By Dick Rieley, Mid-Altantic Sales Manager, Ophir Photonics Group
A manufacturer was asked to produce a high volume of molded devices that have an <100um hole in the center through which in the final assembly a specific amount of material will pass. Since the product cannot be tested until fully assembled, any device found to have the incorrect hole size, must be rejected and reworked, thereby reducing productivity. Being able to inspect and sort out acceptable from unacceptable devices prior to final assembly can represent a significant cost savings.
Limitations of Vision Inspection
A vision inspection system, measuring both sides of the <100um hole, was tested. Although this approach could detect the hole diameter accurately, it could not detect obstructions inside the hole from the forming process. Not only was it necessary to have a hole of the correct size, but the hole needed to be free and clear of any internal obstructions.
Ophir-Spiricon, using the 4.4um pixel SP620 USB camera running BeamGage® software, was asked to test and inspect these devices to determine if the correct hole size could be detected as well as insuring the hole was free and clear of any obstructive material. For this test, Ophir- Spiricon was provided with devices that had been pre-inspected as acceptable and other devices that had been pre-inspected as rejectable.
The proposed test approach would use a stationary light source, positioned about 2” above the device, and centered over the <100um hole. The light would penetrate the hole and then diverge. The Spiricon camera was positioned beneath the device, facing toward the light column coming through the hole. The concept was that from a hole of an acceptable size, the diameter of the beam of light would be constant and repeatable. With a hole that is too large ( which was typically the failure mode ) the beam would be larger and, by precision measurement, sorted out as unacceptable. This approach was presented as a near dynamic test, i.e., light passing through the hole, similar to a material passing through the hole as in the final assembled product.
Using the pre-inspected acceptable devices, the correct diameter to the center hole -- <100um -- was established. These discs were used to establish a baseline that was considered numerically acceptable as compared to the devices that had been determined unacceptable where the diameter of the hole was too large by specification.
Figure 1. Ophir-Spiricon USB-SP620U Camera with 4X Expanding Lens Attached.
In this set up, the device was positioned face down with a light source entering the device from the back side. The following image shows the fixed positioned light source, the device, and the SP620 camera beneath the device.
Dual Measurement Approach It was determined that there were two possible measurements to better qualify a device. 1. The Ophir camera can provide a measurement on the size of the hole. 2. The camera can also measure the amount of light intensity coming through the hole.
It was recommended that this dual measurement approach would be more conclusive since the amount of light coming through the hole was more closely associated with the actual function of the device, i.e., allowing a certain amount of flow through the hole necessary to meet specification once installed in the final assembly.
Hole Size Measurement
Using the BeamGage software, the data analysis from the camera can easily measure the diameter in both the X and Y dimensions based on the light coming through the hole. This can be done with 1% accuracy.
Light Intensity Measurement Through the Hole
The BeamGage software can also measure the light intensity in the form of power (mW’s) or just a numerical counts as was used here. This approach can determine exactly how much light based on the size of the hole is coming through the hole. There is a direct correlation between beam size and light intensity measured.
Typical Measurement of Each Device
The following graphic is representative of the image and measurement values of the beam size and beam intensity from each of the discs measured.
Under the Power/Energy section, ‘counts’ of light are shown coming through the hole. This is an absolute value, repeatable and consistent. This value can also be calibrated to live power and displayed in the appropriate power values.
The second category is the Spatial data, where the hole diameter is shown using a 4-sigma measurement formula for both X and Y dimensions based on the light beam penetrating the hole.
Each of the devices was scanned. An XL chart was prepared showing the actual values for intensity, as well for the dimensional measurement of the beam. The values from the good devices were used to calculate an average; that average was compared against the actual values of the rejectable devices. Also shown was the percentage of difference off the average. In all 12 devices the data for BOTH the intensity measurement and the dimensional measurement tracked with the pre-inspection results.
Analysis of Data: Power Measurement
The three (3) good devices were used to establish a baseline of acceptability; an average of 1,791,000 intensity counts were seen. ALL of the eight (8) reject devices measured on the average as 61% greater.
Analysis of Data: Beam Size Measurement
The three (3) good devices were also used to establish a baseline of acceptability for hole diameter, giving an average of 1.515e-2 for X and 1.548e-2 for Y. Whereas the reject devices measured on the average 26% greater on the X and 22% greater on the Y.
Summary and Conclusion
Based on this limited sample of acceptable and rejectable devices, using both the intensity measurement as well as the beam diameter measurement, there was a sizeable separation between these two populations.
In a production environment, the BeamGage software allows minimum/ maximum limits to be established on measurements of this type, while offering a variety of outputs to manufacturing production control