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Process Control Strategies for EMS Assemblers
Matrix X3 Series AXI x-ray system.

Besides improper design methods, it is well documented that the Assembly setup and placement process contributes to 15 to 20 percent of the most common SMT defects, such as missing parts, wrong parts, misplaced parts, and wrong polarity. In addition, 50 to 65 percent of Board Assembly defects result from uncontrolled solder paste printing and solder formation processes which lead to defects such as opens (co-planarity, head-in-pillow), shorts, tombstoning, and excess or insufficient solder. While solder voiding is not a defect but rather a process variation, voids can lead to field reliability issues.

By using simple tools to automate the inspection process, EMS providers can achieve higher yields and less defective boards being shipped to the end user.

FocalSpot offers several production assembly inspection tools to aid in improving process control to improve yields and find defects quickly and easily to ensure customer satisfaction.

First Article Inspection
Let's start with the data necessary to build a PCB board. EMS providers struggle with collecting the proper data from the OEM to build a board and wind up manipulating data as they prepare the programs for the screen printer and placement machines. When the customer data first comes in, it often does not include a bill of material and may even lack a proper assembly drawing. EMS providers must cobble together the necessary data to quote the job in the first place and then prepare the equipment files for the assembly machines to build the product. Much can be and often is lost in this very first step. While the centroid file tells the placement machine where to place each part including its rotation, it does not normally include a part description and possibly even the part number physically marked on the component. A bill of material (BOM) will always include this information.

Now using FocalSpot's FA Inspector Software a simple software tool can be used during the quotation process or assembly data setup to check the assembly data file and compare it to the BOM. If the two files do not match, the EMS provider can quickly communicate the information internally or with the OEM to resolve these issues before building the assembly programs. If a PDF or JPG of the assembly drawing exists, the FA Inspector also has a tool to visualize the data and ensure that all the proper parts data from the centroid file, including polarities, align onto the drawing as an effective way to check component polarity and data integrity. The result of this labor is that there is a complete description of the components to be placed onto the board, including SMD values.

The Inspection Process
Every first board built goes thru a first article visual inspection process prior to completing the full run of boards whether this is a one board build or thousand boards. In most manufacturing environments this is a manual process including using hard copy print outs of the program list of each reference designator and bill of materials. Operators check every reference designator part visually by eye or using a microscope to check the board for presence/absence of parts, position of the parts, and polarity. Each and every reference designator is marked off the list using color highlighters as well, if the drawing exists. This is where it's important to check the drawing, using a printed drawing showing the reference id and polarity marks. This process is time consuming and after the fact only paper documentation of the inspection exists. On the inspection side what is often missed at this stage is whether the right part number has been mounted.

Checking part values is an important step prior to running production boards . This can be accomplished with taking parts out of the real and probing them to measure their values. This is not completely ideal but better than not checking them at all. You can also check part values on parts which are marked using cross reference lists of standard SMD components. The FA Inspector software uses a simple and easy-to-use drop-down SMD cross reference list to check against the part number. But that alone does not fully ensure that the proper part values have e been mounted. A good technique is using the sticky board with a combination of probing and automatic part checking.

Using the FA Inspector can automate this process step including checking parts on an unsoldered board for proper values. After loading the centroid data, BOM and available drawing, the FA Inspector can scan the first board to provide a single-pass full image of the PCB. Using the FA Software each reference designator is automatically presented to inspect either visually on the screen or by simply drawing a region of interest around the part label to create a part picture template that the machine will test. These picture templates, which are linked the part number, form the basis of the inspection library together with the package models to allow for automated first article testing or AOI inspection.

Pre-Reflow Inspection
Using the In-line FA Inspector pre-reflow can automatically identify missing parts, wrong parts, off-position parts, and improper polarity. While traditional AOI machines are usually deployed after the oven as they work hard to attempt to positively find solder defects — which is no easy task for even multiple camera systems — a machine focused on component defects takes less time and is easier to set up than for post-reflow inspection. Have you ever seen an AOI machine located post-reflow where the solder inspection algorithms have been turned off?

