mobile | classic
Dataweek Electronics & Communications Technology Magazine





Follow us on:
Follow us on Facebook Share via Twitter Share via LinkedIn


Search...

Electronics Buyers' Guide

Electronics Manufacturing & Production Handbook 2019


 

X-ray dose considerations
27 March 2019, Manufacturing / Production Technology, Hardware & Services

The benefits of using X-ray technology in the electronics industry to inspect printed circuit board assemblies are well proven.

This article summarises X-ray radiation dose, and the techniques that can be employed to reduce radiation exposure to samples in a Nordson Dage Quadra X-ray inspection system.

Quadra X-ray geometry

First we need to consider how the Quadra X-ray inspection system works. Quadra allows the internal structure of samples to be seen by shining X-ray light through a sample (Figure 1). This creates a shadow image that is detected in real-time using a high-resolution Aspire flat panel X-ray detector. The darkness of the shadow cast by any point in the sample depends on how much X-ray light has been absorbed by that part of the sample.

Figure 1. Quadra X-ray tube, sample and detector geometry shown at low magnification (left) and high image magnification (right).
Figure 1. Quadra X-ray tube, sample and detector geometry shown at low magnification (left) and high image magnification (right).

High image magnification is used to see smaller features clearly. This is achieved by moving the sample closer to the X-ray tube. For the highest magnification, the sample tray is lowered to the top of the X-ray tube.

Figure 2. Oblique viewing angles are achieved by moving the detector in a hemisphere around the sample.
Figure 2. Oblique viewing angles are achieved by moving the detector in a hemisphere around the sample.

Viewing the sample from different angles allows more defects to be observed. The Aspire flat panel detector moves around two oblique axes to allow this. X-rays from the QuadraNT X-ray tube are emitted over a wide cone angle so there are plenty of X-rays available off-axis.

Different materials absorb radiation to differing degrees (Figure 3). Lead is very good at absorbing X-ray radiation which is why it’s used to shield the X-ray cabinet. Silicon and aluminium are relatively poor at absorbing X-rays so images tend to be very bright. Tin and copper, commonly used in solder, are somewhere in between.

Figure 3. X-ray absorption by different materials of the same thickness.
Figure 3. X-ray absorption by different materials of the same thickness.

The shadow created at any point also depends on the voltage and power used to drive the X-ray tube (Figure 4). An X-ray tube operating at a voltage of 80 kV creates X-rays over the entire energy range up to 80 keV. Increasing the voltage to 160 kV doubles the X-ray output energy range from 80 keV to 160 keV.

Figure 4. X-ray photon energy output for different tube voltage and power settings.
Figure 4. X-ray photon energy output for different tube voltage and power settings.

Increasing the tube power increases the number of X-rays emitted at every point over the range. In general, higher energy X-rays are more likely to pass through a sample than lower energies. High energy beams are useful for looking at thick samples, or materials that strongly absorb X-rays, for example lead. Lower energy X-rays are better for thinner samples, or materials that weakly absorb X-rays, for example copper or silicon.

What is X-ray dose?

The X-ray dose for any part of the sample is a measure of the X-ray energy absorbed per unit mass, and is measured in units of gray (Gy), where

1 Gy = 1 J/kg = 100 rad.

Since different materials absorb incoming X-rays with differing efficiency, their absorbed dose will vary when placed in the same X-ray beam. A material like lead, which absorbs X-rays well and casts a dark shadow in an image, will absorb a higher dose than a material like silicon which absorbs X-rays weakly.

The absorbed dose also varies with:

• Distance to the X-ray source: The amount of X-ray radiation reduces as 1/r² in the same way a light appears brighter the closer you are to it.

• Time in X-ray source: X-ray dose is cumulative so the longer spent in the X-ray beam, the higher the absorbed dose.

• X-ray tube output: The higher the tube power or voltage, the larger the absorbed dose.

How to reduce sample dose

Reducing the magnification used and minimising the time the X-ray tube is switched on are effective ways to reduce X-ray dose. Quadra’s low dose mode can be used to switch X-rays off when it detects the sample is no longer being manipulated.

Figure 5. X-ray photon energy output without (top) and with (bottom) additional filtering. The energies below 30 keV do not contribute to the final image but still contribute to absorbed dose.
Figure 5. X-ray photon energy output without (top) and with (bottom) additional filtering. The energies below 30 keV do not contribute to the final image but still contribute to absorbed dose.

Another method is to filter out lower energy X-rays (Figure 5). These X-rays do not contribute to the final image for two reasons:

Figure 6. Absorbed dose and image quality for a typical silicon sample with filtering tray (left) and without (right).
Figure 6. Absorbed dose and image quality for a typical silicon sample with filtering tray (left) and without (right).

• Absorption by sample: At low energies, particularly below 20 keV, X-rays are strongly absorbed by most samples. They contribute to sample dose, but are less likely to make it through the sample to the X-ray detector.

• Detector sensitivity: Flat panel X-ray detectors, like AspireFP, are insensitive to X-rays below ~20 keV. Any X-rays that manage to pass through the sample have a lower likelihood of being detected.

Filtering out low energy X-rays is an effective way to reduce sample dose without compromising image quality. A filtering sample tray is available for Quadra which incorporates zinc strips to absorb low energy X-rays while letting higher powers pass through to the sample. This can reduce overall X-ray dose by up to 80% (Figure 6).

Masking is a simple way of reducing X-ray dose for any components on a board that do not need to be inspected at all by X-ray. A portion of dense material, for example lead, can be attached to the sample tray immediately below the sample to mask the sensitive component from X-rays.

