Reflow Ovens Information

Reflow ovenReflow ovens are electronic heating devices used to mount electronic components to printed circuit boards (PCB) using surface mount technology (SMT). The electronics manufacturing industry maintains SMT as an industry standard due to the advantage it offers of simpler construction of electronic devices.  Reflow ovens vary in size and type. Commercial reflow ovens range in cost from thousands to tens of thousands of dollars. The option to construct homemade reflow ovens reduces costs; however, it also limits both functionality and durability.

The invention of the reflow oven solved the problem of excessive time consumption involved in manually soldering electronic components to printed circuit boards. Modern convection reflow ovens feature high thermal transfer efficiency, allowing for shorter profiles and more consistent, even heating compared to earlier models.

Reflow soldering involves melting a paste of solder and flux to form a permanent bond between electronic components and printed circuit boards. A typical reflow solder process is carried out as follows. The process begins by laying a stencil with holes cut out for individual pads over a PCB and applying solder paste to the PCB with a screen printer. A pick and place machine or other placement equipment then positions electronic components onto the PCB, aligning component leads with the solder paste pads. The board is then sent through a reflow oven to heat the paste and then cool it, forming a permanent bond between the components and the PCB. The board can then undergo cleaning, testing, packaging, or further assembly into a completed product.

A typical reflow soldering process follows a temperature profile that characterizes the optimum rate of heating and cooling that the solder paste and components should experience. The four main zones of the thermal profile are preheat, soak/preflow/dryout, reflow, and cooling.

  • The preheat zone involves heating the entire assembly at a controlled rate between 1 – 4°C to temperatures from 100 to 150°C. The rate of heating in this zone is critical to avoid thermal shock to the components.
  • The soak zone holds the temperature at a steady level for up to two minutes between 150 to 170°C. This allows fluxes to activate and for the temperature to stabilize throughout all components.
  • The reflow zone heats the assembly to a temperature higher than the solder’s melting point for 30 to 60 seconds to ensure reflow for every soldered lead.
  • The cooling zone lowers the temperature at a controlled rate between 1 to 4°C to evenly form solid solder interconnections between components and the board, with ideal grain size and structural strength.

Types

A common type of reflow oven is the infrared convection oven. This type of oven accomplishes heat transfer to the components and solder by radiation of heat from heating elements.  Fans provide circulation to improve heat transfer efficiency.

Vapor phase ovens use the thermal energy in a layer of vapor to achieve soldering. A liquid perfluoropolyether is boiled to create a layer of vapor in which the PCB is immersed.

In comparison to convection ovens, vapor phase ovens have high heat transfer efficiency, which can result in less power use. In addition, they can provide improved solder wetting and a small physical footprint. They are suited to low volume production due to their batch process nature. Vapor phase ovens, however, can be more expensive due to the cost of specialized vapor materials.

Reflow ovens vary in size, type of heating, and features. The most common types of reflow ovens are: bench top reflow ovens, batch reflow ovens, and in-line conveyor reflow ovens.

Specifications

Today’s reflow ovens have a variety of features tailored to the intended use, the duration of production, and the desired outcome. Selecting a reflow oven requires consideration of all aspects that affect the process of production. Considering the following criteria can be helpful when choosing a reflow oven:

  • Thermal performance
  • Throughput
  • Maximum temperature rating
  • Heating technology
  • Reflow oven type
  • Entrance clearance
  • Maximum PCB width and height
  • Speed of conveyor
  • Conveyor design
  • Process gas
  • Computer software and PC interface
  • Power supply
  • Software
  • Reliability
  • Serviceability
  • Maintenance downtime

Homemade reflow ovens can be fashioned from household appliances such as toasters or microwave ovens that use infrared rays as the source of heat. Drawbacks of this method, however, include the inability to set up a proper profile or establish cooling methods. Homemade reflow ovens satisfy simple, smaller jobs and are not adequate for ongoing, larger projects.

Applications

Reflow soldering is often used to attach surface mount components to a circuit board. In addition, it can be used for through-hole components (THT), although wave soldering can be simpler and less expensive for pure through-hole boards. A reflow oven, however, can help reduce the assembly costs of boards containing a mix of both through-hole boards and surface mount boards. It aids in the protection of electrical components, removing the potential for damage caused by overheating during the soldering process.

Profiling

Thermal profiling establishes techniques that prevent defects in the final product. The term describes the act of measuring multiple points on a circuit board to determine the ideal rate of heating and cooling. The measurement of thermal temperatures inside an oven includes a complex set of time and temperature data. This data defines profiles referred to as ramp-soak-spike or ramp-to-spike. Ramp, soak, and spike suggest the temperatures needed to change the structure of solder and the time it takes to occur.

Weaker joints form as a result of improper profiling. Inadequate temperature settings can damage components by heating and cooling at a rate that is too high. Ensuring proper reflow oven profiling can help to avoid costly production problems.

Standards

A reflow oven is the ideal choice for several soldering needs. Optimum results can be achieved by identifying all aspects of the product characteristics, including component types, solder and board requirements, volume, pace, size, and thermal necessities.

To help in choosing the reflow oven that appropriately meets your needs, the following document describes the practices and requirements for the manufacture of soldered electrical and electronic assemblies: IPC/EIA J-STD-001

 

Video credit: Manncorp

 

Image credit: 

Heller Industries Inc. / CC BY-SA 3.0