Surface Mount Tech.

Surface Mount Technology

INTRODUCTION Compact size of electronic equipment is due to surface mount technology. Surface mount technology allow packaging of electronic components so that the overall assembly is compact.

In SMT both components and conductive tracer (i.e. connections) are installed on the same side of substrate or surface. Substrate can be ceramic, paper plastic, rigid and flexible PCB’s.

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Surface Mount Technology

SURFACE MOUNT COMPONENTS Many but not all through whole components have a surface mountable counter part. Due to some physical limitations so conventional components cannot be manufactured as SMC (Surface Mount Components). e.g. High capacity capacitor and power transformers, still most circuit can be assembled, using SMT, even though some conventional through hole components are required. It is important for SMT surface designer and service technician to know the general physical configuration and operating parameters of various SMC’s. All SMC’s in familiar are available separately SMC’s used for automatic assemblies are supplied in rills of paper or in magazine. DETAILS ABOUT SOME (SMC’S) 1. Chip resister : These are most widely produced of all SMC’s. They are originally developed for used in hybrid micro circuit. The chip registers are leadless registers. Constructionally deice is similar to thick film resister. The typical size of resisters are 1.6 × 3.2 mm., 1.4 × 2 mm, 2.6 × 3.3 mm. The resistance range of most chip resistor is 10 Ω to 2.2 MΩ some companies manufacture up to the value of 10 MΩ or higher than this.

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Surface Mount Technology

2. Chip Capacitor : There are originally developed from use in hybrid micro circuits. There are basically three types of surface mountable chip capacitor. a) Multi layer Ceramic b) Electrolytic c) Tantalum Ceramic multi layer chip capacitors are commonly used. They are stable and highly reliable. The capacitance value available are from 1 PF to 1 µF. Package style is identical to chip resister. The size of chip capacitor depends on the value of capacitor. For high capacity electrolytic and tantalum capacitor are used. Tantalum capacitors are available for the values of 0.1 µF to 100 µF. Aluminium electrolytic capacitors are larger than that of tantalum. Tantalum capacitors are available in the values of 1.5 µ F to 47 µ F. 3. Potentiometer : Both single and multiterm trimming potentiometers are available in surface mountable configuration. They are made from ceramic or high temperature plastic so as to protect than from emertion soldering. The dimension of smallest single term trimmer is 4 x 4 mm. Multi terms trimmers are not designed for repeated adjustments. They can be adjusted mostly 10 times. The trimmers are designed to be adjusted by means of miniature screw driver or special tools.

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Surface Mount Technology

4. Inductors : Many kind of surface mountable lead and leadless inductors and even transformers are available. Inductance’s value ranges from few 10’s of nano henry to 1 milli Henry ( 10’s of nano h to mh ) 5. Descript Semiconductors : Many diodes, transistors and descript semiconductors are available in miniature surface mountable packages. The S O D ( Small outline Diode ) package is leadless cylinder used for diodes. The SOT ( Small outline transistor ) packages are used for transistors, diodes various up opto electronic components. Package configuration determines power dessipation of any semiconductor. 6. Integrated Circuits (IC’s) : Surface mountable IC’s are developed by Texal Instruments. The most popular surface mountable IC packages is small – outlet (SO) configuration developed by Philips. It resembles, a miniature ( Dual In Line Package ) DIP. An 50 i.e. small outlet device occupies 1/4th board space of an equipment ( Dual In Line Package ) DIP. 7. Other Surface Mountable Components : There are many other surface mountable devices available like photo transistor opto isolators/couplers, infrared LED’s, ceramic filters, switches and relays.

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Surface Mount Technology

SMT ASSEMBLY METHODS Surface mountable components like conventional through hole components can be placed an a board or soldier in place either by hand or by machine.

Hand assembly is used by home experimental and electronic companies for the production of prototype SMT circuit boards.

AUTOMATIC SMT ASSEMBLY Automatic placement equipment can select the position on circuit board from 1000 to 5 lakhs components per hour. There are three other categories of automatic SMC placements equipment (Mass placement).

CONDUCTING BONDING Soldering is a cheap method for bonding SMCs and socket terminals to circuit board pads. Conductive adhesive also used, both methods are important.

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Surface Mount Technology

TYPES OF SOLDERING There are various methods of soldering

1)

Hand soldering.

2)

Wave soldering.

3)

Reflow soldering.

In hand soldering the soldering paste is applied to the board and the SMCs are placed on it by hand, so it is called as hand soldering.

Reflow soldering is most important conductive bonding method for SMC’s. The simplest form of reflow soldering occurs when tinned terminals and thickly tinned pad is heated by soldering iron or other means. Until the two tinned layers melt and merged together.

CONDUCTIVE ADHESIVE BONDING : Conductive adhesives are used to bond the terminals of components to the conductive traces. They are relatively easy to use and they eliminate thermal shock of soldering. Numbers of families of adhesive are available, all of them contain conductive powder in one or two part base. The most common conductive powder in order of increasing resistance includes gold, silver, copper, nickel, carbon and graphite. Adhesive base includes urathene, polyster and one or two part epoxies.

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Surface Mount Technology

Conductive adhesives can be applied by hand using squeezable dispenser, just like tooth paste and automatically metered gringe or a piece of wire. They can also be applied by screening.

