Cross section (electronics)

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In electronics, a cross section, cross-section, or microsection, is a prepared electronics sample that allows analysis at a plane that cuts through the sample. It is a destructive technique requiring that a portion of the sample be cut or ground away to expose the internal plane for analysis. They are commonly prepared for research, manufacturing quality assurance, supplier conformity, and failure analysis. [1] [2] Printed wiring boards (PWBs) and electronic components and their solder joints are common cross sectioned samples. The features of interest to be analyzed in cross section can be nanometer-scale metal and dielectric layers in semiconductors [3] up to macroscopic features such as the amount of solder that has filled into a large, 0.125in (3.18mm) diameter plated through hole.

Contents

Preparation

Cross sections can be prepared by several methods typically chosen based on the scale of the feature of interest because the technique affects the smoothness of the final polish. Smoother polishes allow an analysis of smaller features but can also take longer or be more expensive to prepare. Cross sectioning hard materials such as alumina might require a different technique than a soft material like gold or soft plastic.

Mechanical grinding and polishing

Mechanical grinding and polishing is a common method of preparation to analyze features on the order of 1s to 10s of microns [4] to macroscopic features. Samples may first be cut down in size, for example, around a via in a PWB or around a ceramic capacitor soldered to a PWB. Samples may be prepared by encapsulation in a rigid material such as epoxy to keep the sample intact during grinding and with a vacuum step to fill in air gaps and create a solid sample with no voids. However, cross sections of some samples can be prepared with no encapsulation.

Encapsulated samples are prepared using a rough grinding medium to remove material from the sample until just before the plane of interest is reached. Equipment can help automate the process by holding grinding and polishing media firm and then spinning it so a sample can be pressed against it. Typical grinding media are silicon carbide and diamond, which can be in the form of disposable discs impregnated with the grinding media or a slurry applied to a reusable pad. Successively finer media are used to finish grinding to the plane of interest and to polish at the plane of interest. Each successively smaller grit is used to remove the scratches and damage caused by the previous grit.

Mechanical cutting or milling

Some equipment allows for preparation of cross sections by direct cutting or milling. [5] [6]

Other techniques

Focused ion beam, ion beam milling, and cleaving [3] are common techniques in the semiconductor fabrication industry.

Printed wiring boards

Manufacturers of substrates used in electronics prepare cross sections of a final product for quality assurance. [7] In cross section, the quality of drill holes can be assessed and the plating quality and thickness in vias can be measured. Voids in the substrate materials may be seen which show the quality of the lamination process.

Electronic components

Viewing the internal structures of electronic components by cross section can reveal problems with manufacturing and material quality. In integrated circuits, a cross section can reveal the die, and its active layers, the die paddle, and 1st level interconnect (wire bonds or solder bumps).

Solder joints

Cross sections of component solder joints are commonly prepared to assess the quality and extent of the metallurgical bond. This analysis can be used to help determine any issues during the soldering processes that could lead to solder fatigue and failure. Solder joint cross sections are also commonly prepared during failure analysis to see cracks in the solder. Crack morphology can lead to identification of the type of stress and ultimately the root cause of the solder joint failure. [8] [9]

Analysis techniques for cross sections

Analysis of polished cross sections can be performed with a variety of techniques. Images are commonly taken with optical microscopy and scanning electron microscopy. Chemical analysis can be done with energy dispersive x-ray spectroscopy (EDS). Hardness testing can also be performed.

Related Research Articles

<span class="mw-page-title-main">Printed circuit board</span> Board to support and connect electronic components

A printed circuit board (PCB) or printed wiring board (PWB) is a laminated sandwich structure of conductive and insulating layers. PCBs have two complementary functions. The first is to affix electronic components in designated locations on the outer layers by means of soldering. The second is to provide reliable electrical connections between the component's terminals in a controlled manner often referred to as PCB design. Each of the conductive layers is designed with an artwork pattern of conductors that provides electrical connections on that conductive layer. Another manufacturing process adds vias, plated-through holes that allow interconnections between layers.

Electrical connector Device used to join electrical conductors

Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection between parts of an electrical circuit, or between different electrical circuits, thereby joining them into a larger circuit. Most electrical connectors have a gender – i.e. the male component, called a plug, connects to the female component, or socket. The connection may be removable, require a tool for assembly and removal, or serve as a permanent electrical joint between two points. An adapter can be used to join dissimilar connectors.

Surface-mount technology Method for producing electronic circuits

Surface-mount technology (SMT) is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

Stripboard

Stripboard is the generic name for a widely used type of electronics prototyping material for circuit boards characterized by a pre-formed 0.1 inches (2.54 mm) regular (rectangular) grid of holes, with wide parallel strips of copper cladding running in one direction all the way across one side of on an insulating bonded paper board. It is commonly also known by the name of the original product Veroboard, which is a trademark, in the UK, of British company Vero Technologies Ltd and Canadian company Pixel Print Ltd. It was originated and developed in the early 1960s by the Electronics Department of Vero Precision Engineering Ltd (VPE). It was introduced as a general-purpose material for use in constructing electronic circuits - differing from purpose-designed printed circuit boards (PCBs) in that a variety of electronics circuits may be constructed using a standard wiring board.

Metallography

Metallography is the study of the physical structure and components of metals, by using microscopy.

A via is an electrical connection between copper layers in a printed circuit board. Essentially a via is a small drilled hole that goes through two or more adjacent layers; the hole is plated with copper that forms electrical connection through the insulation that separates the copper layers.

Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, modify electrical conductivity, remove burrs and other surface flaws, and control the surface friction. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item. An unfinished surface is often called mill finish.

