Optical Measurement Techniques
How does Akrometrix measure warpage?
Akrometrix uses three optical full-field techniques to measure warpage: shadow moiré, fringe projection, and digital image correlation (DIC). The best choice for a particular application depends on a number of factors, but, in general, shadow moiré works best for medium and large samples, fringe projection for smaller samples (< 10 mm), and DIC where in-plane strain as well as out-of-plane warpage are required.
What size samples can be measured?
We routinely measure warpage on samples from 2 x 2 mm up to 600 x 600 mm
How sensitive are the measurements?
In our equipment, resolution is generally on the order of 1 micrometer for samples <=25 mm for all three techniques, which also sets the lower limit for accuracy and repeatability. Resolution becomes larger as the sample size increases, especially with fringe projection and DIC where it is a linear function of the field of view. Shadow moiré resolution is constant (2.5 microns or better) regardless of sample size.
What is the imaging resolution?
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 1.3 Mpixels and a field of view 17 mm wide, imaging resolution for fringe projection is approximately 13 micrometers. For a 600 mm wide sample measured with shadow moiré, the value grows to half a millimeter. For DIC, imaging resolution is typically an order of magnitude larger. Akrometrix has a variety of camera and lens combinations to optimize this value for a particular operation.
How long does it take to acquire and analyze data for a measurement?
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.
How is the data presented?
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 two- and three-dimensional graphical formats. It can further analyzed to extract coplanarity values, strains, or other derived parameters, depending on the software. In our production test systems, it can be used to generate a pass/fail signal relative to a predetermined threshold.
How is the sample heated and cooled?
We have equipment for heating the sample by infra-red radiant elements or forced air convection. Cooling is by forced air convection using either room temperature air or chilled air.
What is the available temperature range?
Normal operation is from room temperature to 300 C. With our Convection option, temperatures down to minus 55 C are possible.
How fast can the sample be heated or cooled?
This depends very strongly on the sample size, thermal capacity, thermal conductivity and other properties. Typically we can achieve 1-3 degrees C per second, with the heating/cooling rate getting lower as the extreme temperature limits are approached.
If I use convective heating in the real process, should I use it in the warpage measurement?
The Akrometrix approach is to try to duplicate the effects of the heating/cooling cycle we are trying to simulate (including heating rates and uniformity), not the causes. 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.
What kind of temperature profiles can be run?
Akrometrix temperature profiling capabilities are very flexible, allowing for a wide range of simulation opportunities. Dynamic temperature profiling is a core capability of all our systems. Dynamic profiling enables both manufacturing and operating environments to be very accurately reproduced in the instrument. Reflow processing, cure cycles, harsh environment exposure, reliability temperature cycling - each are examples of the system capabilities.
How much warpage is too much?
This is an active area of study in the industry. Individual companies and industry standards committees are trying to set specifications based on empirical experience and theoretical calculations, taking into account sample dimensions, pad pitch, and other factors. We'll keep you informed to the best of our abilities on the state of the art, through our regular website features.
Can multiple samples be measured at one time?
Yes, as long as all the samples fit within the camera field of view. Akrometrix software is designed to crop out individual areas of interest at the analysis stage. 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.
Can you measure solder bump coplanarity? Solder paste volume?
Yes, but the very fine imaging resolution required to characterize sub millimeter features implies a very small total field of view for the measurement. Akrometrix has extensive experience at making such measurements during thermal cycles, but does not offer a product designed for high volume inspection of these features in a production environment.
Can you measure the height of a component relative to the substrate?
With shadow Moiré and fringe projection, there are limits to the size of a discontinuous step that can be unambiguously characterized. These techniques rely on "fringe counting" along a continuous path across the surface and abrupt discontinuities can create limitations in the analysis. Digital image correlation (DIC) does not have these particular limitations, but may not be appropriate for all applications.
Can you measure glass? Ceramics? Silicon wafers?
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.
Can you measure coefficient of thermal expansion (CTE)?
Yes, with DIC, we can measure in-plane strain over temperature intervals up to 300 C, so CTE values are readily calculated. The normal in-plane strain resolution of 100 micro strains translates to a precision of +/- 1 ppm/deg C over a 100 degree interval.
Can you measure stress?
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.