BY

S.NAVYA PRAVALIKA

AND

M.L .SINDHURI

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)

•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

Feed mechanism used to load components

into a pick-and-place machine

SMD pick-and-place machine (with

simulated motion blurs)

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.

INFRARED SOLDERING

CONVENTIONAL HOT GAS SOLDERING

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

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

removal

2. Residual solder removal

3. Printing of solder paste on PCB,

direct component printing or

dispensing

4. Placement and reflow of new

component

•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

•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

•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.

•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.