Trusted Name In Electronics Components

Beiträge vom September, 2011

Leadless Chip Carrier Castellations

Friday, 23. September 2011 3:24

For the most demanding applications, where cost is not a major constraint, a three-layer construction is used with a flat gold plated lid sealed using a gold-tin solder preform. This style of package is still used for many RF devices, although cheaper alternatives have been devised, as indicated in Figure 8. Glass sealing, using a pre-glassed ceramic lid with a three layer chip carrier, results in some cost saving. The use of a single layer chip carrier having a pre-glassed cavity and a ceramic cup-shaped lid gives a device of about half the price. Even further economies are obtained, if there is no need for hermeticity, by encapsulating the device on its ceramic base, in epoxy resin.

LCCCs were originally designed to be soldered to the ceramic substrate of hybrid circuits. In this application the thermal expansion coefficients of substrate and component are matched, but this is not the case with assemblies on FR-4 laminate. Consequently, for PCB applications, the LCCC has been superseded by devices with compliant leads, to compensate for CTE mismatch between the component and the board.

Thema: semiconductors | Kommentare (0) | Autor:

The Chip Carrier Concept

Friday, 23. September 2011 3:22

The term ‘chip carrier’ refers to a range of IC packages that are square or rectangular, with terminations brought out on all four sides. The first of these to be developed was the leadless ceramic chip carrier (LCCC). This can be envisaged as the useful active centre of a hermetic DIP, with all the leads and excess packaging material discarded. Since it is constructed of the same materials and in the same manner as hermetic DIPs, it is at least as reliable. Leadless ceramic chip carriers were commonly available in sizes from 5 mm square to 25 mm square and above, with 1.27 mm (0.050 in) lead pitch, and from 20 leads to upwards of 100.

Leadless ceramic chip carriers are constructed in a variety of ways that are dictated by the end product use and the cost of manufacture. The principle of the construction is that the IC chip is bonded to a ceramic base and connections are made with fine wires to metallisation patterns that are brought out to external solderable contact pads (castellations) as shown in Figure 8.

Thema: semiconductors | Kommentare (0) | Autor:

Leaded IC Packages

Thursday, 22. September 2011 2:06

The small outline transistor (SOT) and the small outline integrated circuit (SOIC, or simply SO) packages have a longer history of use than other surface mounting devices. The SO package was developed inEuropein the mid-1970s particularly for the emerging electronic watch market.

The SOIC is a plastic package, available in 6, 8, 10, 14, and 16 pin versions with a body width of 4 mm, and in 16, 20, 24 and 28 pin versions with a wider body of 7.6 mm. The flattened leads are on standard 1.27 mm (0.05 inch) centres and are formed outwards in a ‘gull wing’ fashion, so that the tips of the leads lie in contact with the PCB. The package outline and typical dimensions of the SOIC range are given in Figure 6 and Table 7. They may vary very slightly from one manufacturer to another except for the lead pitch which must be 1.27 mm.

The low lead-count SOICs require less than half the area of their DIP equivalents and weigh only one-tenth as much.

Table 7: Dimensions of SOIC packages

Type

Leads

Pitch
mm (p)

Width
mm (w)

Length
mm (L)

Body width
mm (A)

Device width
mm (W)

Height
mm (C)

SO-6 6 1.27 0.4 3.75 4.0 6.2 1.6
SO-8 8 1.27 0.4 5.00 4.0 6.2 1.6
SO-10 10 1.27 0.4 6.25 4.0 6.2 1.6
SO-14 14 1.27 0.4 8.75 4.0 6.2 1.6
SO-16 16 1.27 0.4 10.00 4.0 6.2 1.6
SO-16L 16 1.27 0.5 10.50 7.6 10.65 2.6
SO-20 20 1.27 0.5 13.00 7.6 10.65 2.6
SO-24 24 1.27 0.5 15.60 7.6 10.65 2.6
SO-28 28 1.27 0.5 18.10 7.6 10.65 2.6

SOIC-20

The SOP concept has more recently been extended to include a wide range of thinner, smaller packages, many with smaller pitch centres.