Thermoplastic conductive adhesives can be rework using heat from ordinary soldering iron or hot air gun. The SMC’s can be removed after adhesive becomes soften. A new SMCs can be bonded to the same location by reheating the adhesive.

The major draw back of conductive adhesive is cost, self life, remains for 6 to 12 month and hazardous vapour.

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Surface Mount Technology

ADVANTAGES OF SMT There are various advantages of SMT. 1. Reduce Circuit Board Size : The compact size of SMC’s considerably reduces area of circuit board. 2. Light in weight : SMC’s are lighter than their through hole counter port. e.g. 8 – pin DIP, LM 308 M opamp weighs 600 mg. The so package vargen of same IC Weighs 60 gm. 3. The low weight of SMC’s and smaller circuit board together gives 5 to 1 weight advantages over conventional board. Also SMT boards are thin therefore they gives 8:1 volume advantage over conventional boards. 4. Double – Sided Circuit Board : SMT can also used double sided board with an advantage that components can be placed on the both sides. 5. Subminiature Circuits : SMT circuits are nearly as tiny as hybrid integrated circuits. 6. Automated assembly : SMCs are much more compacted with automatic assembly equipment. The time consuming in drilling holes in circuit board is climinated. SMC’s have no wire leads to cut, bend and insert. Therefore SMT boards can be automatically assemble quickly than conventional boards. 7. Lower Cost : The SMC’s cost is generally more, till SMT can reduce over all board cost for variety of reasons. e.g. saving of 40% cost results from elimination of drilling hole equipment. 8. Other Advantages : Some advantages of SMT are less obvious than above. i)

Compact size of SMT so it is used in mobile.

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Surface Mount Technology

DISADVANTAGES OF SMT 1.

SMC Standardisation : The same SMC from two different manufacturers may have different dimensions, which may create a problem. Hence standardisation is necessary. Till today no standardisation is made.

2.

SMC Availability : Some 15,000 components are available as SMC but not all of them may be available when needed i.e. wanted.

3.

High start up expenses : The start up cost of SMT for both manufacturers and individual experimenters can be high. For manufacturers automated production equipment is most expensive investment.

For experimenters requires new

assembly tools and stock of surface mountable resisters, capacitors, Diodes, transistors, ICs, etc.

Thus the overall cost is very high.

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Surface Mount Technology

INSPECTION, TESTING AND REPAIRING Due to small size of components SMT board requires very careful inspection particularly for solder ball, improperly soldered joints and missed soldver connections, etc. Some components are specially difficult to inspect like quad PLCC’s (Plastic Loaded Chip Carrier) i.e. IC having J – profile pins along each four sides and it has more than 28 pins. Complete SMT board can be tested by hand or automatic testing equipment. Whether testing is done by hand or automatically the test probes should be touch to SMC’s solder pad or their conductive traces. The test probes should not be touch to terminals of SMC’s properly designed SMT boards have test point location. Replacing defective SMC’s required more patience and care because they are very small and are placed very closed to other components. While desoldering care must be taken to prevent over heating of board and nearby SMC’s. When solder melts SMC’s should be twisted before it is lifted from the board to broke the solder surface tension otherwise solder pad may comes out of the mode. Installing new SMC is not difficult just place the SMC and heat its terminal for best result and old solder should be removed using desoldering tools. Thus the repairing of SMT circuit is difficult, so we can install new circuit instead repairing faulty circuit.

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Surface Mount Technology

CONCLUSION From the above discussion I conclude that SMT is more advantages than the conventional circuit board. Because it reduces the size of circuit board, cost, weight etc. Due to above advantages SMT circuit can be mount on the coin or on the postal stamp. Also the circuit can be mounted on both sides of the circuit board. So now a days it is used in mobile, calculator or the circuit where small space circuit is required.

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Surface Mount Technology

REFERENCE 1.

W.W.W. Smta. Org

2.

Electronic Packaging and Integrating Hand book. - By Harper

3.

Electronics for you – July 1995

4.

Electronics for you – Oct. 1999

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Surface Mount Technology

Internal Electrode Secondary Electrode Protective Glass Overcoat External Electrode (Solder)

Ceramic Substrate

Register INSIDE A LEADLESS CHIP REGISTOR

External Electrode (Solder)

Ceramic Substrate Interleaved Electrodes Dielectric INSIDE A CERAMIC CHIP CAPACITOR

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Surface Mount Technology

Gull Wing

J - Lead SOP

PLCC PCB Discrete Component

MSP Butt Lead VARIOUS SHAPES OF THE PINS DIP

Discrete Component PCB

Through Hole DISCRETE

COMPONENTS

Soldered SOLDERED ON

OPPOSITE

SIDE

OF

COMPONENT SIDE SOP

PLCC

PCB Discrete Component MSP SURFACE MOUNT COMPONENT SOLDERED ON FOOT PRINTS OF PCB

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Surface Mount Technology

Solderability Testing

Board Prebake

Solder Paste Cure

Component Pretin

Board Surface Preparation

Reflow

Component Preform (Leaded)

Stencil Solder Paste

Component Assembly

Deflux Assembled Board

Component Coat REFLOW SOLDERING PROCESS PLAN

COMPONENTS

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SUBSTRATES SOLDER ELECTRICAL INSPECTION REFLOW DEVICE PASTE CLEANING BAKE PLACEMENT SOLDERING &SCREEN REWORK TEST

15 INVERT BOARD

Surface Mount Technology

FLOW CHART FOR SMT

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