Reflow Technologies for Reducing Solder Voids

Why Reduce Voids?

Reducing voids in solder joints can improve both manufacturing yield and product performance. Voids occupy part of the effective soldering area and can increase current density in the remaining conductive region. They also reduce the heat-dissipation capability of the component or solder joint.

• Improved thermal dissipation in components and solder joints, since voids can increase current density and impede heat flow.
• Improved long-term solder-joint stability and reliability under thermal dissipation, vibration, and shock conditions.
• Potential performance improvements in high-frequency applications.

Vacuum-Assisted Reflow Soldering

• Vacuum-assisted reflow has been shown to reduce solder-joint voiding by up to 99%.
• During the liquidus stage of the soldering process, pressure is reduced to 5–20 Torr.
• When the vacuum is applied, existing voids escape through the liquid solder.

The mechanism can be understood as follows:

• As pressure decreases, the volume of trapped gas bubbles increases.
• Larger bubbles are more likely to collide with other bubbles and eventually reach the edge of the liquid solder, where they can escape.
• The greater buoyancy of larger bubbles accelerates their movement and makes escape more likely.

Standard Reflow and Vacuum-Assisted Reflow

The process sequence for vacuum-assisted reflow can be represented as:

Printed solder paste → reflow soldering with liquid solder → vacuum application during reflow → cooling with solidified solder

The pressure inside a trapped bubble changes according to the Young–Laplace relationship. The relationship between surface tension and bubble radius affects the pressure required for a bubble to expand or contract.

In this relationship, γ represents surface tension and r represents the bubble radius. The bubble size can then be determined from the bubble pressure using the ideal-gas law.

Convection Reflow with an Integrated Vacuum Module

• A vacuum module can be inserted directly into a convection reflow oven line.
• The module is positioned immediately after the reflow peak, in the liquidus region.
• Infrared heating is combined with the vacuum function to maintain the liquidus condition after the vacuum is applied.
• The convection reflow process with an integrated vacuum module operates continuously and allows the thermal profile to be transferred directly from a non-vacuum reflow application.
• Continuous operation helps reduce operating costs and increase hourly throughput.

Inline Vacuum Reflow System Layout

Vacuum Reflow Test Conditions

• The intended vacuum time was 60 seconds, but the setting was mistakenly configured for 120 seconds.
• Chip size: 7 mm × 10 mm, square package.
• Liquidus peak temperature: 250 °C.
• Alloy: SAC305, with a melting point of 217 °C.

Test 7: No Vacuum

Test 1: 5 Torr for 30 Seconds

Worst-case void ratio: 0.21%

Stencil Design Result 2

Comparison of a QFN "MLF 100" package processed with and without vacuum.

Type 2: no vacuum; vacuum condition: 5 Torr for 60 seconds.

Vacuum Reflow Void-Reduction Summary

• Vacuum-assisted reflow can be implemented by adding a vacuum module to a reflow oven line.
• Vacuum-assisted reflow with convection heating uses a continuous thermal profile, supporting lower operating costs and higher hourly throughput.
• An advanced pump package with a high pumping speed can provide rapid evacuation.
• Recent customer demonstrations showed a tenfold reduction in voiding and achieved a requirement of less than 1% total void area.
• Only 15 seconds of reflow under vacuum was required to achieve a total void-area ratio below 1%.
• All tested pressures below 20 Torr met the requirement of less than 1% total void area.

Void-Reduction Reflow Technology 2: Ultrasonic-Assisted Reflow

Overview

An ultrasonic solder-void reduction solution applies ultrasonic energy to a printed circuit board during solder reflow. The ultrasonic energy is applied while the solder is in the liquid state.

• Cavitation is theoretically intended to stretch voids and bubbles so that they combine with other voids and reach the outer surface of the solder, allowing the trapped gas to escape.
• During the compression phase, cavitation can impact bubbles and break them into many very small bubbles.
• Ultrasonic energy also causes bubble movement and mixing, which promotes coalescence and subsequent removal.

Ultrasonic Workstation in a Convection Oven

Ultrasonic Reflow Thermal Profile

Sample: 4

Vibration duration: 10 seconds

Soak time, 160 °C < T < 180 °C: 60 seconds

Time above melting point, T > 217 °C: 70 seconds

Peak temperature: 238 °C

Total time: 200 seconds

Ultrasonic activation time: 10 seconds 

Void Results Without Ultrasonic Activation

Sample 3: total void ratio = 25.18%; maximum void ratio = 4.14%.

Sample 2: total void ratio = 13.16%; maximum void ratio = 2.82%.

Sample 1: total void ratio = 14.03%; maximum void ratio = 2.08%.

Void Results with Ultrasonic Activation for 10 Seconds

Sample 3: total void ratio = 4.26%; maximum void ratio = 0.15%.

Sample 2: total void ratio = 3.77%; maximum void ratio = 0.13%.

Sample 1: total void ratio = 4.53%; maximum void ratio = 0.58%.

Ultrasonic Reflow Void-Reduction Summary

• Applying ultrasonic energy to the PCB after the solder reaches the liquidus temperature has been shown to significantly reduce both individual void size and total void percentage.
• The ultrasonic energy is applied at a very low level and for a short duration, such as 10 seconds with micrometer-scale displacement.
• An ultrasonic coupling station can be integrated into a convection reflow oven to promote solder-void reduction.