Key Advances in Void Reduction in the Reflow Process

Using Multi-Stage Controlled Vacuum

This paper explores new advances in the reflow soldering process includingvacuum technology and warpage mitigation systems.

The first topic for discussion will be the implementation of a vacuum processdirectly in a conventional inline soldering system. The paper discusses thesignificant results that have been achieved in reducing voids to < 1%.

Another key area to be discussed is the maximizing of the unit per hour (UPH) ofthe system while still achieving the void rate reduction < 1%.

Another key capability that will be explored is the elimination of solder splatterduring the process of vacuum purge down.

The second topic we will present is the mitigation of warpage on substrates or wafers. This is one of the key technical challenges facing the industry.

We will explore direct warpage mitigation as well as independent substrate/waferclamping systems as well as inline and recirculating automation solutions. 

Key Advances in Void Reduction with Vacuum Technology

One of the challenges facing the industry is the requirement to significantly reduce voids from the process.

The proven method of reducing voids has been through the implementation of vacuum chambers in conventional convection reflow machines.

Why Use A Vacuum In Reflow Soldering?

There are five ways to eliminate voids that will improve product performance.

1. Improve heat dissipation of components or solder joint structures (i.e., current density increases with voiding)

2. Improve long-term stability and reliability of solder joint against heat dissipation and vibration/shock

3. Improve chip performance in high-frequency applications

4. Maintain impedances within specification for components (e.g., power modules)

5. Mitigate or eliminate solder problems (e.g., bridging, solder splashes, etc.) at μBGAs

Advantages of Vacuum-Assisted Convection Reflow

The advantages of vacuum-assisted reflow are:

Vacuum-assisted reflow has been shown to reduce the voids in a solder joint by 99%

Vacuum pressure is reduced to 1-5 Torr during liquidous of the soldering process

Existing voids escape externally through the solder when a vacuum is

applied.

Trapped gas bubbles increase in size as pressure is reduced

Larger bubbles are more likely to collide with other bubbles and ultimately collide with the edge of liquid solder to escape

Larger bubbles are accelerated by stronger buoyancy forces making

them more likely to escape 

The pressure inside trap gas per the Young- Laplace Equation

Pressure trapped inside a gas bubble changes according to Young-Laplace Equation

Pbubble = Pambient + 2g / r

(where “g” is surface tension and “r” is the radius of the bubble)

The ideal gas law using Pbubble determines the actual bubble size. 

 

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