Thema: semiconductors | Kommentare (0) | Autor:

The Dual-in-line Package of Integrated circuits

Thursday, 22. September 2011 2:06

The dual-in-line package (DIL or DIP) has a number of disadvantages which began to become apparent as the pressure increased for higher lead-count devices:

  • A DIP with large numbers of pins in a double row at a (0.1 inch) pitch becomes relatively expensive because of excessive size and material use
  • The DIP format results in long internal lead lengths for the pins towards the ends of the package, with consequent higher inductance and inter-lead capacitance, limiting device performance
  • The package occupies more board area than is necessary, since board manufacturing technology has advanced to accommodate much smaller lead pitches. Figure 5 shows a typical design plan of a 64-pin DIP and demonstrates how inefficient is its use of board area
  • Larger sizes of DIP become progressively difficult to handle robotically and to insert automatically into plated through-holes.

In short, it is the mechanical properties of the package that have physically limited the size to which the DIP can grow.

As the technology of surface mounting has developed, a range of packaging types has therefore emerged, some by transposition from the hybrid microelectronics industry and some by development in their own right.

Thema: semiconductors | Kommentare (0) | Autor:

Power Devices

Wednesday, 21. September 2011 2:06

As you will have deduced from the different Small Outline packages for transistors, higher power means that more heat energy has to be removed from the package, and this usually means the incorporation of a larger metal structure in order to conduct heat from the die where it is dissipated.

Full consideration of this topic is beyond the scope of this module, but you should be aware that, in power devices, adaptations have to be made to the ways in which bonds are made to the die reverse and to the top surface. Typically requirements for external heat sinking must be observed in order to prevent the component ‘burning out’. Power devices are also generally associated with higher-than-average currents, and this aspect is one more factor to be considered during board design.

Thema: semiconductors | Kommentare (0) | Autor:

Diode Formats

Wednesday, 21. September 2011 2:04

Discrete diodes are frequently packaged in SOT-23 format: one of the three contacts may be redundant, although diode pairs with either anodes or cathodes connected together are common.

However, several two-terminal hermetically sealed glass-to-metal packages have been developed especially for diodes, the two most popular both being cylindrical. The MELF is so-called because its appearance and dimensions are similar to those of the earlier MELF (metal electrode face bonded) resistors. With adequate cooling, power dissipation can be as high as 2 W.

MELF (Metal Electrode Face Bonded) diode
The SOD-80 (‘Small Outline Diode’) encapsulation (or ‘MiniMELF’) is 3.6 mm long and 1.6 mm in diameter. It is cheaper, lighter, and requires less board space than the SOT-23, but has no flat top surface and can be more difficult to handle. Its construction is specifically designed for small diode chips and the package dissipation is limited to 250 mW.

Cylindrical packages have no flat top surface and are sometimes difficult to handle. The manufacturing costs of glass parts may also be higher than for plastic encapsulations. For these reasons, moulded plastic packages such as the DO-214  are becoming increasingly common. Different sizes of package, with more or less heat sinking, are selected according to the die size and current/power rating of the application.

Thema: semiconductors | Kommentare (0) | Autor:

Transistors and diodes

Tuesday, 20. September 2011 2:00

Discrete transistors and diodes are generally available in standard Small Outline Transistor (SOT) packages originally designed for use in hybrid microelectronics assembly. Although there are moves to introduce smaller variants, the most common of these is the SOT-23 (now renamed in the American JEDEC standard as TO-236). The construction of a typical SOT-23 package is shown in Figure 1. The SOT-23 can accommodate almost any semiconductor with a die size up to about 0.75 mm square, and its power handling capabilities make it suitable for small-signal transistors.

SOT-23 (Small Outline Transistor or Diode) package
There have been many criticisms of the design, which was an early-1970s compromise, and comparatively large. Improved internal constructions have made it possible to develop a range of more compact devices, which are more compatible with the smaller multi-layer ceramic capacitors now in common use. The smallest of these currently has a moulding size of 1.6 × 0.8 mm, with a seated height of 0.7 mm, and lead centres on 1.0 mm pitch, occupying only 30% of the mounting area of a standard SOT-23.

There are a number of similar packages, differing in their size, power handling capacity and number of leads, but using the same basic concept. In all cases the trend is towards smaller packages, and finer lead pitches.

Dissipating more heat generally demands a heavier metal leadframe. As an example, semiconductors on chips up to about 1.5 mm square can be packaged in the SOT-89 format, shown schematically in Figure 2. Its three leads are all along the same edge of the package but the centre one extends across the bottom to improve the thermal conductivity. Whilst the SOT-23 can dissipate typically 200 mW in free air at 25°C, the SOT-89 can handle up to 500 mW under the same conditions.

