Mounting of Passives Electronic Components

Passive Electronic Components

Lecture 3

Page 1 of 19

02-Apr-2017

Mounting of Passives Electronic Components

Lecture Plan

1. Point-to-Point Mounting
2. Through-Hole MountTechnology (TMT).
3. Surface MountTechnology (SMT).
4. Printed Circuit Board (PCB).
5. Soldering.
6. Gluing and welding.

1. Point-to-Point Mounting

The technologyis characterized by:

• type of component terminals: (a) wire, (b)lug, (c) clamp, (d) plug;
• component mounting by: (a) screwing, (b) clamping, (c) soldering to lugs, (d) insertion in a socket;
• interconnections are performed using: (a) hook-upwire, (b) chassis.

Point-to-Point Mounting technology started together with the first electrical and electronic appliances.

Fig.1. Point-to-Point Mounting

Fig.2. Different types of terminals in electronic components suitable for point-to point mounting

Method of soldering in the case of Point-to-Point Mountingis manual soldering using solderingiron.

1. Through-Hole Mount Technology (TMT) is the method of mounting of electronic components with wire or wire-like stamped terminals by insertion of terminals in special holes in printed circuit board followed by soldering of the terminals to metalizes pads that surround the mounting holes.

The technology is characterized by:

• Needle-like (wire or stamped) terminalsof the components;
• Dielectricpanel with holes (printed circuit board, PCB);
• Mounting pads around the holes and interconnections between the pads formed from copper foil laminated to surface of dielectric panel;
• Components mounting by insertion of wire terminals through the holes in PCB;
• Wave Soldering Technology.

Dielectricpanel and copper foil patterns laminated to its surface constitute Printed Circuit Board (PCB).Through-Hole Mount Technology is inseparably linked with PCB.

Fig.3. Cross-section of PCB with through-hole mounted components (a).

Top view on PCB with mounted components(b).

Bottom view onPCB – "printed" copper conductors (c).

2.1.PCB history.

2.1.1.In 1903, Albert Hanson, a Berliner living in London, filed a “printed” wire patent aimed at solving the telephone exchange need. It was proposed to produce conductive metal patterns by cutting or stamping of copper orbrass foil. Then the patterns were bonded to dielectric plate(additive patterning).

2.1.2.Thomas Edison proposed idea of selective applying of glue and dusting of conductive (graphite or bronze) powder.

2.1.3.In 1913, Arthur Belly filed for a patent describing the method in which metal was etched away (subtractive patterning). Resists layerhad to be applied to metalized surface of the board before etching in places where metallization has to remain.

2.1.4.In 1918, Max Schoop proposed flame-spraying process.

2.1.5.In 1942 newly invented in Englandbomb fuse had printed circuit instead of wiring.

2.1.6.In 1947, “A Circuit Symposium” conference in Washington, D.C. sponsored by U.S. Aeronautical board and the NBS was held. Six processes were endorsed by the governmental technical representatives:

1. Painting by metal-filled inks.
2. Metal spraying.
3. Chemical deposition.
4. Vacuum deposition.
5. Die stamping and bonding.
6. Dusting conductive powder on tacky ink.

Subtractive patterningprocess that nowadays has become the principal process of PCB technology was not mentioned. It was supposed to be an auxiliary process.

2.1.7.In 1948 the U.S. authorities ruled that all electronic circuits for airborne instruments were to be printed.

2.1.8.In the 50s photoimaging and etching processes were introduced together with copper-clad material in PCB manufacturing. These improvements were connected with name of inventor Paul Eisler, Austrian Jew that was forced out of work by the fascists in 1934 and left for England.

In the beginning the components were inserted in PCB manually. Later semiautomatic and even automatic insertion equipment and solderwave soldering equipmentwere developed. Nowadays TMT remains in use in some limited applications in the form of mixed technology (TMT + SMT). It is used in some products that should not be miniaturized (TV, videos, home audio, etc.). The reason is a cost consideration. Semiautomatic TMT equipment hastypical insertion rate of 750 cph (components per hour). Automatic equipment can reach about 15,000 cph speed but needs special pre-process: terminals forming and components taping in special sequencer machine.

