PRESENTED BY,V.GANESH

ME CIMROLL NO:200821524

DESIGN FOR ASSEMBLY

DEFINITION"a process for improving product design for

easy and low-cost assembly, focusing on functionality and on assemblability concurrently.“

The aim of design for assembly (DFA) is to simplify the product so that the cost of assembly is reduced.

It improve quality and reliability, and a reduce in production equipment and part inventory

DFA GUIDELINESMINIMIZE THE NUMBER OF PARTSSTANDARDIZE AND USE COMMON PARTS

AND MATERIALSDESIGN FOR EASE OF FABRICATIONDESIGN FOR PARTS ORIENTATION AND

HANDLINGMINIMIZE FLEXIBLE PARTS AND

INTERCONNECTIONSDESIGN FOR EASE OF ASSEMBLYDESIGN FOR EFFICIENT JOINING AND

FASTENING

MINIMIZE THE NUMBER OF PARTS

Minimize the number of parts: It reduce the number of assembly

operations. -Each part should be examined to see if

it can be eliminated or combined with other parts. -Two parts need to be separate only if there is relative motion between them

STANDARDIZE AND USE COMMON PARTS AND

MATERIALSStandardize: Standardize PCB

dimensions so that standard fixtures can be used during wave soldering, cleaning, etc.

Standardize the location of tooling holes and board thickness.

When smaller boards suffice, design them such that multiple quantities can be cut from a larger standard board.

DESIGN FOR EASE OF FABRICATIONSelect processes compatible with the

materials and production volumes.

Select materials compatible with production processes and that minimize processing time while meeting functional requirements.

Avoid unnecessary part features because they involve extra processing effort and/or more complex tooling.

DESIGN FOR PARTS ORIENTATION AND HANDLING

To minimize non-value-added manual effort and ambiguity in orienting and merging parts.

Basic principles are:-It is stable and self-aligning-Parts easy to grasp and insert-Avoid parts that are sticky or slippery -Design the work station area to minimize the distance to access and move a part. -Parts presented in the right orientation-Avoid parts that can be easily damaged or broken

MINIMIZE FLEXIBLE PARTS AND INTERCONNECTIONSAvoid flexible and flimsy parts such as belts,

gaskets, tubing, cables and wire harnesses

Their flexibility makes material handling and assembly more difficult and these parts are more susceptible to damage.

Use plug-in boards and backplanes to minimize wire harnesses

DESIGN FOR EASE OF ASSEMBLYBy utilizing simple patterns of movement

and minimizing the axes of assembly

Complex orientation and assembly movements in various directions should be avoided

Assembly should proceed vertically with other parts added on top and positioned with the aid of gravity

DESIGN FOR EFFICIENT JOINING AND FASTENINGThreaded fasteners (screws, bolts, nuts and

washers) are time-consuming to assemble and difficult to automate.

Where they must be used, standardize to minimize variety and use fasteners such as self threading screws and captured washers

Use integral attachment methods (snap-fit)

DESIGN PRINTED CIRCUIT BOARDS FOR ASSEMBLY

Minimizing component varietyStandardizing component packagingUsing auto-insertable or placeable componentsUsing a common component orientation Component placement to minimize soldering

"shadows” Selecting component and trace width that is within

the process capabilityUsing appropriate pad and trace configuration and

spacing to assure good solder joints and avoid bridging

Using standard board and panel sizesUsing tooling holes Establishing minimum borders, and avoiding or

minimizing adjustments.

PCB DESIGN PROCESS

DESIGN PROCESS FLOW

1.Requirements allocation and functional partitioning

It defining the configuration of a new PCA are taken long before detailed circuit design and layout activities begin.

Performance requirements are based on what the end product , how it is to do it, and under what conditions it will be expected to fulfill its intent.

Allocation of functional elements of a system/product.

PCA PARTITIONINGConstraints for partitioning:1. Circuit performance

considerations may involve- Length of signal connections -Segregation of digital and analog circuitry -Power distribution -Component power dissipation

2. Physical constraints may include- Size and shape of the space allocated for PCAs-Area needed for components and interconnections- Quantity of inputs and outputs required- Environmental conditions- Heat removal

3. Producibility and testability needs may include- Circuit density (components and interconnections)- Ability to identify and isolate circuit faults- Component packaging and mounting- Characteristic tolerances (circuit parameters and physical features)

4. Maintainability requirements may have to be-accommodate- Ease of PCA removal and replacement- Access for adjustment and test (in place and out of the product)-Provision of features for repair and modification

2.CIRCUIT DESIGNFirst identify the inter operability of the

functions

Determine how signals are to flow between them and functions external to the PCA.

