. Many hardware containing DIP and SMD components often require hybrid soldering. To cope with this situation, selective wave soldering was proposed and applied, which is the same as the wave soldering principle, just to add a tray to isolate the parts which are no need to be soldered from the wave, or use the nozzles to solder.

 

In short, reflow soldering and wave soldering deal with different situations and both of them are necessary. Moreover, the soldering method is just one of many options for electronics manufacturing, in which we must consider more sides. To know more details on how to make high-quality boards, please contact us.

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