Typical method of soldering in TMT process is wavesoldering technology (see “Soldering” paragraph below).

1. Surface Mount Technology (SMT)is the method of mounting of electronic components on flat surface of PCB without mounting holes.

The technology is characterized by the following features:

• Stamped “legs” in so called leaded component or metallized edgesof so called leadless component(chips) constitute the terminals (see picture below).
• Component placement on PCB and soldering are completely automated.
• PCB is similar to TMT PCB but without mounting holes.

Fig.4. Surface mountedcomponents on PCB

In the early 80s the industry began to replace the traditional Through-Hole Mounting technique (TMT) with the Surface Mount Technology (SMT). Special Surface MountableDevices (SMD) replaced the traditional wire-leaded components. The historical roots of SMT can be traced back tothe middle of 70-s when hybrid and microwave circuits were developed on the base of so called “leadless components” mounted on ceramic substrates with screen printed interconnections.

SMT components may be leaded and leadless (chips). They are placed on PCB by pick-and-place machine (Chip Shooter) with placement rate up to 40,000 cph. Placement precision is about 0.05 mm at 3. The outline dimensions of a smallest two-terminalchip component are 0.3mm×0.15mm.Fine pitch multi-terminal components (ICs) have a pitch down to 0.3 mm.

Advantages of SMT components (when compared with TMT components):

• Smaller dimensions of components make it possible to increase mounting density.
• Smaller dimensions of components result in better HF performance (low parasitic inductance, low package propagation delay, low electromagnetic interferences).
• SMT components commonly have lower cost.

Problems related to SMT components:

• Rigid terminals of leadless chips make solder joints more prone to cracksthat may result from PCB bending, vibrating or thermal cycling.
• “Tombstone effect” (see the end of the lecture) is possible duringsoldering of chip components.
• Manual assembly of prototype circuit is problematic.
• Surface mounting is unsuitable as the sole attachment method for components that are subject to frequent mechanical load (connectors, mechanical switches).
• Labeling (marking) of small components is problematic or even impossible.

Thermal fatigue of solder joints .

Material fatigue is a structural damage suffered by materials when they are subjected to periodic mechanical stress. The periodic stress may result from:

• application of periodic mechanical force;
• temperature cyclingandmismatch of component's and PCB's thermal coefficients of expansion.

We shall consider the last case.

Suppose that a chip resistor having length is soldered to the surface of a PCB as it is shown in the figure below. The solder layer thickness is. Ambient temperature is T1.

Suppose that ambient temperature changed to : . Even in case when chip resistor does not dissipate power a shear strain in the solder joint arises because of different thermal coefficients of expansion of chip and PCB.

Chip length after temperature change will be . At that, the length of underlying part of PCB will be. It will result in shear stress in the rectangular solder blocks that represent solder joints. It is known from the Strength of Materials that shear strain is measured by the anglethat represents the change ofthe initiallyright angle as shown in below picture ( when ambient temperature is T1).Let us neglect elastic strain in chip and PCB. At that, may be found geometrically.

Commonly this angle is very small and therefore may be approximatelyreplaced by its tangent:

Multiple (cyclic) deformation of solder joint results in its cracking (Fatigue Phenomenon).

The first-order approximation of the relationship between the strainin soldering material and number of cycles to the material failure is given by Coffin-Manson relation:

,

where - fatigue ductility coefficient, c - fatigue ductilityexponent. They are two dimensionless physical constants that characterize particular soldering material.

Equation (1.2) is applicable to Low Cycle Fatigue - the loading that typically causes failure in less than 104 cycles and is associated with relatively large plastic (non-elastic) strain in metals. This situation is typical for solder joints.Equation (1.2) may be represented graphically as a straight line in coordinates (see graph below).

Plotted experimental data for 60Sn40Pb solder

Let us substitutein (2) by (1):

.

In the case of 60Sn40Pb solder it may be supposed that , .

For lead-free Sn3.0Ag0.5Cusolder , [4].

Numerical example. Suppose that d = 6.310-3 m (the biggest size of standard chip resistor), T = 35 K,  = 510-5 m (common thickness of solder in a joint). Number of cycles to failuremay be calculated as the following.

Nf - ?

