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Introducing
The Testability Director Version 3.2
 

Complex Semiconductor Packages Increase the Need for 3-D X-Ray Inspection

By Paul Walter, Managing Director, Dage Precision Industries, Inc.

New Package Types Demand More from Automated X-Ray Inspection

By Stacy Johnson, AXI Product Manager, Agilent Technologies, Inc.

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This issue's focus is X-Ray Inspection. 
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Computerized tomography (CT) is a well established medical diagnostic technique that is now being employed for the inspection of advanced three dimensional (3-D) semiconductor packages and complex electronic devices.  Until recently, the use of CT for semiconductor and other inspection applications has been limited by slow computer processing times, low system resolution and relatively high investment.  While some CT systems have been used for semiconductor inspection, these conventional CT systems typically had not provided the analytical performance necessary for critical, high-density applications.  

Computerized Tomography
Computerized tomography is an imaging method in which computational geometric processing is used to generate a single three-dimensional model of an object from a series of individual two-dimensional x-ray images.  The CT model is computer developed from a series of 2-D x-ray images taken as the sample, or semiconductor device, is rotated within an x-ray beam.  The variations in the density within those images, and how their locations change as the sample rotates, are evaluated by a computer reconstructing a 3-D model of the sample.  This 3-D model can be viewed and manipulated, to provide analytical images, or slices, through any plane within the CT model.

Recent advancements in CT technology have greatly improved the imaging speed, increased the resolution to a suitable level to allow analysis of the most detailed features, and delivered systems at a reasonable price.  Because of these improvements CT has become an ideal inspection methodology for complex 3-D packages since it generates a three-dimensional model of the entire electronic package.  The resulting 3-D model can be viewed in real-time so that interconnections normally obscured by other joints or components within the package can be diagnosed, assuring complete package inspection.

Semiconductor Packaging
As sophisticated package designs become more common, advanced 3-D packages are replacing traditional lead-frame packages.  Size constraints of final products require semiconductor packaging with smaller footprints and lower profiles while at the same time the board level interface and assembly processes drive interconnect density.  This trend drives device integration into one package, reducing the number of I/O on the circuit board and promotes the use of subsystems.

Semiconductor designers encouraged the development of system-in-package (SiP) device integration including multiple stacked die packages, including package-in-package (PiP) and package-on-package (PoP).  These devices typically contain multiple stacked die with multi-level wire bonding, or wafer bumping, internal to the device.  Circuit board assembly requires surface mount technology, meaning that packages need to interface with circuit board solder land pads or solder bumped pads.
While these 3-D packages offer increased functionality in a smaller footprint, they also place a greater demand on any required x-ray inspection and provide their own unique set of challenges for package inspection and process qualification during package, assembly and test.  Traditional analysis using 2-D x-ray imaging is often limited with these new package types since the multiple layers within the device are seen at the same time. This can be confusing with the multiple dies and multiple layers of wire bonds appearing to overlap each other in the image.

Three-Dimensional Modeling
Because of the limitation of 2-D x-ray imaging systems, CT is increasingly used to inspect semiconductor devices for die attach quality and the quality and effectiveness of wire bond integrity within complex 3-D packages.  The critical elements of producing a CT model include acquisition of the necessary 2-D x-ray images, computerized reconstruction from the 2-D images into a CT model, and visualization and manipulation of the completed 3-D model.  The resolution of the reconstructed 3-D matrix is generally defined in terms of volume pixels, or voxels.  Newly developed CT systems have dedicated parallel processors to manipulate such huge volumes of data as quickly as possible.  Therefore, high-resolution CT models are available for viewing within moments of the image acquisition being completed (Figure 1).

Figure 1: CT image of microvias within BGA substrate

In addition to analysis of die attach quality and wire bond integrity within complex 3-D packages, many of these newly developed CT systems can also perform high-resolution 2-D inspection functions. Therefore, the user has a common inspection platform which has the flexibility to rapidly and easily convert their x-ray system between 3-D to 2-D modes to satisfy manufacturing inspection needs. The latest x-ray tube advances means that 2-D and 3-D analysis is possible with the greatest grey scale sensitivity and feature recognition down to as little as 250 nanometers (Figure 2).  

