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Mechanical Product Design Basics

Created: 7/6/2020Last Updated: 7/04/2023Authors: Steve Shi, Adwaita Dani

INTRODUCTION

Product design (PD) is the primary job of the mechanical engineering team whose main responsibility is to design mechanical structure of the product including, enclosure, materials selection, thermal design, tolerance analysis, component space constraints, design of experiment etc. in order to extend product life cycle.

Close collaboration with the product design team is critical for system hardware design engineers during product development. Therefore it is important for designers to understand basic product design process, terminology, tooling schedule, and cost.

BACKGROUND

CAD

Mechanical Controlling Outline (MCO): Mechanical outline includes PCB shape, component keep out regions, etc.

Drawing Interchange Format (DXF): Auto CAD file for outlining mechanical component dimensions.

Datum: It’s a measurement reference (e.g., point, line, plane, or even a hole) used in mechanical drawing to show relative distance from the reference point to the part.

Mechanical Parts

Boss: It’s the extruding piece of mechanical shape resembling an mounting hole; often it is used to screw the PCB into the mechanical enclosure.

Bracket: It’s a mechanical part that is used to fix/joint two large discrete components together. Often it is used to screw the side of the PCB to the edge of the mechanical enclosure. One instance is that if the side metal of the ensure is used as an antenna and bracket is used to join the antenna to the feed point on the PCB.

Chamfer: It’s the flatten edge of a previously sharp edge; often it is used to reduce sharp edges for safety reasons as well as aesthetic reasons for product industrial design.

Shielding Can: It’s an aluminum metal cover soldered down and mounted over a cluster of high speed or sensitive electronic components distributed in the PCB. Its primary function is to act as a Faraday cage to both reduce electronic noise emission or increase immunity against external interference by the cluster of electronics.

Connectors: These are the mechanical ports that are used to interface with product such as USB ports found on computers or HDMI ports found on TVs.

Enclosure: It’s the cabinet/housing for electronic components. There are two types of enclosures: external and internal enclosures:

- External enclosure is the exterior case/ housing to protect internal electronic components as well provide openings openings for ports, screens, etc. during assembly.
- Internal enclosure is used for internal sub-assemblies to house internal electronics such as battery, PCBs, Display Module, etc.

Mechanical Materials

Pressure Sensitive Adhesive (PSA): Adhesive that activates on applying pressure, and does not require temperature or humidity. If not specified, the adhesive may be electrically insulating. Conductive PSAs (such as this) can be used to make an electrical connection as well as mechanical adhesion between surfaces.

Thermal Sensitive Adhesive (TSA): Similar to PSA, but requires some pressure and temperature to activate the adhesive.

Thermal interference materials (TIM): These materials are used to extract and dissipate heat effectively from the heat source such as a power regulator to the external environment in order avoiding over heating and result in processor slowing down and safety reasons for human contact.

Manufacturing

Mechanical tool: It is a mechanical machine that makes the mechanical part. It can be a mold for plastic or the jig & fixture that holds the working piece for a CNC machine in the metal enclosure production.

Typically there are soft and hard tools:
- Soft tools are used in earlier product design stages to validate the initial product mechanical design. It is made of metals not heat treated so it’s mechanical reliability for usage is worse compared to that of hard tools.
- Hard tools are more reliabilable needing less replacements and are used at the mass production stage to ensure manufacturing accuracy and tolerance.

Process capability index (Cpk): It indicates manufacturer's capability to meet component tolerances: a higher CPK means that manufacture has tighter tolerances, hence less part to part tolerance variability during manufacturing process. A typical CPK used in industry is 1.67.

Root Summed Squared (RSS): It is a tolerance analysis method that uses individual components tolerances to find an accumulated components stacking error.

Production Part Approval Process (PPAP): A process to verify manufacturing capability of a supplier for a specific component. Various measurements are recorded for built components, compared against specified upper and lower limits, and the Cpk is calculated. The Product Design/Mechanical Engineer signs off if the capability meets requirements.

Structural Reliability

Service loop: It is often a term describing the flexibility of module mating interface to the main logic board such as a camera module made with long flexible printed circuit board. The longer or windier the flex, the less tension is applied to the mating connector resulting in more robustness against accidental drop.

Bend Radius: It's a metric to evaluate the level of bend of a flexible printed circuit board (FPCB) without any damage. The goal is to minimize bend radius without Delamination of the FPCB layers and limit the excessive yank/pull force exhibited on the mating connector. Generally rule of thumb for bend radius 4-5X thickness of 2 layer FPCB or 2-3X of 4 layer FPCB.

- Note: Bend radius can be estimated in the CAD tool to look at the arc angle and partial perimeter.

Analysis

Basic development flow

- Conceptual stage (proof of concept)

MEs work with Industrial Designers to come up with the concept of the product. System EEs provide requirements for power dissipation, EMC shielding, RF antenna locations and I/O port connectors.

- Design stage (validation of concept)

Detailed design and engineering validation builds. Usually soft tools are used in this stage to reduce cost and allow iterations and desing of experiments (DOEs).

- Manufacturing stage (mass production)

Increasing manufacturability of the product often boils down to decrease assembly time by having the right enclosure design, flex design, mounting structure design, choose the right part tolerance, etc.

Design example

USB flexible printed circuit board (FPCB)

USB ports are ubiquitous in portable electronics. Often USB ports are found on the side of the product enclosure and are mounted on FPCB which then is used to connect to the main mother board. In the mobile phone design with limited space, the board to board connections that is used to mate between USB flex to the main motherboard (commonly referred as main logic board) needs to be bent (sometimes a 180 degrees bend in needed) in certain ways to ensure a reliable connection

How do we ensure reliable connection that can withstand drops or long term bending stresses inflicted on the connector and the pcb materials?

The common techniques are increasing the service loop of the FPCB, appropriate bending radius in the flex needs to wrap around to make a connection, and use a mid mount USB connector to make a reliable connection to the FPCB.

SUMMARY AND CONCLUSION

Mechanical parts are used to house and joint components together within a product to form a protective and reliable structure for extend product life
Mechanical materials are used to supplement/ease the mechanical assembly for products.
Manufacturing metrics, CPK and RSS used in tolerance analysis, provides the designer the confidence in the reliability of the final product in the mass production.

As an EE with system role and responsibilities, it’s essential to understand the basics design methodologies and jargon used by mechanical engineers counterparts in order to successfully working collaboratively jointly to make a great product. Often through these close collaboration, both parties gain insight to restrictions of each party;. One such example on the system EE side is PCB areas for placement and routing as well height and XY dimensions are needed in the early stage for both party to make the informed and discreet trades-offs without degrading product life cycle that generally comes with avoiding the responsibilities in early design stage.