Wednesday, May 23, 2018
Publication Date: 12/1/2007
Archive >  December 2007 Issue >  Special Feature: Test and Measurement > 

X-Ray Inspection: Making It Work
Low magnification x-ray provides large field-of-view.

The benefits of x-ray inspection are broad in scope because of the ability of x-rays to see through packages, including: encapsulation, heat sinks and metallic shielding to reveal obscured connections and identify potential quality issues non-destructively. X-ray inspection is particularly beneficial for applications that involve advanced packaging technologies such as: BGA, CSP, FC, WLP, POP, SIP, QFN. This is true because these components all are difficult if not impossible to assess by alternative inspection methodologies that include Vision 2D/3D, AOI, Laser, and ICT.

X-ray inspection has become an irreplaceable tool for design evaluation, process improvement, quality assessment and rework verification. While, x-ray image processing, fault detection and failure analysis tools provide the ability to quantify and fine-tune manufacturing processes, improve quality and yield, reduce scrap and decrease warranty returns. In short, having x-ray inspection capability can positively affect the entire lifecycle of a product. For these reasons, more and more OEMs are making x-ray inspection a process requirement and a prerequisite for CM and rework service vendors, by favoring or mandating in-house x-ray inspection capability.

Detectable Faults
Solder joint defects are caused by a variety of unique thermal and mechanical conditions that occur during the manufacturing process. These defects are: shorts or bridging, voiding, delamination, missing part or connection, open, misalignment, poor coplanarity, unacceptable size or shape.

Each type of defect has its own related cause. For example, shorts or bridging can happen because of excess or uneven solder paste deposition, damaged solder mask, solder splash, movement during reflow — aka a "disturbed joint", poor reflow, overheating. Voiding, aka "outgassing." happens because of trapped gas or contaminants inside a solder joint while cooling., If gas escapes during the solder process this can cause air bubbles, pin holes or blow holes.

Delamination or separated layers within a device structure is an open area void of adhesive located under the die of a component. This is caused by stress, impact, contaminants or excessive heat during the manufacturing process.

A missing part or missing connection point (i.e., solder ball) usually occurs during the part handling process (pick-&-place) or by poor quality control in reference components delivered on tape-and-reel, or caused by a chip shooter mechanicsm malfunction (turret head, table and feeder carriage) miss-feed or dropped part. A missing (or cold) connection of solder across conductors or lands that should be electrically joined created an "Open" condition. This can be due to insufficient heating during reflow phase, poor thermal stability of the PC board and/or component, faulty solder paste deposition, disturbed joint, contamination, or a missing connection such as a solder ball.

Misalignment, either showing lifting or tombstoning is caused by placement error. This happens when the component orientation is other than intended. Misalignment can result in low joint strength and poor electrical connection, or defects such as bridges, opens, etc. The cause is mechanical part placement error, uneven solder deposition, disturbed joint.

Poor coplanarity (open/misaligned) — a solder joint or series of solder joints on a component is out of alignment and fails to make contact with the land. Also, contact points in an area array that do not make contact with the intended geometric plane (ball to solder and matching land). The causes for this are the same as for either "Missing or Cold" or "Misalignment".

Unacceptable size and shape variations in solder volume result from u or excess/insufficient solder deposition, damaged mask, insufficient heating during reflow phase, poor thermal stability of the PC board and/or component, disturbed joint, contamination.

There are several ways of using the x-ray inspection system as a microscope for real-time manual inspection. Fast and accurate quality assessment can be achieved using a combination of low, medium and high magnifications.

Low Magnification (0-10X): provides a large field-of-view (FOV), and is a fast method of detecting gross anomalies. By using low magnification, an operator is able to scan a large inspection area while searching for fault indications. This technique is useful for detecting gross anomalies such as: bridging (shorts), excess and insufficient solder, misaligned and tombstoned components, unacceptable size and shape variations in solder joints, etc. For best practice, use this as first pass inspection to get the job done quickly and efficiently.

Medium Magnification (10-75X): is a moderately fast method of detecting gross anomalies, since it reveals additional detail and information.

High Magnification (100-1000X): provides a slow but detailed method for detection of subtle anomalies. High magnification provides greater visual detail and presents more information about a fault. This approach is generally employed by operators after seeing something suspicious during a scan at lower magnification. The best balance of throughput and magnification depends upon the application and the experience level of an operator.

Low X-Ray Energy
Using low x-ray power provides quick detection of bridges, poor reflow indicators (shape) and registration of the solder-to-pad characteristics.