Figure 2: CT image of cracked microvia, cracks within a micro-BGA solder ball and a slice through BGA balls located near the pad/ball interface

Summary
New advancements in computerized tomography make it an ideal technique for the inspection of advanced 3-D packages since it allows complete viewing of interconnections within the package that otherwise would be obscured by other features when viewed in 2-D x-ray inspection images.  These new CT systems allow rapid volume reconstruction to be carried out at the same time as image acquisition providing complete package inspection assuring die attach quality, wire bond integrity and improved semiconductor package performance.

 Stacy Johnson, AXI Product Manager, Agilent Technologies

The latest printed circuit boards (PCBs) are full of new and innovative high speed interconnects.  If we look at the types of packages being placed, we find more pins packed into a single chip.  A lot more quad-flat pack, area array packages, and a big increase in the use of high density packages is evident and yet, the industry continues to pursue the reduction in average selling prices for chips, packages, the boards themselves, and electronic systems. All of this cost pressure and increased density is putting a greater burden on traditional test methods, like in-circuit test (ICT).

In general, boards also have more joints.  In the past a board with 20,000 joints could have been considered to be on the upper end, and now we have cases of 60,000 joints and that number is only rising!  The products in general are becoming more complex, with more high density interconnects, more hidden joints, less test points and accessibility and overall this is increasing the need for automated X-ray inspection (AXI).

The latest trends in printed circuit board assembly are pushing more customers to consider and utilize X-ray inspection.  This presents several challenges to X-ray solution providers.  One challenge arises from the increasing capabilities and features housed within package types.  DirectFET is an example of a new package well suited for X-ray.  The leadless package design and the proportionally large land patterns allow for a very wide variety of joint formations thus providing new challenges to X-ray structural testing.  

Another example is the Quad Flat No-Lead (QFN).  The utilization of QFN packages in the PCB market is increasing largely because these devices are relatively low cost.  The relatively inexpensive nature of leadframe-based chip scale packages (CSPs) such as QFN’s without solder balls has led to the popularity of these devices being utilized.  Currently, only generic test requirements for the QFN exist in the industry.  More quality testing and inspection is needed due to the increase in utilization.  Since the QFN is a hidden joint, X-ray is the best way to inspect it in process.  Customers utilizing QFN’s were not able to test this package type.  To solve this issue Agilent partnered with a major electronics Original Equipment Manufacturer (OEM) to develop an algorithm for the Agilent 5DX automated X-ray inspection system to address the QFN and reduce false call rates. 

In summary, as package types change X-ray solution providers need to be responsive with a technical solution.  As X-ray is the best fit for many of these new package types, the vendors must address new device and package types.

The good news is that even with the ever-changing technology, the market is getting better at building printed circuit boards (PCBs).  More utilization of solder paste inspection and pre-reflow automated optical inspection (AOI) with the ability to prevent defects is helping.  This is a trend in the industry, however, there is still no perfect process.  The defect rates are still quite high and therefore, for a medium to high complexity product defect containment needs to be present and the defects need to be caught in order to meet the end-user quality and manufacturer cost requirements.

One of the fastest ways to improve profitability is to improve the end user quality and that is where automated X-ray inspection (AXI) can be a perfect defect containment solution.   There are a lot of different test strategies.   Finding a strategy that balances cost with the right level of quality is complex.  Defect prevention and defect containment are the two positions with regards to the defect landscape.  X-ray focuses at defect containment. 

AXI is still the single best solution for defect containment.  If an X-ray solution is integrated correctly the manufacturing costs can be reduced. Some customers look at their manufacturing flow with and without X-ray and realize that with a proper implementation of X-ray the manufacturing costs can be reduced, even after the capital investment.  Partnering with other test technologies and striking the balance with AOI, in-circuit test (ICT) and functional test is the key and in most cases offers a further reduction in costs and increase in product quality.   In closing, as cost pressure continue, technology evolves, and defect happen, the role of automated X-ray inspection becomes solidified.

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