Selecting Surface Mount Packaging Formats for Digital Environmental Sensors
Selecting surface mount sensor packages requires balancing land-pattern mechanical stress and reflow limits against bus integration effort and unit yield cost.
Voiding Ratio
Quality Metric
Area measurements of trapped gas within a metallic bond provide an assessment of the structural integrity of hidden solder joints. This voiding ratio is commonly used to assess the quality of bga and qfn attachments where the solder is hidden beneath the component body. A high value suggests potential issues with thermal conductivity or mechanical strength.
X-ray inspection is the standard method for calculating this value after the reflow process.
Calculation Method
Image processing software analyzes the grayscale density of an x-ray image to distinguish between solid metal and air pockets. To determine the voiding ratio, the area of all identified voids is summed and divided by the total area of the solder pad. The result is expressed as a percentage.
Modern inspection systems can automatically calculate this for every joint on a board and flag those that exceed the programmed limit.
Acceptance Limit
Industry standards such as IPC A 610 set the maximum allowable amount of empty space for different classes of electronics. For many applications, a voiding ratio of up to 25 percent is considered acceptable, though specific requirements for power electronics or high reliability systems may be stricter. Large voids located near the interface between the pad and the solder are more critical than small, dispersed bubbles because they interfere with the primary path for current and heat.
Evaluation of these limits determines whether a board requires rework or can be shipped to the customer based on the intended environment and safety requirements of the final product. Verification of the bonding area through cross-sectional analysis provides a secondary method to calibrate the automated x-ray inspection system.
Careful Control
Careful control of the choice of solder paste and the reflow temperature profile helps to manage the formation of these bubbles. Modifying the soak time or using a vacuum reflow oven can significantly reduce the voiding ratio in dense packages. Consistent monitoring of this metric helps to identify drift in the assembly process before it leads to a drop in yield.