Advantages of Multiple Resolutions in Automated 3D X-Ray Inspection
Test Research, Inc.
Aaron Johnson
Barbara Koczera
February 20, 2012
Introduction
Today’s consumers expect smaller, more compact products such as notebooks and mobile devices.
In response to this, Printed Circuit Board Assembly (PCBAs) designers continue to focus their designs on smaller form factors to meet these demands. In order to achieve “miniaturization” of their end products, PCBA designers are incorporating more “fine pitch” parts with hidden joints –more commonly referred to as Bottom Termination Components (BTC) – into their designs.
The most commonly used BTCs include micro-BGAs, Quad Flat No-Lead (QFNs), and various MOSFET devices such as Land Grid Arrays (LGAs). The use of these types of BTCs is leading to a significant increase in the use Automated X-Ray Inspection (AXI) – and especially in AXI systems that offer high resolution inspection required to effectively inspect these fine-pitch parts.
The TR7600 SII is the only system on the market that offers, among others, the distinct advantage in offering “multiple” resolutions for any given AXI inspection test program.
Accompanied with a unique Off-Line Repair Station, the combination of theTR7600 AXI system and Repair Station effectively detects defects and enables Operators to review defects in both 2D and 3D, which (1) minimizing escapes which often lead to Field Failures and damage to the OEMs reputation in Product Reliability, and (2) minimizes unnecessary repairs, ergo, increasing Yields, and, more importantly, minimizing damage inadvertently incurred during the repair process.
Full
Range
of Resolutions
Unfort
unately, high resolution X-Ray inspection, if used to inspect the entire PCBA, can drastically increase the cycle time of the inspection, preventing the AXI system from keeping up with the Production Line’s “Beat Rate”. Therefore, it is important that manufacturers restrict high resolution X-Ray inspection to only those areas requiring it, i.e. the areas for which the QFNs or LGAs reside.
For the remainder of the board, it is quite effective to utilize a lower resolution magnification – thus minimizing the overall AXI cycle time. Having the option to select from multiple resolutions when creating an AXI program gives manufacturers the greatest flexibility to match capabilities with requirements in the most efficient manner.
Some AXI systems offer multiple resolutions, however, only “one” resolution can be used for the entire X-ray inspection.
For systems of this type, AXI programmers are forced to make an “either/ or” (e.g. either 5µm or 10µm) decision on what resolution to select—a decision which will most definitely negatively impact cycle time “or” test effectiveness.
Other AXI suppliers only offer one single resolution – period. This makes inspection of very fine pitch BGAs or 01005 passive devices nearly impossible.
If the X-ray images are too small at the Repair Station (see Figure 1) it is difficult for the Repair Operator to confirm subtle but critical defects such as BGA “Opens”.
Only the TR7600 SII currently offers the most comprehensive range of selectable resolutions (5µm, 7.5µm, 10µm, 15µm and 20µm) to give manufacturers the flexibility to create programs using “multiple” resolutions most suitable for the device under test.
Reduced Cycle Times
With the TR7600 SII any given AXI program can be developed to utilize one or more resolutions, allowing the program developer to minimize cycle time. The lower 20µm resolution can be used for the majority of the board while the higher resolution (5/7.5/10µm) can be programmed to inspect fine pitch components.
Some AXI vendors offer 5 or 10µm, but “only” 5 or 10µm.
For AXI inspection, inspecting a PCBA at such a high resolution would extend the cycle time to extreme lengths.
It would be impossible in most cases for the AXI system to keep up with the production line “beat rate”. This forces the manufacturer to put the AXI system offline in a standalone mode. This would likely turn the AXI into a “bottleneck” in the production line, potentially causing schedule delays.
Inspection of Fine Pitch Parts
More PCB designers are including the use of 01005 components in their designs.
For AXI inspection, it is nearly impossible to inspect these fine pitch parts with a resolution lower than 10µm. A number of AXI suppliers offer only one resolution, and some only offer resolutions only greater than 15µm. This may not be seen as problematic for manufacturers that do not “currently” encounter 01005 components on their boards, but given the sustained push towards miniaturization, it is only a matter of time before they will find themselves lacking the capability to inspect these increasingly common components.
Repair Station Efficiency
Another advantage of having multiple resolutions is having larger X-Ray images available at the Repair Station for the Repair Operators to properly disposition defects.
Some quick background on the Repair Station:
once a board is scanned using AXI only the images of potentially defective solder joints are sent to the Repair Station for review.
The Repair Operator reviews each image and decides if rejection of the joint is valid.
It’s important that the image at the Repair Station be large enough for the Operator to visually see the defect in the x-ray image.
For very fine-pitch parts, such as a
.3 mm micro-Ball Grid Array (µBGA) or Chip Scale Package (CSP)
is it very difficult to see the X-ray images at a 20µm resolution.
Open BGA joints are sometimes only slightly smaller in diameter than “good” joints so if the image isn’t large enough to see a variation in size, the Repair Operator cannot properly disposition the defect.
Inspection
Automated X-ray Inspection requires no manual visual inspection at all.
The PCBA enters the system on a conveyor.
Once the board is clamped in place the X-ray tube and cameras scan the board in sync in a serpentine-like path.
The board is first scanned using the lowest resolution selected by the user.
After the board is scanned using the initial resolution, the X-ray tube will move closer to the board to inspect at the next higher resolution.
The system then scans the parts or areas of the board that the user has selected for a higher resolution inspection.
The TR7600 SII will not scan the entire board again, only the areas specified by the user for Higher Resolution inspection.
Summary
There are several Automated X-Ray Inspection models available from a number of suppliers.
It’s important for the manufacturer who is considering purchasing an AXI System to fully understand the features and limitations of the different solutions available.
With the continued trends toward miniaturization and a reduction in production “Beat Rates”, the TR7600 SII’s multiple resolutions, high-speed cameras and effective and reliable defect detection provide a comprehensive solution that was recently recognized
with a 2012 Test and Measurement World Best in Test Award for in the Machine Vision/Inspection category.1
1
The TRI TR7600 SII Automated Optical Inspection System won a 2012 Test and Measurement World Best in Test Award in the Machine Vision/Inspection Category. The program comprises three main categories – the Best in Test Product Awards, the
Test of Time awards, and
Test Engineer of the Year award. The 2012 Best in Test Product Awards recognize innovations released between November 1, 2010 and September 30, 2011 that have made an especially significant contribution to the test industry.
Each year finalists are selected from among the nominees by Test & Measurement World’s editors, after which Test & Measurement World’s engineering audience are invited to vote for the winners using an online ballot. Test & Measurement World then uses a combination of these audience votes and balloting by its editorial staff to determine the Best in Test and Test of Time winners. For additional information on the awards program, visit: http://www.tmworld.com/article/520453-Finalists_for_T_MW_s_2012_awards.php
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