Amfax delivers a new generic ATE for testing aerospace pilot display unit modules. The system enables testing of all module types using the same tester
Amfax have recently designed, built and delivered core PXI based automated test systems to a major aerospace manufacturer. The brief was to come up with a design that could test all the pilots display unit (PDU) modules using a single automated tester. Previously the client had a number of ATE's that each tested just one module. This meant that previously the client not only had to invest in multiple testers but had to obtain support for each unit separately. The ATE used a fully loaded PXI rack to house all of the instrumentation needed to ensure test coverage of all the measurements required for each module. The modules tested included a solar cell, electronic display, interface unit and auxiliary PCBs.
Challenge: To design and develop a revolutionary 3D measurement based Printed Circuit Board Assembly (PCBA) inspection technology to help companies improve the quality of their product.
Solution: Combining the benefits of the NI Compact RIO FPGA platform and the user interface qualities of LabVIEW helped Amfax develop the world’s most accurate PCBA inspection system enabling OEM’s and CEM’s to significantly reduce their lifecycle costs.
Amfax, are a global Platinum Alliance Partner of 23 years who specialize in designing and manufacturing test engineering solutions for our customers worldwide. Our extensive customer base includes blue chip companies in the aerospace, rail, transport, energy, telecommunications and consumer electronics market sectors.
The challenge we have addressed by developing the a3Di PCBA inspection system is that many PCBA manufacturing companies were struggling to solve a problem of accurately and repeatedly inspecting their PCBA to ensure correct component placement and good quality solder joints to improve reliability in use.
Historically, humans have been used to visually inspect PCBA’s at the end of the production line. This is prone to poor repeatability and consistency of inspection and as such many companies have switched to Automated Optical Inspection (AOI) systems.
These automated systems use cameras in place of the inspectors to capture images of the board and then subjectively compare these images against a reference standard to check if the PCBA has any faults. Any difference between the image and reference standard is flagged by the AOI as a potential failure. The local system operator must then make the final decision as to whether it is a “real” defect or not. There is always a tradeoff in these systems between generating potential failures that an operator reviews, which are known as false fails or relaxing the rules and allowing false passes. Both incur additional operating cost and can potentially affect the products quality.
These systems certainly have their place as they provide significant benefits over the human inspection method and have been the system of choice for PCBA inspection for a number of years.
The team at Amfax felt there must be a better, more reliable and ultimately more cost effective solution to the inherent problems associated with AOI systems, so set out to design and build the Amfax a3Di.
What makes a3Di unique?
The Amfax a3Di system takes a completely different approach to address the challenges raised by AOI systems. It users a twin metrology based measurement technology to take millions of measurements with accuracies of under 5 microns to build a compete 3D representation of a PCBA or any other object that can fit inside the a3Di. The system scans a PCBA in a matter of a few seconds and the measurements are tested, one to one, against the original CAD dimensional data to determine whether the board has any problems relating to the component or lead location, orientation and size. Solder joints are also reviewed for shape and volume and can be matched to IPC Class 1,2 and 3 recommendations. Uniquely it measures the complete PCBA and thus can find foreign objects between packages or excessive board warpage on the PCBA that can prevent the PCBA mating with its housing at a future stage.
As a3Di is performing real measurement testing, there is no need for an operator as there are no false calls, either the board passes the test or not. That is the advantage of testing against real 3D measurements instead of relying on a comparative methodology such as AOI.
The result is zero false calls and no subjective decisions needing to be made by an operator. This means that the users of a3Di save the cost of operators, increase product throughput and more significantly improve their product quality.
NI cRIO – The heart of the machine
The heart of the a3Di is the control system used to control all aspects of the machines operation. The a3Di was a brand new design for Amfax, so as a Platinum Alliance Partner our first thought was that we must use cRIO. The control system chosen for A3Di is a NI cRIO system using FPGA and the NI 9375D I/O hardware.
The performance of this cRIO solution is staggering as it is controlling all of the following I/O and sensors on the a3Di.
Optical position sensors
Up and Downstream SMEMA control
Operator manual controls for width PCB control
System side EStop notifications
The cRIO has proven to be a completely dependable, reliable and cost effective solution for this high performance, ground breaking application. Using cRIO as the product management system significantly reduced our development time and helped us get the various autonomous state machines of the multiple product control cells run with far tighter timings than the normal 1ms tick of most PLCs.
During development, our developers, who are CLD and CLA level engineers, partnered with the technical support teams in the local NI office in the UK who helped the team overcome some of the detailed control issues raised in the systems design.
LabVIEW – The obvious choice for user ergonomics
A decision was taken early in the a3Di product specification phase to use LabVIEW to not only provide the control code but to control the system from the user interface perspective also. The ability to design product quality operator interfaces and the flexibility of LabVIEW for creating an engaging user interface environment for the operator makes the software front end of a3Di a unique selling point.
The impact of using NI components within the a3Di product enables Amfax to offer a world class, unique and well supported solution to those OEMs and CEMs looking to improve their PCBA assembly inspection process and significantly reduce their operational costs.
The a3Di is also revolutionizing the way PCBA manufacturers can now compete for their customers business. By using a3Di, these manufacturers have a unique selling proposition to their own customers. They can pass on savings made by using a3Di and guarantee that the boards being manufactured are being tested by the most accurate system available to do the job.
