Wednesday, September 28, 2016
VOLUME -24 NUMBER 12
Publication Date: 12/1/2009
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Finding the Missing Link with X-Ray
Good BGA solder joints (transmissive view).

In many of the world's leading Electronics Manufacturing facilities x-ray inspection is a must-have item. Yet, of all of the machines on the production floor, x-ray is quite often the most under-utilized piece of capital equipment. From the simplest to the most complex of systems, x-ray is typically purchased to inspect connections and solder fill that cannot be visually examined. During the purchasing cycle, the capabilities of x-ray inspection are explored and demonstrated, but once installed on the manufacturing floor, only the basic capability of gross defect inspection is usually used.
Bad BGA solder joints (transmissive view).


While determining gross defects is an important capability, it represents only half of what x-ray inspection can do. Gross defect recognition consists of inspecting mounted components in a flat, horizontal plane. The types of defects that can be determined are bridges (shorts), missing solder ball joints, misalignment, inconsistent sphere sizes and voids. If these defects are found, then the particular components are called out as "failures" and will need to be removed and replaced. Flat plane inspection is certainly an important first step of quality verification; however the "good or passed" inspection calls still leave room for some very costly future failures.
Good collapse and wetting (oblique view).


The characteristic process of manual x-ray inspection is "through transmission" viewing. This means that the x-ray beam passes through whatever is in its path to the image capture device, be it a coated camera, image intensifier or flat panel digital detector. The x-ray beam is attenuated by the various densities of an object as it passes through to the detector. The higher the density the darker the gray-level of the final image; while the lower density areas produce a lighter gray level in the image.
Incomplete reflow with opens (oblique view).


In a top down flat view of a Ball Grid Array (BGA) there could be an open between the pad and a solder ball that will not be seen as it does not change the overall density in the path of the beam. To see an open, lack of wetting, or shape of the solder ball, an off-axis view (tilted and/or rotated) is required. This view shows the true integrity of the solder ball joint attachments to the pads. This type of off-angle viewing can be used for "failure analysis" to discover the root cause of failures and also for "quality verification" to confirm quality of solder joints during process development.

Good or Bad?
In a typical case, images may have been determined to be "good" in a flat plane inspection; however all that would have been shown is that they were free of "gross defects." An off-axis view of the same assembly shows the true condition of the solder joints.
Misalignment from pick-and-place (oblique view).


When the board under test is tilted, the ball shape and gray scale change at the rim of the pad (meniscus) will show up (in the cases shown here) first that there was a complete collapse and proper wetting to the pad. This confirms that these are truly good solder joints and a good process. In another case, however, the board would have also been determined to be "good" in a flat plane view, but it shows that there was incomplete reflow during the oven process. This is indicated by the random column-shaped joints mixed with rounded solder spheres that did not wet to the pad. From the image or "X-ray signature", the process engineer would know that they would need to go back to the oven profile and adjust the peak temperature and/or the time above liquidous to correct this problem in the production line.
Partial reflow and cold joints from improver oven profile (transmissive view).


Many, but not all, manual x-ray systems have the capability to perform off-axis imaging. These systems will rotate and/or tilt the part or rotate the image train to provide off-axis viewing. In a surprising number of electronics production environments, implementation of off-axis viewing for process development and control is not being performed. The use of angled viewing should be applied for quality verification as standard practice. This is the "Missing link" in the utilization of manual x-ray inspection systems.
Improver screen print and reflow (transmissive view).


One reason off-axis viewing is not used is because operators do not fully understand what they are looking at or looking for. It is important to first understand the orientation of the image train to know where the top of a joint is and where the pad area is located. Once the operator knows the orientation, it is then possible to look for the indicators of the ball shape and wetting to the pads which are the signatures of good solder joint quality. With proper training on the X-ray inspection system, operators can interpret the images and determine quality of the joints (quality verification) or the origin of any defects (failure analysis).

Another reason off-axis viewing is not practiced in a production environment is the added time it takes to complete the inspection. After all, "time is money", but poor quality solder joints can be even more costly when failures are not found prior to sending completed product out to the field. Catching these quality defects before they become a part of the finished product will return large dividends, saving troubleshooting time, parts costs and field failures, not to mention the company's production quality reputation. This is where the value of x-ray inspection comes into play.

Is Manufacturing Process Okay?
A process for screen printing, placement, AOI and reflow may be developed and boards are run. The process engineer then relies on in-circuit and/or functional test to check the process. If the finished boards fail the process engineer will look for indicators to correct the process issues. X-ray may be used to inspect boards in a flat plane for gross defects. This inspection may point back to screen printing, placement or reflow issues. On a "failed" board, the operator may even utilize off-axis viewing for failure analysis and discover the root cause of the failure. However, if the initial boards pass in-circuit and/or functional test, the process engineer declares the boards ready for full production run and it's full steam ahead.
Shorts and partial connections (oblique view).


Unfortunately in-circuit test and functional test do not provide conclusive determinations for the solder joint integrity. The oblique view may show up incomplete reflow and possible "cold" solder joint(s). Because these solder balls are making partial contact with the pads, it's possible that they can pass in-circuit test/functional test. Out of fifty boards there may be one or two that eventually fail, calling out a bad BGA. This board will then be sent to repair and the BGA will be removed and replaced using a profile on the BGA Rework system. The board will then come back to in-circuit test, pass, and the process engineer declares the problem "fixed", but is it really?

The other boards that have passed in-circuit/functional test are installed and mounted into the finished product and shipped. Because of flexing, thermal cycling or vibration one or two more boards fail in the assembled unit. These boards are returned and sent back to rework the bad components and are finally declared "fixed".

Costly Field Failures
Field failures are always extremely costly, both in product quality reputation and actual replacement and physical repairs. The result is spending far more time and money troubleshooting and repairing the few captured failures than would have been spent implementing off-axis quality verification during the production process — a procedure that would have caught these initial solder quality issues before product was actually shipped.

If off-axis x-ray inspection were implemented for quality verification as an integral part of process development, failures could be captured before the full production run is completed. Through the proper interpretation of x-ray images, a process engineer can identify from the x-ray signature precisely where in the production line the problem is occurring and corrective measures can be taken before final production is completed and shipped.

When and only when, these practices are implemented can a company start to experience the true value in its capital investment in x-ray inspection systems.

Contact: FocalSpot, Inc. 9915 Businesspark Avenue, Ste. A, San Diego, CA 92131 P: (858) 536-5050 Fax: (858) 536-5054 Web: www.focalspot.com  

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