At higher x-ray energy, x-rays pass through traces and pads as well as the most of the solder revealing the hidden voids.

Rotated view (angular board manipulation) involves rotating a sample (PC board) within the x-ray imaging path to display oblique (off-axis) viewing of the inspection area. The purpose of oblique viewing is to reveal the shape, size and location of solder connections and potential faults within. This is especially important for double-sided boards, where top and bottom side components may obscure clear viewing of object details.

X-ray inspection systems vary broadly in their designs and capabilities, yet all have similar fundamental components which make them x-ray systems. Therefore, the selection of one system over another depends on how the system will be used.

Sample handling and board size are important; is it machine loadable? Does the board fit into the sample holder of the machine? This is generally the first question asked when qualifying a system for a range of PC board inspection applications.

Angular Sample Manipulation
What about the system's ability to rotate a sample (PC board) within the x-ray imaging path to display oblique (off-axis) viewing of the inspection area? The purpose of oblique viewing is to reveal the shape, size and location of faults with solder connections. This is especially important for double-sided boards, where top and bottom side components may obscure clear viewing of object details. A rotation angle from 0 to 40° is ideal for this application. Manual or mechanical board manipulation will influence machine cost and operator convenience but makes little difference to the effectiveness of the inspection.

Magnification vs. Field-of-View
Magnification and field-of-view have opposing correlations. Ultimately the goal should be to obtain a good balance between sufficient magnification (inspection detail) and sufficient FOV (inspection view area) to view as much of the inspection area to minimize the time required for a complete board inspection. When using high magnification, the field-of-view (FOV) becomes smaller thereby reducing the speed at which a board can be completely inspected. Therefore, throughput and magnification have opposing objectives and the best balance of each depends upon the application and operator experience.
Rotated view results from angular board manipulation.

For all x-ray inspection systems, spot size, spot circularity and applied power are components of x-ray source resolution. The most commonly used specification is spot size.

Because spot size varies in relation to the applied electron beam power, a nanofocus x-ray source may be restricted to relatively low power operation — or else the spot size will grow to microfocus size as the power is increased to penetrate materials of relative density and/or thickness. Also, all systems are essentially nearly equal in resolution below 40X magnification, regardless of spot size due to the unsharpness effect.

X-Ray Source Power
X-ray source power is measured in kilovolts. By adjusting the flow of current, the number of emitted x-ray photons can be controlled to provide the required degree of material penetration. Low energy reveals traces and solder connection shape, while higher energy is used to reveal hidden internal characteristics such as voids. The typical power range of real-time x-ray inspection systems is between 50kV to 130kV. X-Ray tube power selection depends on the materials being inspected. Printed circuit boards and most SMT components require between 50kV to 90kV to adequately inspect the entire board. However, heavily shielded components, castings and mechanical assemblies may require 90kV to 130kV or more depending on the materials and density of the parts.

For all x-ray systems, the image processor and software have a direct impact on speed, accuracy and repeatable analysis results. Typical software packages include basic image quality enhancement tools such as image averaging and visual improvement filters. By way of this collective "image processing," visual detail is improved, thereby making it easier for an operator to quickly evaluate overall quality and identify subtle anomalies. Other software tools such as data collection, measurement and analysis reporting tools provide interactive diagnostics that further isolate, quantify and document faults for corrective action.

Most users only use the minimal tools needed to get the job done and some advanced visualization features — such as 3D rendering — are not essentially useful or considered necessary.
High magnification x-ray has small field-of-view, but provides more information about fault.

A good x-ray inspection strategy should employ a balance of low magnification to view as much of the inspection area at one time to quickly identify gross faults; and high magnification to provide adequate inspection detail for in-depth failure analysis. The best balance of each depends upon the application and operator experience.

How suitable is the system for the application — sample handling, tube power, etc.? For the most part, this is application-specific, with the stipulation that system selection should clearly identify actual requirements and system use. In most cases, the actual use model is something like this: "Get the job done, quickly, easily and efficiently, while using the minimal tools necessary to accomplish this task. Buy what you need and what you will actually use. Also ask if the system fits the available budget. Beyond the initial purchase price of a complete system, budget should include the long-term cost of ownership — service, support and warranty — as well as operational simplicity and ongoing maintenance, as well as ease-of-use and minimal training requirements.

For more information, contact: FocalSpot, Inc., 9915 Businesspark Ave., Ste. A, San Diego, CA 92131 858-536-5050 fax: 858-536-5054 E-mail: Web:

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