Pay attention that the power of -2 = -1/0.5 is not integer value! It is approximate value with single significant digit.

1. Printed Circuit Board.

4.1.PCB construction and parameters

PCBs are generally defined by:

• type of dielectric material,
• number of conductor layers,
• copper thickness that is traditionally characterized by “copper weight”.

Copper Weightis measuredin Ounces of Copper Per Square Foot (m = 1oz.= 28.35g, S = 1ft2 = 0.3052m2). The common range of the parameter is 1/4… 3 oz. Typical value is 1 oz. Higher numbers are usedin high current and high frequency applications.

Copper density is = 8920 kg/m3. Let us calculate 1 oz. copper layer thickness h.

Suppose that copper weight number has unlimited precision (idealization):

h = ?

It is commonly supposed that 1 oz. of copper per square foot corresponds to35 m of copper thickness.

Let us calculate so called sheet resistivity of 1 oz. copper layer. Sheet resistivity is resistivity of a square pattern of a film or a foil of given thickness. Resistance of rectangular pattern may be calculated as the following:

,

wherel, b, h, S – respectively length, width, thickness [m], and cross section area [m2] (perpendicular to the length direction) of the pattern, - material resistivity [m]. Conclusion: Resistance of a PCB foil pattern depends on its length/width ratio and does not depend on absolute values of its linear outline dimensions.

Suppose that (square pattern). Its resistance we shall call “sheet resistivity” and designate as .

= ?

Sheet resistivity =. Itmust have[] dimensions. But for better understanding of the matter it is commonly designated as[/square], [/sq.],or [/□].

Layering

Layers of copper tracesin PCB are divided by dielectric layers. Number of copper layers may vary from 1 to 30. External layers (top and bottom) are covered by a screen printed solder mask.

Commonly board thickness is 15 mil…250 mil or 0.38 mm…6.35 mm(1 mil=25.4 m).

Dielectric thickness is 1.3…2 mil or 0.03 mm…0.05 mm.

Line width and spacing between lines – down to 5 mil or 0.13 mm. Line (trace) thickness and width are selected depending on the expected current. Spacing between the traces depends on the expected voltage.

Vias(metallized holes for interlayer connections).

Types: (a) blind, (b) through hole, (c)buried, (see picture below).

Vias: a) blind ; b) through hole; c) buried art_bb.htm

4.2.Dielectrics for PCB manufacturing

Glass-reinforced polymer (magnified cross-section)

Glass-reinforced polymer laminates.The most common combination of resin and reinforcement material isepoxyand woven glass (FR-4, FR-5). In special applications another materials are used like polyester resin, non-woven glass, etc.

Ceramic PCBs .

Three ceramic based technologies are used to manufacture ceramic PCB: Thick Film Multilayer (TFM), High Temperature Co-fired Ceramic (HTCC) and Low Temperature Co-fired Ceramic (LTCC).The same technologies are used for manufacturing of Multi-Chip Modules (MCM).

Thick Film Multilayer (TFM).Multilayer circuits formed by sequential screen printing of conducting and dielectric thick-film materialson ceramic substrate. After printing each layer is dried, and fired. Double-sided and multilayer circuits with up to four circuit layersare manufactured in high volume for commercial applications (automotive, telecommunications). For military and aerospace applications, circuitswith six to eight circuit layers are manufactured in a moderate volume.

Co-fired ceramics (HTCC and LTCC). Sheet dielectric ceramic material in not fired (“green”) stateis cast as a flexible tape and used as a substrates for screen printing of conducting thick-film materials. Printed layers are laminated (stacked and pressed together), and then co-firedto receive rigid and monolithic board.The advantage of co-firing process is possibility of inspection of all printed layers prior to lamination. This insures better capability of the process compared to sequential screen printing of Thick Film Multilayer (see above paragraph).

High Temperature Co-fired Ceramic (HTCC).Ceramic “green” tape materialiscommonly based on alumina. Interconnections are screen printed using inks based on refractory metals: tungsten (W) and molybdenum (Mo)that can be fired in hydrogen atmosphere at high temperature 1600…1800 C required for alumina substrate firing. Unfortunately, their electrical conductivity is significantly lower when compared to silver conductors used in LTCC.The advantage of HTCC is almost order of magnitude higher thermal conductivity than in LTCC.