Thema: semiconductors | Kommentare (0) | Autor:

Semiconductor ‘back-end’ processes

Tuesday, 20. September 2011 1:58

We will learn much more about semiconductor ‘back end’ processes in subsequent modules, but you need to understand at least something of the terminology and what is involved in turning a ‘chip’ into a finished package1.

1 It is always very dangerous to point people at sources of information on packaging.

The starting point for semiconductors is the wafer, which contains a (very large) number of devices, separated by small gaps, and electrically isolated from each other as part of the processing. Starting with a blank wafer of extremely pure silicon, building up layers by deposition techniques, etching patterns, and implanting dopants into the silicon structure using high energy particles, the semiconductor ‘fab’ ships a wafer which is partially probe-tested, but needs terminations in order to communicate with the outside world. These ‘front end’ processes attract the headlines, but the back end of the pantomime horse is just as important!

The ‘back end’ process consists of sawing the wafer into individual dice (the terms ‘chip’ and ‘die’ are equivalent), mounting the die on a lead-frame or other mount using conductive adhesive, and finally making fine wire connections to the top surface – this ‘wire bonding’ process uses gold and aluminium wires typically 25-33 µm in diameter. Because wires and semiconductor are relatively fragile and easy to contaminate, the die will then be protected in some way.

A silicon wafer

Lead-frame for a 128-pin QFP
Silicon die wire-bonded onto a ceramic substrate
The first solid state devices were incorporated into high reliability military and telecommunications applications, and needed a hermetic package to prevent junction leakage and degradation of transistor gain caused by moisture and contamination.

Packages were made either of metal, with glass-to-metal seals isolating the leads, or of ceramic: both technologies were expensive, and often relatively large and mechanically fragile. However development of silicon planar technology, improvements in methods of passivation for the die surface, and advances in polymer formulation and purity, combined to make it possible to mount a silicon die on a free-standing lead-frame, encapsulate the assembly in resin (usually by transfer-moulding), and create a protected device reliable enough for most applications.

Thema: semiconductors | Kommentare (0) | Autor:

Polymer types

Monday, 19. September 2011 1:59

Polymer science is a broad field that includes many types of materials which incorporate long chain structures with many repeated units. One useful way of categorising polymers for the requirements of electronic assembly is by functional behaviour. In the strictest sense these categories are not fixed, or even particularly precise, and you should be aware that some materials can fit into more than one category:

  • Elastomers are flexible or ‘rubbery’ materials which can readily be deformed, and return rapidly to almost their original shape and size once released from stress, thus making them able to form reliable seals. Natural and synthetic rubbers are common examples of elastomers
  • Plastics are materials which can be shaped or moulded under appropriate conditions of temperature and pressure, and then hold their shape. In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity
  • Some plastics, such as nylon and cellulose acetate, can be formed into fibres. These have different mechanical characteristics and are often regarded as a separate class of polymer.

The stress-strain relationships for these groupings are substantially different, as may be seen in Figure 1.

Elastomers

ASTM D-156611 defines an elastomer as a ‘macromolecular material that returns rapidly to approximately the initial dimensions and shape after substantial deformation by a weak stress and release of the stress.’ Such elongations typically exceed 100%.

The earliest-used elastomer was natural rubber, obtained from the sap of the rubber tree, which contains around 95% of a polymer whose repeating unit is isoprene. As polymer chemists evolved more and more polymers resembling natural rubber in properties, the term elastomer has grown to represent these materials, rubber being reserved for its original use.

Elastomers have three main functions in electronic assemblies:

  • to form an environmental seal
  • to provide mechanical strain relief
  • to give a means of conducting heat away from sources within the assembly.

Although used for many centuries in its raw form, a significant step forward was made when Charles Goodyear succeeded in ‘vulcanising’ natural rubber by heating it with sulphur to induce what is now understood to be cross-linking. The significance of the great performance improvement resulting from this treatment has led to the term ‘vulcanisation’ often being loosely used to describe the cross-linking of any elastomer.

Elastomers consist of long chain-like molecules, linked together to form a three dimensional network. Typically, an average of about 1 in 100 molecules are cross-linked: when this number rises to about 1 in 30, the material becomes more rigid and brittle. Most elastomers are thermoset materials, and cannot be remoulded, an exception being the class of materials known as ‘thermoplastic elastomers’.