Focused ion beam Device

Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a scientific instrument that resembles a scanning electron microscope (SEM). However, while the SEM uses a focused beam of electrons to image the sample in the chamber, a FIB setup uses a focused beam of ions instead. FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams. FIB should not be confused with using a beam of focused ions for direct write lithography. These are generally quite different systems where the material is modified by other mechanisms.

Electron cryotomography

Electron cryotomography (CryoET) is an imaging technique used to produce high-resolution (~1–4 nm) three-dimensional views of samples, often biological macromolecules and cells. CryoET is a specialized application of transmission electron cryomicroscopy (CryoTEM) in which samples are imaged as they are tilted, resulting in a series of 2D images that can be combined to produce a 3D reconstruction, similar to a CT scan of the human body. In contrast to other electron tomography techniques, samples are imaged under cryogenic conditions. For cellular material, the structure is immobilized in non-crystalline ("vitreous") ice and allowing them to be imaged without dehydration or chemical fixation, which could otherwise disrupt or distort biological structures.

Flat no-leads package Integrated circuit package with contacts on all 4 sides, on the underside of the package

Flat no-leads packages such as quad-flat no-leads (QFN) and dual-flat no-leads (DFN) physically and electrically connect integrated circuits to printed circuit boards. Flat no-leads, also known as micro leadframe (MLF) and SON, is a surface-mount technology, one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. Flat no-lead is a near chip scale plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB. Flat no-lead packages include an exposed thermally conductive pad to improve heat transfer out of the IC. Heat transfer can be further facilitated by metal vias in the thermal pad. The QFN package is similar to the quad-flat package (QFP), and a ball grid array (BGA).

Thick-film technology

Thick-film technology is used to produce electronic devices/modules such as surface mount devices modules, hybrid integrated circuits, heating elements, integrated passive devices and sensors. Main manufacturing technique is screen printing (stenciling), which in addition to use in manufacturing electronic devices can also be used for various graphic reproduction targets. It became one of the key manufacturing/miniaturisation techniques of electronic devices/modules during 1950s. Typical film thickness – manufactured with thick film manufacturing processes for electronic devices – is 0.0001 to 0.1 mm.

Scanning SQUID microscopy

Scanning SQUID microscopy is a technique where a superconducting quantum interference device (SQUID) is used to image surface magnetic field strength with micrometre scale resolution. A tiny SQUID is mounted onto a tip which is then rastered near the surface of the sample to be measured. As the SQUID is the most sensitive detector of magnetic fields available and can be constructed at submicrometre widths via lithography, the scanning SQUID microscope allows magnetic fields to be measured with unparalleled resolution and sensitivity. The first scanning SQUID microscope was built in 1992 by Black et al. Since then the technique has been used to confirm unconventional superconductivity in several high-temperature superconductors including YBCO and BSCCO compounds.

Failure of electronic components Ways electronic components fail and prevention measures

Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.

Pad cratering is a mechanically induced fracture in the resin between copper foil and outermost layer of fiberglass of a printed circuit board (PCB). It may be within the resin or at the resin to fiberglass interface.

Glass frit bonding, also referred to as glass soldering or seal glass bonding, describes a wafer bonding technique with an intermediate glass layer. It is a widely used encapsulation technology for surface micro-machined structures, e.g., accelerometers or gyroscopes. This technique utilizes low melting glass and therefore provides various advantages including that viscosity of glass decreases with an increase of temperature. The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching or deposition. A low viscosity promotes hermetically sealed encapsulation of structures based on a better adaption of the structured shapes. Further, the coefficient of thermal expansion (CTE) of the glass material is adapted to silicon. This results in low stress in the bonded wafer pair. The glass has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures occurs. The usual temperature of achieving flowing and wetting is between 450 and 550 °C.

Physics of failure is a technique under the practice of reliability design that leverages the knowledge and understanding of the processes and mechanisms that induce failure to predict reliability and improve product performance.

Stencil printing is the process of depositing solder paste on the printed wiring boards (PWBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are a stencil, solder paste, and a printer.

Digital image correlation analyses have applications in material property characterization, displacement measurement, and strain mapping. As such, DIC is becoming an increasingly popular tool when evaluating the thermo-mechanical behavior of electronic components and systems.

Dye-n-Pry, also called Dye And Pry, Dye and Pull, Dye Staining, or Dye Penetrant, is a destructive analysis technique used on surface mount technology (SMT) components to either perform failure analysis or inspect for solder joint integrity. It is an application of dye penetrant inspection.

Conductive anodic filament, also called CAF, is a metallic filament that forms from an electrochemical migration process and is known to cause printed circuit board (PCB) failures.

References

  1. ↑ Robertson, Christopher T (2004). Printed Circuit Board Designer's Reference: Basics. Prentice Hall Professional. p. 45. ISBN   9780130674814.
  2. ↑ "Circuits Assembly Online Magazine - Using Cross-Sectioning for PCB Assembly Qualification".
  3. 1 2 "From transistors to bumps: Preparing SEM cross-sections by combining site-specific cleaving and broad ion milling | Solid State Technology". Archived from the original on 2017-11-14. Retrieved 2017-10-30.
  4. ↑ "PCB Microsectioning – Circuit Board Cross Section | saturn". Archived from the original on 2017-12-01. Retrieved 2017-10-30.
  5. ↑ "Allied High Tech Products - Mechanical Milling".
  6. ↑ "Abrasive Cutting Machines & Precision Cutting Machines | Buehler".
  7. ↑ http://www.circuitinsight.com/pdf/alternative_methods_cross_sectioning_smta.pdf [ bare URL PDF ]
  8. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2017-08-29. Retrieved 2017-10-13.{{cite web}}: CS1 maint: archived copy as title (link)
  9. ↑ https://nepp.nasa.gov/docuploads/D084F3EB-BFA9-4733-8F535584A99095F9/Dernning_BGAInspection.pdf [ bare URL PDF ]
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