Low TemperatureCo-fired Ceramic (LTCC).The tape material is composed of a glass and alumina.High conductivity silver-basedthick-film material is used for screen printing of conductive traces. Multilayer PCBs with 50 or more layers have been manufactured using LTCC technology.This technology is well-suited for constructing RF modules for wirelessapplications with capacitors and inductors integrated into the same substrate. The shortcoming of LTCC is lower thermal conductivity than in HTCC.

Thermal conductivity of PCB materials

LTCC

1. Soldering.

Permanent connection of mechanical parts may be done by using welding, soldering, and gluing. Their essence is introduction of liquid phase between the parts to be connected (molten base metal in welding, molten solder in soldering, glue in gluing). The liquid wets the parts, gets in intimate interfacial contact with them arising intermolecular attractive forces. Then the liquid solidifies keeping the parts connected.

Soldering is a process by which the parts are joined together using material (solder) with lower melting point than melting points of materials of joined parts. Welding is performed by melting of materials of joined parts. Glues are liquid materials that solidify as the result of chemical reaction or evaporation of solvent.

5.1.Soldering materials.

Solder is metal (elemental or alloy) or glass. Glass is used, for example, to attach optic cable to metal connector or to assemble ceramic and metallic parts of hermetic IC package.

Metallic solders may be split in two families:

• Hard solders are copper, zinc or silver alloys with melting temperature above 400C. High melting temperature makes them not suitable for electronic assembly.
• Soft solders have lower melting temperature (but lower mechanical strength) when compared to hard solders. The most of soft solders are tin alloys.

Tin is unique metal that wets vast majority of the metals. Six not wetted by tin (and therefore not solderable by tin alloys) metals are:

• Cast iron,
• Chromium (Cr),
• Titanium (Ti),
• Tantalum (Ta),
• Magnesium (Mg),
• Beryllium (Be).

The fact that some metals that are not wetted by solder is important. These metals are used in soldering machinery for manufacturing machine parts that get in touch with molten solder.

Lead-containing solders. They were basic solders in electronic industry for many years. They have relatively low melting temperature, excellent mechanical properties but comprise toxic lead. Nowadays they are gradually replaced by lead-free solders.

The most commonly used tin-lead alloys (currently they are used mostly in military and aerospace applications).

Lead-free solders.

The most frequently used lead-free soldersarecomposed of 3…4% of silver,0.5…0.7% of copper, andtin (the rest). This family of solders is commonly called SAC (Sn, Ag, Cu) solders.Melting point of respective eutectic ternary alloy is 217C (34 higher than melting point of eutectic tin-lead alloy).

The most commonly used SAC alloy is Sn-3.5Ag-0.75Cu.

Issues with lead-free solder (when compared with Sn63Pb37 solder).

Higher melting point of lead free solder (217C versus 183C) pushes peak reflow temperatures from 220C to 260C. It results in:

1. Energy consumptionincrease and impact on the ambiance.
2. Total product cost increase.(More expensive solder, higher energy spending).
3. More rigorous requirements tosoldered components.(They are exposed to higher temperature).
4. More rigorous requirementsto fluxes.
5. Higher probability of fatigue cracks in solder joints.

5.2.Fluxes. Commonly wetting of the surfaces of soldered parts by molten solder is limited by oxides and contaminations. Fluxes are special materials thatremove oxide film and contaminations fromsoldered parts and prevent further formation of oxide films during soldering process. Furthermore, the fluxeslower the surface tension of the solder and promote wetting.In the past flux residues had to be removed after soldering in the most cases. Modern “no-clean” fluxes do not require removal of flux residues.

5.3.Wave soldering. The process is commonly used for soldering of through-hole mounted components to PCB. Sometimes it is used for soldering SMT components. But they have to be glued to PCB in advance. The basic equipment is a wave soldering machine. It comprises conveyorthat moves the PCBs with inserted components through three different zones: (a) fluxing, (b) preheating, (c) soldering (see picture below). Soldering is performed by “solder wave” device. It is a pan of solder equipped with a pump that produces the actual wave of molten solder (see picture below).