The most common elastomers used in electronics are silicones (Section 0), which are supplied either as thick pastes which can be dispensed, or as fully cured preforms. Silicones may be chosen because of their chemical inertness or, more commonly, because of their thermal performance. Silicone elastomers may also ‘double’ as adhesives.

Moulded cured elastomers can also be supplied with conductive filler materials, such as silver, copper, nickel and graphite. These gasket materials are intended to combine an environmental seal with shielding against electromagnetic interference (EMI).

Plastics

‘Plastic’ is a term which can cover a wide range of polymer materials, all of which can be moulded, for example to produce the body of a QFP component, the casing for a computer keyboard, the hand set of a mobile telephone or the encapsulant cover for a PLCC.

There are two main groups of plastic polymers, thermoplastics and thermosets:

Thermoplastics

Thermoplastics are supplied fully polymerised and remain permanently fusible, melting when exposed to sufficient heat, and potentially they can be recycled and reused.

Although some thermoplastics can have a crystalline microstructure, the essential feature of their structure is that there are relatively weak forces of attraction between the chains. These are overcome when an external force is applied (resulting in the plastic deforming) or when the material is heated, so that it becomes first soft and flexible and eventually a viscous melt. For each thermoplastic there is a specific temperature at which the material will start to distort, which is known as the ‘heat distortion point’. However, when the material is allowed to cool it solidifies again. This cycle of softening by heat and solidifying by cooling can be repeated more or less indefinitely and is the basis of most processing methods for these materials.

Thermoplastics are usually supplied in the form of granular feedstock, which is heated, melted and moulded, and removed from the mould only when it has cooled below its ‘heat distortion temperature’.

Examples of thermoplastics are polyethylene, poly(vinyl chloride), polystyrene, nylon, cellulose acetate, acetal, polycarbonate, poly(methyl methacrylate), and polypropylene.

Thermosets

A thermoset material is produced by a chemical reaction which has two stages. The first results in the formation of long chain-like molecules similar to those present in thermoplastics, but still capable of further reaction. This second stage of inter-linking the long molecules takes place at the point of use and often under the application of heat and pressure.

Since the cross-linking of the molecules is by strong chemical bonds, thermoset materials are characteristically quite rigid and their mechanical properties are not heat sensitive. Once cured, thermosets cannot again be softened by applying heat: if excess heat is applied to these materials they will char and degrade – as with eggs, once hard-boiled, they cannot be softened! Examples of thermosets are phenol formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies, and some polyesters.

Thermoset raw materials are supplied in an uncured or partially cured state and fully cured during fabrication. The various stages of cure of a catalysed thermoset resin are known as ‘A-stage’ (uncured), ‘B-stage’ (partially cured), and ‘C-stage’ (fully cured). Many moulding compounds and laminating fabrics can be processed whilst in the B-stage, and some must be kept refrigerated until ready to use.

Compared with thermoplastics, thermosets are much less soluble in organic solvents, and have harder surfaces. Inherently somewhat brittle, thermosets can be combined with reinforcements such as fibre-glass to form very strong composites.

Many thermoset mixes produce an ‘exothermic’ reaction, that is they give off heat during curing. The amount of heat generated will depend on the material and the amount of catalyst used. This effect needs to be taken into account when processing thermosets, particularly when producing large castings.

Adhesives

Adhesives can be classified by the method used for curing, and a number of different mechanisms have been developed to suit different applications:

  • Anaerobic adhesives are single-component materials which cure at room temperature when deprived of contact with oxygen. The curing component in the adhesive will not react with the adhesive as long as it is in contact with oxygen. The capillary action of this type of liquid adhesive carries it into even the smallest gaps to fill the joint.
  • In ultraviolet curing adhesives, the chemicals which would initiate curing are present, but are bound together and are inactive until exposed to UV light. The degree of cure depends on the UV intensity, and in some applications light may be physically blocked from reaching the polymer. In order to resolve this ‘shadowing effect’ problem, UV curing adhesives may have secondary curing activation systems, often using heat to ensure that the cure is complete.
  • Anionic reactive adhesives polymerise when in contact with slightly alkaline surfaces. In general, ambient humidity in the air and on the bonding surfaces is sufficient to initiate curing.
  • Activation adhesives are two part adhesives, where the solvent-based activator is applied to the surfaces to be bonded, often by spraying. Polymerisation begins as soon as the adhesive is applied to the activated surface.
Adhesive types

The lack of a detailed understanding of the adhesion process has not hindered progress in developing very strong adhesives for most materials. The only problem is that the wide range of chemical structures makes it impossible to produce an adhesive which is compatible with all polymers. It is always prudent to check recommendations on suitable adhesives and surface preparation with the material manufacturers.

