SURFACE MOUNT TECHNOLOGY

(SMT)

INTRODUCTION

An SMT component is usually smaller than its through Hole counterpart because it has either smaller leads orno leads at all . It may have short pins or leads of variousstyles , flat contacts, a matrix of solder balls ( BGAs ),or terminations on the body of the component.

Surface - mount technology (SMT) is a method for producingelectronic circuits in which the components are mounted orplaced directly onto the surface of printed circuit boards (PCBs).

An electronic device so made is called a surface mount device (SMD). In the industry it has largely replaced the through - hole technology constructionmethod of fitting components with wire leads intoholes in the circuit board.

HISTORY

• Surface Mounting was originally called “Planar Mounting".• Surface-Mount Technology was developed in the 1960s ,became widely used in the late 1980s. Much of the pioneering work in this Technology was by IBM which Launched Vehicle Digital Computer( autopilot ) used in the Instrument Unit that guided all Saturn IB and Saturn V vehicles.• Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB.• Components became much smaller and component placement on both sides of a board became far more common with surface mounting than through-hole mounting, allowing much higher circuit densities.

TECHNICAL TERMS..

SMD Surface-mount devices (active, passive and electromechanical components)

SMT Surface-mount technology (assembling and mounting technology)

SMA Surface-mount assembly (module assembled with SMT)

SMC Surface-mount components (components for SMT)

SMP Surface-mount packages (SMD case forms)

SME Surface-mount equipment (SMT assembling machines)

PROCESSES INVOLVED

• SURFACE MOUNT DESIGN• SOLDER PASTE APPLICATION• COMPONENT PLACEMENT• SOLDERING• CLEANING• REPAIR/REWORK

SURFACE MOUNT DESIGN

It depends on a number of factors

• Market needs• Function• Package moisture sensitivity• Thermal and solder joints

reliability• As the packaging density

increases, thermal problems are compounded, with a potential adverse impact on overall product reliability

1. Design for manufacturability:

Includes the considerations of land pattern, placement, soldering, cleaning, repair, and test are important issues.

2.Land Pattern Design:

•The surface mount land patterns, also called footprints or pads, define the sites where components are to be soldered to the PC board.• The design of land patterns is very critical, because it determines solder joint strength and thus the reliability of solder joints, and also impacts solder defects, clean ability, test ability, and repair or rework.

3.Design for Testability:

In SMT boards, designing for testability requires that test nodes be accessible to automated test equipment (ATE).

SOLDER PASTE APPLICATION

Solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process/jet-printing mechanism

COMPONENT PLACEMENTComponents to be placed on the boards are usually

delivered to the production line in either paper/plastic tapes wound on reels or plastic tubes. Numeric Control pick-and-place

machines remove the parts from the tapes, tubes or trays and place them on

the PCB. There are different types of auto-placement machines :In-line: It employs a series of fixed-position placement

stations. Sequential: Components are individually placed on

the PC board in succession.

Placement of Components on PCB SMD Pick and Place Machine

SOLDERING

Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a relatively low melting point.We have further 2 types of soldering in Surface Mount Devices :Wave SolderingReform Soldering

WAVE SOLDERING• Wave soldering is a large scale soldering

process, used to solder the electronic components on PCB.

• Then name is derived from the use of waves in molten solder.

• The process uses a tank to hold a quantity of molten solder; the components are inserted into or placed on the PCB and the loaded PCB is passed across a pumped wave or waterfall of solder.

• Used for both through-hole printed circuit assemblies, and surface mount.

• If a wave soldering process is used in SMT, then the parts must be glued to the board prior to molten solder wave to prevent them from floating off when the solder paste holding them in place is melted.

REFLOW SOLDERING

•They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. •The boards are then conveyed into the reflow soldering oven. •The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board. •If the circuit board is double-sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place.

Steps Included In Reflow Soldering:

Pre Heating→→ Soldering→→ Cooling

INFRARED SOLDERINGDuring infrared soldering, the energy for heating up the solder joint will be

transmitted by long or short wave electromagnetic radiation

ADVANTAGES•Easy setup•No compressed air required•No component-specific nozzles (low costs)•Fast reaction of infrared source

DISADVANTAGES•Central areas will be heated more than peripheral areas•Temperature can hardly be controlled, peaks cannot be ruled out•Covering of the neighboured components is necessary to prevent damage, which requires additional time for every board•Surface temperature depends on the component's reflection characteristics: dark surfaces will be heated more than lighter surfaces

CONVENTIONAL HOT GAS SOLDERINGDuring hot gas soldering, the energy for heating up the solder joint will be transmitted by a gaseous medium. This can be air or inert gas (nitrogen)

ADVANTAGES•Simulating reflow oven atmosphere•Switching between hot gas and nitrogen (economic use)•Standard and component-specific nozzles allow high reliability and reduced process time•Allow reproducible soldering profiles

DISADVANTAGES•Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted•A rework process usually undergoes some type of error, either human or machine-generated, and includes the following steps:1. Melt solder and component

removal2. Residual solder removal3. Printing of solder paste on

PCB, direct component printing or dispensing

4. Placement and reflow of new component

REFLOW v/s WAVE SOLDERING

• Reflow soldering is the most common method of attaching surface mount components to a circuit board, although it can also be used for through-hole components.

