Donna Bibber, Technical Partner, microPEP, East Providence, RI, USA
Micro Manufacturing is a $36 billion (£18 billion) market and is forecast to grow to a $68 billion (£34 billion) market in 2010. Medical, electronics, industrial, military, aerospace, automotive, and microfluidics market sectors are demanding critical applications such as implantables, pressure sensors, and control modules. If such products fail, the consequences can be catastrophic, even potentially life threatening.
Because of the critical nature of such micro component applications, component tolerances require precise inspection equipment to properly validate them.
Due to the microscopic size of micro components, non-contact measurement is an economical and tactically feasible method to measure them. Systems such as measuring microscopes, vision measuring machines, and optical digitisers and scanners, or a combination of these systems, are necessary to properly validate micron size and tolerance components having both 2D and 3D shapes and features.
Measuring Microscopes
Microscopes are used for dimensional measurement. Measuring microscopes have a precise measuring platform and provide bright and sharp images based on the magnification and resolution level of the object under them. Microscopes come with monocular, binocular, trinocular, and dual head eye pieces, and many include digital displays, mechanical stages, fine focus, and embedded software. Such equipment is typically used for the inspection of micro electronic parts like integrated circuits, and also for the analysis of micro structural samples and assemblies.
Vision Measuring Machines
Vision measuring machines consist of a wide range of products including video-based systems, expanded-pupil systems, and multi-sensor based systems. Video-based systems rely on video edge detection methodology for accurate measurement of parts. Increased speed of motion, edge detection techniques, optics, and charge coupled device (CCD) cameras provide higher throughput. In simple terms, magnified images of the part are analysed to determine dimensional characteristics. Multi-sensor measurement machines are aimed at measuring all the critical features and dimensions of the part in a single set-up. These machines typically use two or more sensors to completely measure parts. Sensors used may include vision sensor, laser sensor, or a touch probe sensor such as the ones used in CMM. Hence, in certain cases, touch probe sensors — which do not necessarily belong to the non-contact category — are being used.
Metrology Issues
Gauge repeatability and reproducibility — Gauge R&R — is a term that is often associated with measuring and validating micro components. In the micro industry, we often hear the phrase “you can’t make it if you can’t measure it”.
Very often, micro components have micron level features and tolerances. Because it is difficult to get a repeatable measurement to micron level, the gauge
measurement error usually represents a large portion of the tolerance being measured. This condition is a reality and presents validation challenges, because remember, “you can’t make it if you can’t measure it”. Slight improvements can be made to measurement errors with extremely accurate fixturing and automated systems.
New Developments - Time Saving, Accuracy
Laser scanning CMM. (See Figure 1). Another way to eliminate gauge measurement error is to 3D scan the micro components. This technology is fairly new, the equipment extremely accurate (to less than 1 micron), and it exports the measurements into point cloud data form that can be compared to the original computer solid model. The difference between the point cloud data and the solid model can be reviewed visually using colour.
CT Scanning. New developments in CAT scanning equipment allows for cellular-like structures to be measured. CAT scans have been used in medical science for many years to investigate in-vitro issues using x-ray technology. CT scanning is now being used to obtain “slices” of measurements, and again building a point cloud to compare to a solid model. This is a “next generation” technology for deciphering the “guts” of micro manufactured components. Stay tuned for more on this technology as it is introduced to the market.
Measurement Equipment
Micro manufacturing companies ill-equipped in metrology find themselves in the awkward position of waiting for information from outside sources to get answers to processing questions.
In our fast-paced world, however, immediate data concerning the validity of the process products being produced is required to feed back into moulding machines or stamping machines. Having a well-equipped metrology lab near your moulding, stamping, or optics process allows continuous process improvement to occur quickly and accurately.
A well equipped metrology lab consists of a variety of optical CMMs, laser interferometers, spectrometers, laser CMMs, laser micrometers, and profilometers for surface finish, optical surfaces, accurate dimensional analysis, and validation protocol adherence.
A metrology lab with sub-micron level measurement requires environmental controls such as +/- 0.5 degrees F (+/- 17 degrees C) temperature stability as well as humidity control to obtain repeatable and accurate measurement data.
Practical Experiences
Why is all of this important? Metrology is often viewed as a non-value added activity, but in the area of micro manufacturing, you cannot wait until the end of the project to think about measurement. Instead, it must be discussed in the quoting stage so that the proper cpk is worked through the tooling and the process to get an acceptable process window befitting of the cpk required.
One practical example of this is the measurement of biopsy forceps. Typically, these products have multiple small holes, 3D contoured shapes, and an extremely tight tolerance fit across the parting line such that the two sets of teeth mesh properly to form a good “bite” for tissue attachment. The geometry of these components has an average of 80 measurements per drawing, requiring multiple set-ups and fixturing to measure them properly and repeatedly. The conventional measurement technique can take up to two months to measure them and conduct gauge R&R studies. In a new “unconventional” method of measuring them using a laser CMM, the point cloud data approach took only two weeks to crunch all the data, compare it to the solid model, and produce gauge R&R studies automatically with internal laser CMM statistical software.
Another case study is connected with micro moulded optical lenses. Optical lenses require extreme accuracy in contoured, multi-axis directions. Measuring lenses requires the use of an interferometer (see figure 2). The error in lenses is measured in nanometers as shown in the diagram, with a wavelength error peak to valley (P-V) of 332 nanometers. Interferometer readings to 0.1 wave RMS are detectable with this equipment.
The key measurement criteria for lenses is the surface deviation, shown in figure 2 as 332.47 P-V nanometers (1 nm= 1 thousandth of a micron). This is less than half an optical wavelength (1 wavelength = 0.425 nm) and is two times better than almost all of the optical inserts available on the market.
Without the capability of metrology immediately during the steel cutting process of the inserts, the feedback to the process would be too slow to develop his technology. The interferometer is located in the lab over from the diamond turning machine to keep “noise” away from the measurement. For the metrology purists, the value is the maximum high to low deviation from a best fit radius. Generally, the target of optical inserts is 0.425 nm, so this example shows a very large improvement over what is currently available on the market.
Conclusion
It has been noted that parts can’t be validated without proper measurement techniques. Metrology to micron tolerances requires a temperature controlled laboratory as well as humidity and vibration control. Failure of micro manufactured products such as implantables, pressure sensors, and control modules can be catastrophic, with potential loss of human lives. Having the proper metrology techniques to measure them and quantify them in validation is a critical path for developing and producing micro devices.
Donna Bibber is a well-known micro manufacturing expert and the Technical Partner in microPEP, East Providence, RI. USA. microPEP provides a one-stop design and manufacturing solution for small and micro component applications. microPEP’s core competencies include micro moulding, micro stamping, micro optics moulding, and speciality plating.
