SURFACE MOUNT TECHNOLOGY

A method of assembling printed wiring boards or hybrid circuits, where

components are attached to pads on the board surface, as distinct from through-hole

technology, where component leads are inserted into holes.

There are 3 major types of Surface Mount Assemblies: Type I

(Full SMT board with parts on one or both sides of the board)

Type II

(Surface mount chip components are located on the secondary side of the Printed Board (PB). Active SMCs and DIPs are then found on the primary side)

Type III

(They use passive chip SMCs on the secondary side, but on the primary side only DIPs are used)

TYPES OF SURFACE MOUNT TECHNOLOGY

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

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

COMPONENT PLACEMENT

Components 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.

Feed mechanism used to load components into a pick-and-place machine

SMD pick-and-place machine (with simulated motion blurs)

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.

TYPES OF SOLDERING TECHNIQUES

1. INFRARED SOLDERING2. CONVENTIONAL HOT GAS

SOLDERING

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

INFRARED SOLDERING

CONVENTIONAL HOT GAS SOLDERING

During 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

HOT GAS SOLDERING

CLEANING SMT ASSEMBLIES•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

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.

Thoroughly clean site and solder new device to PBC

ADVANTAGES OF SMT•Smaller components. Smallest is currently 0.4 x 0.2 mm.•Much higher number of components and many more connections per component.•Fewer holes need to be drilled through abrasive boards.Simpler automated assembly.•Small errors in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads).•Components can be placed on both sides of the circuit board.•Lower resistance and inductance at the connection (leading to better performance for high frequency parts).•Better mechanical performance under shake and vibration conditions.•SMT parts generally cost less than through-hole parts.

DISADVANTAGE OF SMT

•The manufacturing processes for SMT are much more sophisticated than through-hole boards, raising the initial cost and time of setting up for production.•Manual prototype assembly or component-level repair is more difficult given the very small sizes of many SMDs.•SMDs can't be used with breadboards , requiring a custom PCB for every prototype. The PCB costs dozens to hundreds of dollars to fabricate and must be designed with specialized software.•SMDs' solder connections may be damaged by potting compounds going through thermal cycling.