Post Reflow Inspection
In an effort to monitor and control the solder paste process and oven profiles, EMS providers complete a post reflow inspection step of the populated SMT parts post reflow. Many facilities today have elected to use AOI and X-Ray equipment at this stage of the inspection process. Smaller facilities still carry out manual Vision inspection using a microscopes.

Finding print-related and solder composition defects after the solder formation process is not an easy task for an AOI machine. AOI machines simply cannot see the entire soldered area under a lead or in hidden areas (such as Gull Wing heel regions or J-Lead toe regions) completely. X-ray Inspection is more efficiently suited for monitoring and finding these defects as the entire pad/solder connection is visible to an x-ray machine.

Today's typical boards include many hidden joints, mostly as a result of BGA and QFN style packages that are in widespread use. These parts absolutely cannot be inspected by the human eye or AOI cameras. The only way to properly monitor these components to identify process variation (solder voiding), monitor solder formation trends, and screen for solder-related defects (bridging, opens/head in pillow, insufficient and excess solder) is by using X-ray inspection.

Manual X-Ray.
X-ray equipment is not just a defense against defects such as opens or shorts before a board ships. Its greater purpose is used in process setup and development. In this case, x-ray examination of the solder joints provides a complete understanding of the interaction of the oven profile, the solder paste and its position on the pad; it provides a firm understanding of the solder profile. Measuring solder voids is a good example of gaining process knowledge about the interaction of the oven and the paste/flux chemistry.

2D X-ray, while basic, can provide an EMS provider with 80 percent of the critical information regarding the process. The top-down views (2D) provide detail of the solder profile. Small EMS companies can benefit greatly by deploying a basic system to help them attain better process control as well as to screen potential defects before the product ships. FocalSpot's MiniV is a good example of an inexpensive 2D system which can provide quick and useful information to adjust the process and catch defects before they go out the door.

For more advanced process development, solder defect screening such as head-in-pillow defects as well as failure analysis of boards that do not pass ICT, function test or perhaps returned product, a system which provides Oblique X-ray Inspection such as FocalSpot's Verifier Series can reveal further information, although 100 percent detail is still far away. Oblique viewing either by moving the PCB in 2 directions (tilt and rotate) or by moving the imaging train around the stationary PCB, the view of the solder joint can be seen in a 3D perspective. This is made possible by instruments in the FocalSpot Semi-Automated Insight Series, which can determine solder shape, BGA compression, and fillet angles — all are more visible using Oblique viewing.

Post Reflow X-ray
Critical Class III devices and densely populated double-sided PCB assemblies require a major step up in x-ray equipment technology to effectively monitor these assemblies and screen for defects. One of FocalSpot's partner companies, Matrix Technologies GmbH (Munich, Germany), offers a line of high-speed dual mode inspection systems. For double-sided assemblies, the machine's ability to view slices of overlapping joints allows the PCB to be fully tested. The Matrix X3 Series AXI x-ray system provides full handling, traceability, and advanced real time solder joint measurement and defect detection capability.

Final Visual Inspection
Once the board is completely built — including mounting of thru-hole assembly parts and connectors — the board will undergo a final visual inspection process and/or functional test prior to shipment. Most EMS providers, however, do not have functional test equipment necessary to test the product before its packaged and leaves the facility. A simple tool to use on the production floor is a Digital Scanner to provide a comparison between a Reference board (or Golden Sample) and the boards built. Such a tool performs a digital scan and permanently stores an image of the entire card for permanent storage and traceability. This can be useful later, since once the board leaves the facility, it is gone forever, unless it's returned as a result of the OEM finding a problem with it. Using FocalSpot's FA Comparator, the operator is able to see two simultaneous images of the PCB, the board under inspection and its reference board. Today, when operators are visually inspecting a board under a microscope they are only looking at one board, while trying to remember what is supposed to be where. That's pretty hard to do for multiple hours a day.

By deploying a host of practical inspection technologies EMS providers can improve their yields, bottom lines and enjoy stronger customer satisfaction.

Contact: FocalSpot, Inc., 9915 Businesspark Ave., Ste. A, San Diego, CA 92131. 858-536-5050 fax: 858-536-5054 E-mail: sales@focalspot.com Web:
http://www.focalspot.com


MiniV is an inexpensive 2D system which can provide useful information quickly.


 
 
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