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EMS providers can save money and time at the FAI process step. FAI is a critical step in the assembly process, yet it is prone to errors, little documentation, and time wastes. To better support FAI, a new class of equipment enhances existing manual FAI, reducing inspection time, digitally capturing more information, and ultimately improving quality.
By Vinny Carriagan, YesTek Ltd and Frank Silva, FocalSpot Inc.
EMS providers always seek improved margins, faster builds, and 100% quality for end customers. No matter the size of the EMS provider, they perform one operation virtually the same way each time: first board or first article inspection (FAI), carried out by the QC department. First article inspection is mostly a manual process.
In North America and Europe in particular, the SMT process has shifted to high mix/low volume (HMLV), which forces more changeovers per shift. More FAI within this environment increases time bottlenecks. Especially in smaller EMS shops, orders are often characterized with extremely low-volume builds on the order of 25–75 boards. These boards also must be inspected post-assembly.
EMS providers set up machines to build each customer's boards. Customers often provide CAD data or data (such as Gerber files) derived from the CAD files. They may also provide a bill of materials (BOM). This data often arrives in native file format and must be put through a data translator before it is used for parts ordering, printer set-up, etc. There is plenty of room for error in this environment. One key aspect of the part data is rotation, as the supplied data does not state whether the coordinates supplied are clockwise or counterclockwise, which often results in polarity devices being placed 180° out.
First Board Inspection Process
Today, the first article inspector is armed with a 10–20× magnifying lens with ring lamp, several color highlighters, and printed copies of the board layout or assembly drawing and the BOM. Inspectors check every part on the first board for whether the part is present or absence, correct part presence, and if polarized parts are in the correct orientation. Surface mount devices (SMD) are laser-marked with resistor values or part numbers that don't always match the BOM's specs, leading to endless scrolling through databases for part verification.
Figure 1. Benchtop first article inspection machine.
This task must be completed as quickly as possible, since the line sits idle until the operator certifies set up is 100% correct. A board with 300–400 parts may take 30–45 minutes to check manually; boards with over 1000 parts can easily consume 90 minutes at first board inspection.
At the end of the FAI process, the operator signs off an internal document passing the first article. The remaining boards can be built and samples inspected in accordance with the quality control required.
First Article Inspection Systems
EMS providers can save money and time at the FAI process step. FAI is a critical moment in the assembly process, yet it is prone to errors, little documentation, and time wastes. To better support FAI, a new class of equipment has been designed to enhance the existing manual first board inspection process, reducing inspection time, digitally capturing more information including a full picture of the board for later traceability, and ultimately improving quality. Comparably, this class of equipment is half the price of existing benchtop AOI machines.
First article inspection machines are scanner-based versus camera-based (AOI is largely camera-based). A scanner images the entire board without moving it or stitching together overlapping frames to build up the full board picture.
Figure 2. Board assembly file editor.
Unlike manual FAI, where operators do not make an image of the board, scanners save a picture of the entire PCB. Scanners are simple; with image resolution up to 1200 dpi, they allow sufficient inspection resolution for basic validation of 01005s. New-generation canners have digital zooming functions, which improve image quality and avoid fatigue associated with the 20× magnifier. New-generation scanners typically allow for a maximum scan area of 11 × 16". A board this size would take less than 60 seconds to scan and around 20 seconds to inspect at 600 dpi, or 120 sec to scan at 1200 dpi.
FAI equipment capabilities include confirming presence/absence of parts down to 01005 form factors; performing polarity checks; correct/wrong part checks; inspection of tombstoned, billboard, or crocodile off-position parts; and solder bridge inspection.
Part of the FAI process is comparing the BOM file used for procurement/prep with the placement machine file used to mount the parts to the PCB. A text file or Excel file of both the placement machine centroid file (x position, y position, rotation, reference designator) and BOM allows cross-checking data, which can then be merged into a single board inspection file. Figure 2 shows a typical interface of importing CAD or placement file data and the assembly bill of materials.
First Board Inspection Mode
The first board inspection mode (Figure 3) allows the operator to step through each part, one by one. The system displays the PCB scan, with its full description (Ref ID, part number, manufacturer, part type, and part description), including a magnified picture of the part for easy visual identification. This is similar to the existing manual process, except that the operator does not search the board for each reference designator and check it off the inspection sheet after visually validating the condition of the part under a microscope.
AOI mode is more advanced. This mode can help significantly reduce FAI time. It differs from the FAI step-through mode as the operator now draws a box around each unique part number (once) to validate the part. If that part is marked otherwise, the box is drawn around the body of the part such as a 0402 resistor or capacitor (Figure 3). All other similar part numbers are then automatically tested simultaneously.
Figure 3. First board step-thru mode.
AOI mode uses template-matching technology versus typical algorithm-based methods to test the part. Template matching is a simpler method; it provides less diagnostic capability than traditional AOI methods, but avoids lengthy set-up time and higher-level internal organization skill sets.
Performing FAI generates inspection routines automatically. The FAI process is simply drawing a box around the component part number if that part is labeled, or drawing the box around the body of the device such as 0402 capacitor or resistors (Figure 4). A more advanced system can test step-through each part faster or automatically once one part number is verified. The remaining part numbers, if they are good, are automatically inspected using template-matching technology to test the remaining identical part numbers. If there is a discrepancy with the part, inspection stops and operators are prompted to validate or fail the part. Libraries of these part pictures can then be set up for customers that use the same part numbers across multiple designs.
After FAI process completion, once each part number is tested, a complete inspection routine is set up, allowing a fully automated program to be run on the remaining boards produced. This is similar to a more complicated AOI machine's function. Typical inspection time for assemblies with more than 1000 parts is less than 30 seconds.
Figure 4. Generating a FAI routine.
Typical camera-based AOI programs have multiple inspection variables to set up and this process can take hours, often days, with the aid of a golden board. AOI machines and AOI inspection programming processes are not suitable for first board inspection and verification. To date, a gap has existing for electronics manufacturers who need to check the first board on the fly with the aid of digital image processing.
There is no centroid or bill of material data required to set up this inspection mode. FAI is performed by simply scanning a board, identifying the fiducials for alignment, setting a fixed grid of rows and columns (field of views), and capturing the picture of the complete board. This step typically takes less than 5 minutes. Comparator mode works by just scanning in the first board – which may be a bare board – and identifying it as a golden board. This PCB is stored in the system's memory, where it is compared to the first assembly board. This is a manual inspection process and the operator is required to step through each field of view and manually inspect parts for pass/fail. Superimposing the two images on one display enhances the operator's ability to locate differences quickly.
Contrast this with the manual method of looking at just the PCB under the microscope, where an operator has nothing to compare to the physical PCB. What's unique about comparator mode is that operators can quickly and easily identify cosmetic issues on both the component and board, which a traditional AOI machine would not find. SMT
Vinny Carrigan is president of Yestek Ltd, a manufacturer of FAI systems. Frank Silva is VP of sales at FocalSpot, a manufacturer of X-ray inspection systems and distributor of inspection and rework systems.