There are two main classes of adhesive for polymeric materials:

  • Solvent adhesives, which may be either a pure solvent which attacks the surfaces to be joined so that they fuse together, or a solvent containing some of the adherend material. This approach is used for polymers such as polystyrene and polymethyl methacrylate, the choice of solvent depending on practical issues such as the rate of evaporation
  • Organic adhesives based on rubbers or polymeric materials, which may be thermoplastic or thermosetting in nature.

The most versatile range of organic adhesives is that based on epoxy resins, and these are particularly widespread in electronics, although they are relatively expensive. The major advantages of epoxy adhesives are that:

  • No by-products are released on cure, so that only light contact pressure is necessary
  • They can be formulated to work well over a very wide temperature range
  • Epoxies have excellent resistance to moisture and chemicals
  • Shrinkage on cure is negligible, so that residual strains in the joint are small
  • Creep of the cured material is low
  • Epoxies can be cured at room or lower temperatures, although those cured at elevated temperatures are stronger.

There are however, the disadvantages that:

  • Care is needed when handling uncured epoxy resins
  • The shelf life of some formulations is limited
  • High temperature strength can only be achieved by sacrificing ductility.

Epoxy adhesives are sold either as two-part adhesives, where the epoxy resin is mixed with a catalyst just before use, or single-part materials, where the catalyst is incorporated during manufacture. Single-part adhesives are generally less reactive, needing to be heat-cured, and often require refrigerated storage to increase storage life.

Note that refrigerated materials generally need to be brought to room temperature before use, and it is unwise to try and accelerate this process. Remember to read the manufacturer’s recommendations on storage life both before and after thawing!

Other adhesives you may encounter are:

  • Nitrile rubber adhesives: usually copolymers of butadiene and acrylonitrile, these are good adhesives in their own right, but also combine with phenolic resins to produce very good structural adhesives
  • Resorcinol adhesives are particularly good for bonding thermosetting plastics, but with a few exceptions (ABS, nylon, acrylic) are not suitable for thermoplastics
  • Cyanoacrylate adhesives can produce very strong bonds very quickly, even between dissimilar materials. The polymerisation reaction is triggered off by water or other weak base on the surfaces to be bonded. Cyanoacrylate adhesives are quite difficult to work with, having a low viscosity and exhibiting a slight ageing effect.

Thema: Others | Kommentare (0) | Autor:

‘Tailoring’ Polymers

Monday, 19. September 2011 1:54

The range of polymer materials available is enormous, as slight changes in the chemical make-up of the monomers or the conditions of polymerisation can result in dramatic changes in the material characteristics of the end of processed polymer.

Polyethylene is an example of a polymer which can be used in a wide variety of applications because it can be produced with different forms and structures. The first to be commercially exploited was called low density polyethylene (LDPE), which is characterised by a high degree of branching, which forces the molecules to be packed rather loosely. The resulting low density material is soft and pliable and has applications ranging from plastic bags and textiles to electrical insulation.

By contrast, high density (HDPE) or linear polyethylene demonstrates little or no branching, so that the molecules are tightly packed and the plastic can be used in applications where rigidity is important, such as plastic tubing and bottle caps. Other forms of this material include high and ultra-high molecular weight polyethylenes (HMW; UHMW), which are used in applications where extremely tough and resilient materials are needed.

New materials can also be tailored by combining monomers with desirable properties. In some cases, these combinations are just physically mixed polymers, but more typically new ‘co-polymers’ are produced. Some types have a random structure of the constituent monomers, others may have a regular, repeating structure of the different materials:

  • A ‘block’ copolymer is made with blocks of monomers of the same type
  • A ‘graft’ copolymer has a main chain polymer built with one type of monomer, and branches made up of other monomers.

Thema: Others | Kommentare (0) | Autor: