Frequently Asked Questions - FAQ
Optical Measurement Techniques
Akrometrix uses two full-field optical techniques to measure warpage: Shadow Moiré and Digital Fringe Projection (DFP). A third technique, Digital Image Correlation (DIC) is used to measure in-plane strain/CTE. The best choice for a particular application depends on a number of factors, but, in general, Shadow Moiré works best for samples with continuous surfaces, Fringe Projection for samples with step heights or small detailed features, and DIC where in-plane expansion is of interest.
We routinely measure warpage on samples from 2 x 2 mm up to 600 x 600 mm
Shadow Moiré measurement resolution is field of view invariant and ranges from sub-micron for smaller samples (<=20 mm) to 2.5 microns as sample size increases. This also sets the lower limit for accuracy and repeatability. Fringe Projection resolution scales with field of view, ranging anywhere from 2.5 for small fields of view up to 8 microns. DIC measurements are performed at a resolution of 100 microstrain with CTE resolution varying based on temperature range.
Imaging resolution relates to the lateral dimensions of the smallest feature that can be measured. This is a function of the camera resolution and the field of view. For a camera resolution of 5.3 Mpixels and a field of view of 45x36mm, imaging resolution for fringe projection is approximately 18 microns. For a 600 mm wide sample measured with shadow moiré, the value grows to 400 microns. For DIC, imaging resolution is based on multiple camera pixels and is typically around 200 microns. Akrometrix has a variety of camera and lens combinations to optimize this value for a particular application.
With full-field techniques, data for the entire sample is captured in each video frame, so data acquisition is independent of sample size. For DIC, data acquisition times are much less than 1 second. For fringe projection and shadow moiré, multiple image capture requires 1-2 seconds. Analysis typically takes a few seconds more. For example, minimum time between measurements during a temperature profile is set at 5 seconds.
The resulting data from a full-field method is a matrix of out-of-plane displacement values, one for each pixel location within the area analyzed. This matrix can be exported. It can be displayed in a variety of 2D and 3D graphical formats. It can be further analyzed to extract coplanarity values, strains, or other derived parameters, depending on the software.
Temperature Control
Sample heating is accomplished by infrared radiant elements or forced air convection. Cooling is by forced air convection using either room temperature or chilled air.
Normal operation is from room temperature to 300°C. With our Sub Room Temperature Module, temperatures down to -55°C are possible.
This depends substantially on the sample size, thermal mass, thermal conductivity, and other properties. Typically we can achieve 1 – 3.5 °C per second.
Our approach is to duplicate the heating rates and uniformity from a profile, not the heating mechanism. As a practical matter, the behavior of a single chamber convective oven is very different from a convective reflow belt oven, so focusing exclusively on the mechanism of heating can be misleading.
Akrometrix temperature profiling capabilities are very flexible, allowing for a wide range of application testing. Dynamic temperature profiling software in our thermal systems allows many and varied profiles to be accurately reproduced in our ovens. Reflow processing, cure cycles, harsh environment exposure, and reliability temperature cycling are all examples of our thermal system capabilities.
Applications
This is an active area of study in the industry. Individual companies and industry standards committees try to set specifications based on empirical experience and theoretical calculations, taking into account sample dimensions, pad pitch, and other factors. Key warpage standards are found in our Industry Standards section.
Yes, as long as all the samples fit within the camera field of view. Our software is designed around multiple sample testing and automates sample finding through Part Tracking technology. This includes multiple components in an oven going through the same thermal cycle, multiple parts in a JEDEC tray, or multiple BGA footprints on a single PCB.
Akrometrix Fringe Projection techniques can be used to measure solder bump coplanarity and solder paste height, but our software tools do not make volumetric measurements.
Yes, our DFP technique is ideally suited to this sort of application and can measure step heights up to around 50 mm. In addition, a phase bridging technique can be used to supplement Shadow Moiré measurements, provided a reasonably accurate height estimate is known.
All three optical techniques rely on diffuse light scattering from a clearly defined surface. Samples that are transparent (e.g. glass), translucent (e.g. some ceramics or polymer films), or specular reflectors (e.g. polished silicon wafers) don’t have natural optical properties compatible with these techniques. However, all these surfaces can be measured when coated with a thin layer of paint or unbonded pigment.
Yes, with DIC we can measure in-plane strain from room temperature up to 300°C. With strain measured, CTE values can be readily calculated using the change in temperature. The normal in-plane strain resolution of 100 microstrain translates to a precision of +/- 1.0 ppm/°C over a 100 degree interval.
No, our techniques measure displacement or strain. Stress may be calculated from this information with additional assumptions about materials properties and stress-strain relationships, but our software does not do this.
Have further Questions or Concerns?
Reach out to us to learn more about our top-of-the-line products and solutions. Our applications engineering experts are happy to provide you with accurate and timely information for all your Thermal Warpage and Strain Metrology needs.