Quadra includes a dose calculation tool that tracks the average dose acquired by the whole sample during inspection. For the most accurate results, the tool can be calibrated by measuring X-ray dose in the path of the X-ray beam before and after the sensitive component (Figure 7).

Figure 7. Sample absorbed dose can be measured by measuring 
and subtracting dose with and without sample present.
Figure 7. Sample absorbed dose can be measured by measuring and subtracting dose with and without sample present.

Getting better images from thin samples

High energy X-rays are poorly absorbed by thin samples, or by weakly absorbing materials such as silicon or aluminium. Medium energy X-rays give the best image contrast for these samples. However, these energies are typically between 30 and 80 kV and are attenuated by ~90% by the standard aluminium sample tray.

Figure 8. Thin sample tray allows higher contrast images at lower X-ray energies.
Figure 8. Thin sample tray allows higher contrast images at lower X-ray energies.

A thin sample tray is available for Quadra which is manufactured from a thin layer of carbon fibre instead of a thicker layer of aluminium. This allows 10 times more medium energy X-rays to pass through, creating clearer images with better contrast.


Credit(s)
Supplied By: MyKay Tronics
Tel: +27 11 869 0049
Fax: +27 11 907 1547
Email: mykay@iafrica.com
www: www.mykaytronics.com
  Share on Facebook Share via Twitter Share via LinkedIn    

Further reading:

  • How to analyse blind via hole failures
    27 March 2019, Manufacturing / Production Technology, Hardware & Services
    It has become common practice to use blind, filled and stacked vias in many portable electronics products. Experience has shown that this method of interconnection is reliable, provided the fabrication process is well defined and controlled.
  • PCB microsectioning – paying attention to detail
    27 March 2019, Cirtech Electronics, Manufacturing / Production Technology, Hardware & Services
    In our high-tech world, it’s easy to overlook the importance of the humble printed circuit board (PCB). Buried in each electronic gadget or appliance there’s always at least one PCB and each one has ...
  • Product development issues explored in new video
    27 March 2019, Manufacturing / Production Technology, Hardware & Services
    Printed circuit broker, Elmatica, recently released a new film about product development, explaining why cooperating with an experienced partner could be a smart move. “We have several times seen the ...
  • Flux management system earns APEX award
    27 March 2019, MyKay Tronics, Manufacturing / Production Technology, Hardware & Services
    BTU International was awarded a 2019 NPI award in the reflow soldering category for its new Aqua Scrub flux management technology. The award was presented to the company during a ceremony that took place ...
  • Solder dross recovery system
    27 March 2019, Electronic Industry Supplies, Manufacturing / Production Technology, Hardware & Services
    Stannol’s Solder Saver is a mechanical device to reduce dross, which occurs as an unavoidable side effect in wave soldering systems. According to its manufacturer, solder consumption can be reduced by ...
  • Water-soluble solder paste
    27 February 2019, Techmet , Manufacturing / Production Technology, Hardware & Services
    Indium has released Indium6.6HF solder paste – a new water-soluble, halogen-free solder paste that is compatible with both tin-lead and lead-free alloys. It is designed to provide enhanced stencil printing ...
  • Affordable soldering station
    27 February 2019, Electronic Industry Supplies, Manufacturing / Production Technology, Hardware & Services
    The Industa 550 soldering station, available from Stannol, is an affordable unit characterised by easy operation and universal fields of application. The temperature is easy to set using a potentiometer, ...
  • Stencil underside cleaning and manual post-cleaning
    27 February 2019, Testerion, Manufacturing / Production Technology, Hardware & Services
    According to experts, between 60 and 70% of all defects and failures during or after the soldering process are attributable to defects in the printing process (Figure 1). In general, these are faulty ...
  • What you need to know about the Hermes standard
    EMP 2019 Electronics Manufacturing & Production Handbook, Manufacturing / Production Technology, Hardware & Services
    Taking advantage of modern TCP/IP communication and XML data format technologies, Hermes is effectively a replacement of the more than 20-year-old SMEMA standard.
  • Contract electronics manufacturing in SA
    EMP 2019 Electronics Manufacturing & Production Handbook, Leratadima Tellumat Manufacturing, Manufacturing / Production Technology, Hardware & Services
    We asked representatives from two leading South African contract electronics manufacturers to share some of their insights into this complex landscape.
  • BGA optical joint inspection criteria and test methods
    EMP 2019 Electronics Manufacturing & Production Handbook, Manufacturing / Production Technology, Hardware & Services
    BGA failures occur for component, printed board, process or design related reasons, and on some occasions, both X-ray and optical inspection cannot easily provide the answer.
  • Eight things to consider when planning a flex or rigid-flex board
    EMP 2019 Electronics Manufacturing & Production Handbook, Manufacturing / Production Technology, Hardware & Services
    The adoption of flexible circuits is growing because it offers a huge variety for seamless interconnections, lighter weight, improved reliability and compressed constructions.

 
 
         
Contact:
Technews Publishing (Pty) Ltd
1st Floor, Stabilitas House
265 Kent Ave, Randburg, 2194
South Africa
Publications by Technews
Dataweek Electronics & Communications Technology
Electronic Buyers Guide (EBG)

Hi-Tech Security Solutions
Hi-Tech Security Business Directory

Motion Control in Southern Africa
Motion Control Buyers’ Guide (MCBG)

South African Instrumentation & Control
South African Instrumentation & Control Buyers’ Guide (IBG)
Other
Terms & conditions of use, including privacy policy
PAIA Manual





 

         
    Classic | Mobile

Copyright © Technews Publishing (Pty) Ltd. All rights reserved.