Apple Design Challenge

Context:

I was interviewing with Apple for a full time role as a product design engineer and received their classic design challenge they gibe out to candidates. After dedicating a week to this challenge, Apple unfortunately reached out to notify they canceled the position I was interviewing for.

I put a significant amount of time and effort into the challenge, and am proud of my solution, so I decided to showcase it here on my website! Enjoy, and please reach out if you have any questions!

What's the Problem?

Problem Statement:

Design a push-button latch for a battery door on the iPod Classic

Constraints:

Button resides within battery door outline
Downward press unlatches the door to pop up
Should accommodate two AA batteries

What has Worked in the Past?

I took a look at other published solutions as well as some mechanisms as inspiration for my designs. All of these sources are shown and linked below.

Assumptions

120GB Gen 6 iPod Classic
iPod depth can increase to fit 2 AA batteries
Enough space to store all other electrical components
Details on the iPod frame, top, and bottom covers are out of scope
iPod battery life is proportional to power capacity
No major DFM draft angles are needed on parts
The stated maximum operating temperature is the max thermal case

What Does the Customer Need, and How can I Measure Success?

Any product should be easy and enjoyable to use, so I thought about some of the most critical features from the perspective of the user. I then turned them into qualitative and quantitative design criteria to use to rate the different designs.

Customer Needs

Accessible Operation

Simple Operation

Robust Design

Elegant Integration

Low Consumer Price

System Requirements

1.4N Minimum Push Force

Low Part Count

Low User Step Count

20G Drop

500 Cycle Fatigue

Smooth Aesthetic Surfaces

Low Part Cost

How Might We Achieve Success?

I came up with two different mechanisms to compare for down-selection. Each with their own benefits.

Single Spring Slider

User Input

Spring Input

Springless Hinge

User Input

Which Design has Greater Potential?

Single Spring Slider was chosen due to an overall better performance than the Springless Hinge from a comparison of the System Requirements.

Single Spring Slider CAD

How Does it Work?

Button flange keeps it flush with battery door

Opposite end tabs act as a simple hinge for the battery door to pop open

Initial Position

Button Press

Button Release

What are the Risks?

Risks

Clip Won't Always Fit in Latch

De-Risking Strategy

Control Latch Gap

Battery Door Clips Disadhere

Control Thermal Stresses

Button is Too Difficult to Press

Design a Sensitive System

How do We Control the Latch Gap?

Design gaps to accommodate tolerance stackups

Min 0.26 mm gap lets the clip always fit

How do We Control Thermal Stresses?

Choose an appropriate clip material and/or adhesive

Different rates of thermal expansion will thin out and debond the adhesive

With a FOS of 2, ABS (cheapest) can be used with adhesives with 6 MPa or greater bond strength

Use ABS for clip, and use 3M Scotch Weld DP110 with Bond Strength of 17 MPa

How do We Design for Sensitivity?

Control/Minimize Reactive Spring Force

Use a 0.13 lbf/mm Spring (McMaster)

Button Travel:

Bill Of Materials (BOM)

References

[1]Ifixit.com, 2025. https://www.ifixit.com/Parts/iPod_Classic (accessed Apr. 30, 2025).

[2]M. Riddle, J. MacDermid, S. Robinson, M. Szekeres, L. Ferreira, and E. Lalone, “Evaluation of individual finger forces during activities of daily living in healthy individuals and those with hand arthritis,” Journal of Hand Therapy, vol. 33, no. 2, pp. 188–197, Apr. 2020, doi: https://doi.org/10.1016/j.jht.2020.04.002.

[3]“Obtaining service for your Apple product after an expired warranty,” Apple Support. https://support.apple.com/en-us/102772‌

[4]E. Research, “Americans Spend Over Four Hours Daily with Audio,” Edison Research, Dec. 27, 2023. https://www.edisonresearch.com/americans-spend-over-four-hours-daily-with-audio/

[5]“iPod classic (120GB) - Technical Specifications,” Apple Support. https://support.apple.com/en-us/112321

[6]“Free CAD Designs, Files & 3D Models | The GrabCAD Community Library,” grabcad.com. https://grabcad.com/library/aa-battery-10

[7]“Free CAD Designs, Files & 3D Models | The GrabCAD Community Library,” Grabcad.com, 2025. https://grabcad.com/library/aa-battery-holder-2

[8]“McMaster-Carr,” Mcmaster.com, 2019. https://www.mcmaster.com/94125K426/ (accessed Apr. 30, 2025).

[9]“ProtoXYZ |,” protoxyz.com. https://protoxyz.com/materials/metal/Aluminum_6061-T6

[10]“McMaster-Carr,” Mcmaster.com, 2019. https://www.mcmaster.com/92000A120/ (accessed Apr. 30, 2025).

[11]“McMaster-Carr,” Mcmaster.com, 2019. https://www.mcmaster.com/90576A102/ (accessed Apr. 30, 2025).

[12]“Coefficient of Linear Thermal Expansion on Polymers Explained,” Passive Components Blog, Jan. 08, 2019. https://passive-components.eu/coefficient-of-linear-thermal-expansion-on-polymers-explained/‌

[13]“3MTM Scotch-WeldTM Epoxy Adhesive DP110 Translucent | 3M United States,” 3m.com, 2024. https://www.3m.com/3M/en_US/p/d/b40066469/ (accessed Apr. 30, 2025).