The importance of this exciting technology has not gone unnoticed, with the British Governments innovation department recognising a3Di as a significant step forward in manufacturing. The Amfax a3Di was a finalist at the 2016 Innovate UK awards hosted at the British Houses of Parliament.
Requirement: To develop and manufacture an automated end of line inspection system for the fault recognition of the wire button style connectors.
Our solution comprised of a high-resolution vision (camera/lens) system, mounted on a precision X, Y motion stage set-up. A plinth was integrated along with special to type fixtures, very accurately locates the connector to be scanned. The system was housed in a light-tight enclosure that contains a series of controlled light sources, to negate the effect of ambient light on the inspection process. A PXI controller/chassis combination provided PC based control of all aspects of the solution.
Integrated motion tools allowed us to scan the vision system over the area covered by the connectors, at the very short focal length required (approx. 12mm from the connector surface).
The motion system scanned a pre-determined pattern of steps in X & Y, and the vision system acquires an image at each step. The area captured by the vision system enabled testing of a minimum pattern of 16 buttons/plungers, in a 4 x 4 pattern. Further development allowed us to capture a larger image and reduce total inspection time.
The image is then transferred to the vision software and processed against a pre-programmed set of pass/fail criteria, the results of which are buffered until all areas have been scanned. When complete the overall result, Pass or Fail, is displayed as a banner on the system display.
The system was based on the PXI platform from National Instruments. A 4 slot PXI chassis houses an embedded PXI-8174 controller, PXI-1422 iMAQ vision card, PXI-7344 Motion control card and a PXI-6508 Digital I/O card for interlock monitoring etc.
The software was developed using LabVIEW, the Vision and Motion modules for LabVIEW and the Motion assistant software tools, all from National Instruments.
The motion system wasconstructed from two 150mm (300mm optional) precision motorised translation stages, mounted to provide travel in both X and Y for the camera/lens system. The stages had a resolution of 10um, which was more than sufficient for this application. A 20mm manual Z adjustment was also provided. The camera selected was a high resolution digital, colour model with suitable lens.
The software algorithm developed is split into 7 main sections. The 7 sections are as follows:
The function of this part of the algorithm simply extracts a 2D array from the 1D argument array being passed in by the controlling software. These arguments include which pins must be tested, and their names.
Processing of half lit image
This section of the algorithm takes an image of the connector lit with the red led array only. Its function is to isolate the wells on the image, and formulate an array of the wells’ coordinates and radii, which is correlated with the test array formulated in the previous section of the algorithm, ie a sorted array is passed out from this function.
A general error condition is raised if the algorithm is unable to isolate a full row or a full column on the grid of wells, as this would mean that there is no reference with which to sort the array.
The algorithm is capable of making multiple passes of the image to obtain a sorted array, so if the first thresholding value does not yield the correct number of wells, the algorithm will store those values it has already got, reduce the threshold value and try again up to 5 times.
This image shows the ideal illumination for the image lit with the red LED array only.The illumination yields a smoothed histogram which should look like this:
The two maxima must be clearly defined, for a valid thresholding value. This shape of graph is important because the two maxima toward the left of the graph represent the well areas and the substrate area repectively, so the algorithm will threshold somewhere to the left of the 2nd Maxima.
Processing of fully lit image
In this section of the algorithm, the image with both LED arrays illuminated is used. The image should look like this:
Compare the peak to the peak to the right of the histogram with the half lit histogram, this peak represents potential defect material, and has been enhanced with the second LED array.
The Algorithm uses the histogram graph to obtain a new threshold value which falls to the left of the large peak to the right of the histogram graph. A particle filter is introduced also which ignores any particle below a certain threshold. This is a critical point in the code, as the value chosen for this area threshold will drastically affect the results of the tests. The default setting is 90 pixels.
This section of the algorithm passes out a binary image from the above threshold / filter operations for the test part of the algorithm.
This test is for wires appearing outside of the well. A ROI is generated above the well in question using the data in the output sorted array from earlier in the algorithm and a small image extracted from the binary image generated in the fully lit image processing. Each pixel is analysed, if the pixel is high, then its distance is measured from the centre of the well. If that distance is greater than the radius of the well then a variable area is incremented. Each time this variable is incremented then the value of area is checked against a set value area threshold. As soon as area is greater than or equal to area threshold, the pin fails and the test terminates.If this condition is not met then the pin passes.
The critical areas here are:
This test uses the same binary image as the previous test, and is testing to see if there is a pin present in the well, and does this by looking for high pixels within the radius of the connector which form a single blob. The area of this single blob is measured against a constant value. If the area exceeds the value then the test passes, and fails if not. The current setting for the area is 1500 pixels.
Display and report
This final section of the algorithm overlays test results onto the image, and compiles a report to pass back to the calling vi.
The system met all system requirements. The use of the LabVIEW vision and motion tools enabled a much faster and reliable test solution than the old manual test procedure. Additional software has been added to improve the feature set for the customer who is extremely pleased with the result.
Our recent work
We provide regular updates on the latest projects we have been working on. Check out our archive to view all of our case studies and user solutions.