• Because wave soldering can be simpler and cheaper, reflow is not generally used on pure through-hole boards.

• When used on boards containing a mix of SMT and THT components, through-hole reflow allows the wave soldering step to be eliminated.

• As through-hole components have been largely replaced by surface mount components, wave soldering has been supplanted by reflow soldering methods in many large-scale electronics applications. However, there is still significant wave soldering where SMT is not suitable (e.g., large power devices and high pin count connectors).

SOLDER TYPES• Many different types of solder that can be used in wave soldering, tin/lead- based solders are the most common.• But these are replaced by Lead free solder pastes, Because of highly toxic in nature.• The next best metals to use are nickel, brass, aluminium, tungsten, and lastly steel.

Lead free solder Pure tin solder

• Most common composition of solder is 63% tin, 37% lead. And another one is 11% tin, 37% lead, 42% bismuth, and10% cadmium.

CLEANING SMT ASSEMBLIES

• After soldering, the boards may be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads.

• require cleaning regardless of the solder flux type used to ensure a thoroughly clean board.

• Rosin flux is removed with fluorocarbon solvents, which require extra rinsing or drying cycles.

• . Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water.

• Proper cleaning removes all traces of solder flux, as well as dirt and other contaminants that may be invisible to the naked eye.

REPAIR / REWORK•Finally, the boards are visually inspected for missing or misaligned components and solder bridging. •If needed, they are sent to a rework station where a human operator corrects any errors.• They are then sent to the testing stations to verify that they operate correctly.

Reworking usually corrects some type of error, either human- or machine-generated, and includes the following steps:•Melt solder and remove component (s)•Remove residual solder•Print solder paste on PCB, directly or by dispensing•Place new component and reflow.

Thoroughly clean site and solder new device to PBC

Fix problems and add parts that can’t survive the high temperature of the reflow oven

•A specially formulated alloy in wire form is designed to melt at the low temperature of around 136 degrees F, 58 degrees C. It eliminates the potential for damage to the circuit, adjacent components, and the device itself.•Liquid flux and a soldering iron are used to melt this low temperature alloy that is specially formulated to stay molten long enough to react with existing solder. The SMT device can then be easily removed with a vacuum pen.

Apply Low Residue Flux to all the leads on the SMD you're removing

With a soldering iron, melt the low

temperature alloy

Easily lift device off the board with a

vacuum pen

PACKAGESSurface-mount components are usually smaller than their counterparts with leads, and are designed to be handled by machines rather than by humans. The electronics industry has standardized package shapes and sizes (the leading standardisation body is JEDEC).

SURFACE MOUNT ADVANTAGESSurface mount technology has been used on hybrid circuits for many years. Its greatest recent advance has been its use on printed circuit board (PCB) assemblies.The benefits of this technology as applied to PCB’s are substantial:

• Ease in automation results in higher production throughput and better yields.

• Closer spacing of components coupled with the ability to use both sides of the PCB for component mounting results in significant savings in board real estate.

• Reduced board cost by elimination of plated through-holes, a major portion of PCB costs.

• Simpler and faster automated assembly. Some placement machines are capable of placing more than 136,000 components per hour.

• Better mechanical performance under shake and vibration conditions

• Many SMT parts cost less than equivalent through-hole parts.

SURFACE MOUNT DISADVANTAGES

• Manual prototype assembly or component-level repair is more difficult and requires skilled operators and more expensive tools, due to the small sizes and lead spacings of many SMDs.

• SMDs cannot be used directly with plug-in breadboards requiring either a custom PCB for every prototype.

• SMT is unsuitable for large, high-power, or high-voltage parts, e.g. transformers so It is common to combine SMT and through-hole on one side.

• Unsuitable for connectors that are used to interface with external devices that are frequently attached and detached.

• Solder joint dimensions in SMT quickly become much smaller as advances are made toward ultra-fine pitch technology. The reliability of solder joints become more of a concern, as less and less solder is allowed for each joint prone to vibrations and temperatures.

CONCLUSION

At last we conclude that this technology will be very useful & effective in the fabrication of electronics circuits on PCB.

• Through hole component technique has been replaced by SMT, with higher I/O capacity, and high efficiency.• Weight, size and volume reduced.• Surface mounting lends itself well to a high degree of

automation, reducing labour cost and greatly increasing production rates.

• SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of equivalent through-hole parts.