The next step is to define how these functions are to be physically accomplished by selecting part types, logic families, and use of custom-designed

Functional flow diagramA functional flow diagram shows the

relationships of critical hardware and software entities on the PCA.

It describes the internal interfaces that are required for system operation.

An example of a functional flow diagram.

LAYOUT DESIGN RULESGeneral rules and guidelines to be used when designing a family or

group of similar PCAs for a system . they areComponent types and logic families (specific part numbers, values,

tolerances) Circuit design rules (loads, terminations, drivers, power distribution,

clock signal distribution, grounding, EMI protection, part derating) Placement of parts and circuitry (proximity, signal coupling,

orientation) Interconnection layout (min/max trace dimensions, parallel

conductors, stubbing, impedance, order of signal and ground/power layers)

Mechanical design (board dimensions, construction, material,mounting hardware, thermal management, environmental protection, identification marking)

Location of signal and test connectors and key connector pin assignments (power/ground, clock lines)

Test and maintenance (fault isolation, test/adjustment accessibility)

Mechanical designMechanical design of a PCA defining its

physical form and structure.

This includes establishing the assembly outline (i.e., length, width, height), circuit board dimensions and how it is mounted, the method of interfacing it to other circuitry, access for removal/replacement, test and adjustment, heat removal, and environmental protection.

3.Mechanical Design3.1 Board Configuration3.2 Support Structure and Mounting

Hardware3.3 Heat-Sink Design3.4 Connectors and Test Points3.5 Height Restrictions and Keep-Out

Areas

3.1 Board ConfigurationThe primary constraints are

-The size and shape of a circuit board-Where the assembly is to be installed-How it is to be mounted, and the amount and type of circuitry it will contain

Board construction (i.e., material, number of layers , stackup) is mainly influenced by component and interconnection density and circuit performance requirements.

3.2 Support Structure and MountingHardware

Support StructureIt provide protection against

excessive vibration and shock inputs and include adequate heat-removal capabilities.

Mounting Hardware It perform the way the PCA is to

be mounted, the location of the I/O connectors, alignment features, and the mechanical extraction and replacement method

3.3 Heat-Sink DesignIt provide internal cooling mechanismsThree modes of heat removal from

components are conduction, convection, and radiation

Heat sinks are available that use one, two, or a combination of all three modes

The objective of designing a PCA for optimal heat dissipation is to ensure that the maximum allowable temperature of components and board material is not exceeded during operation.

Examples of heat sinks for improving thermal dissipation of PCA component parts

3.4 Connectors and Test Points

Connector selection is usually made based on the following factors:- The quantity and type of signal and power connections required (current, voltage, impedance controlled, shielded)-The space available for these connections (density)-How a PCA physically interfaces with the system (plug-in, permanent mount)-Circuit interfaces with the system (motherboard, hard wiring, flex, directly to adjacent assembly)-Board construction (single/double-sided, multilayer)-Type of component mounting (through-hole, surface-mount)-Environment (temperature, humidity)-Reliability (number of insertions/withdrawals)-Application (military, space, industrial, commercial

Test pointsTest points are required to be accessible

while a PCA is mounted in the system, then they are usually soldered to the board’s surface near its edge

It based on their location and purposeIf a small quantity is needed (8 to 10),

individual points can be soldered to the board

Larger quantities can be obtained as multipont units and soldered to the board in a single operation

3.5 Height Restrictions and Keep-Out Areas

Assemblies will restrict the height of components or hardware mounted on a PCA

Even it prevent parts from being installed in specific areas of the board.

These restrictions may be applied to either or both sides of an assembly and must be respected when laying out a board.

It providing clearance for insertion and withdrawal of an assembly

4 .Board LayoutLayout Design Rule:A designer should have a well-defined and documented set

of design rules and guidelines in hand before starting the physical layout effort.

These rules generally are derived from a number of preestablished requirements and conditions, including-Circuit performance- Types of components included in the circuit- The system operating environment- Manufacturing and test processes- Reliability and maintainability- The circuit board construction and materials- Cost and schedule limitations

REFERENCE

PRINTED CIRCUIT ASSEMBLY DESIGN LEONARD MARKS , JAMES A.

CATERINA McGraw-Hill Publication