A Guide to Successfully Using Vacuum Reflow Ovens in SMT Soldering

2023-08-29

In recent years, the use of bottom-terminated components has increased significantly, with some estimates putting the number at over 50 billion per year. This has raised concerns about minimizing voiding between the thermal pad and the BTC. Over 40% of the thermal pad area can experience solder voids after reflow, increasing the risk of overheating the BTC in the field.

As a result, vacuum reflow ovens have evolved. While these ovens virtually eliminate voiding, incorrect use can result in solder splatter/splash. Below are some critical recommendations to successfully reduce both voids AND solder splatter:

By reducing the pressure in the oven to about 200 mbar (1 atmosphere is 1000 mbar) there is an increased risk of solder splatter. Therefore, minimizing the slope of the slope from 1000 to 200 mbar will help to reduce solder splatter in the oven.
After the solder becomes molten, the vacuum should be pulled for about 30 seconds. This parameter is important as some fluxes require more time and others require less time. The time delay before pulling the vacuum maximizes the chance of solvent volatilization and prevents solder spatter.
The vacuum should be kept on as long as possible after the solder has solidified. Some ovens have only one vacuum chamber in the peak area, so the vacuum should be kept on until the solder is solid.

There have been cases where people have mistakenly believed that splatter from solder was due to the stencil printing process in some cases. A stencil under-wipe process or a board gasket that is not properly attached to the stencil can result in solder being transferred to the board in areas not meant for it to be transferred. It can appear as solder splatter after reflow if the under-stencil is not properly cleaned and the board gasket is not properly sealed. To avoid this defect, ensure that the board is properly gasketted and the under-stencil is properly cleaned.

FAQ

What is a vacuum reflow oven?

The vacuum reflow soldering was originally conducted in batches. Boards were placed in a vacuum chamber and heated to melt the solder. After the board was heated, the chamber was evacuated and the void level was reduced or, in some cases, removed from the solder joint.

What is the vacuum reflow process of soldering?

You need to let the solder wet for a few minutes as it reaches the melting point, after which you want to apply the vacuum to the solder. When everything is molten, the vacuum will draw out the voids in the solder, and once you release the vacuum, you'll have the void free solder joint, and that's when you can begin cooling it.

What is the difference between soldering and reflow soldering?

Soldering and reflow soldering are both methods of joining components in electronics, but they differ in their processes and applications:

Soldering:

Soldering involves heating a filler material (solder) to its melting point and applying it to the joint to create a connection. This is typically done with a soldering iron, torch, or other hand-held tools. Soldering is used for various applications, including electrical, plumbing, and jewelry work. It's suitable for smaller-scale projects and allows for precise control over the solder application.

Reflow Soldering:

Reflow soldering is a technique mainly used in electronics manufacturing, especially for surface mount components on printed circuit boards (PCBs). It involves applying solder paste, which contains solder particles and flux, onto the PCB pads. Components are placed onto the paste. The entire assembly is then heated in a reflow oven, which melts the solder paste to create connections between the components and the PCB. Reflow soldering is well-suited for high-volume production and provides consistent and reliable solder joints.

Key Differences:

Application: Soldering is a general technique applicable to various fields. Reflow soldering is specialized for electronics assembly, particularly for PCBs with surface mount components.
Process: Soldering involves manually applying solder to joints using tools. Reflow soldering uses pre-applied solder paste and controlled heating in an oven.
Components: Soldering can be used for various sizes of components and materials. Reflow soldering is ideal for small, surface mount components on PCBs.
Control: Reflow soldering provides more precise control over the soldering process due to controlled heating profiles in the reflow oven. Soldering can have more variability depending on the skill of the operator.
Automation: Reflow soldering is commonly used in automated electronics manufacturing, while traditional soldering can be manual or automated.

In summary, soldering is a broader technique for joining materials using solder, while reflow soldering is a specialized process used in electronics assembly, particularly for surface mount components on PCBs.

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