# Presentation: 751819

## Drop and Impact of Mobile Telephone

- Jason Mareno
- Sr. Applications Engineer
- Mallett Technology, Inc.
- Research Triangle Park, NC

## Analysis Goals

- Survive everyday drops
- Routine drops from 0.5 to 1.7 meters
- Various Impact orientations
- Design Cycle – Time is Critical!
- Typical product life cycle of only 12 months
- Each week late corresponds to enormous loss
- Balance risk and reward
*Develop a technique that can provide accurate predictive data in a timeframe consistent with the needs of the rapid development cycle.*

## Analytical Challenges

- Complex assembly of many components
- Component interaction: rigid links, constraints, contact, etc...
- Small, intricate, and delicate components; accurate yet efficient mesh is required
- Large deflections and many contact interfaces; nonlinearities abound
*Develop a technique that can provide accurate predictive data in a timeframe consistent with the needs of the development cycle.** *

## Failure Modes

- Majority of failures occur when one component moves/deforms further than anticipated and unexpectedly collides into another component
- Exterior housing deflects so far that it crushes a delicate antenna contact
- Protective frame around LCD transmits significant forces to glass display
- Circuit board deflects so far that solder joints are cracked
*Highly accurate stress calculations not required; only an accurate representation of deformed shape versus time*

## Analytical Goal

- Quickly and accurately predict component interaction and deflection.
- Global model must quickly predict "what will hit what" and "how far things will bend"
- If needed, use submodels to calculate stresses as necessary.
*The mesh must represent *_*stiffness *_*accurately, and stresses approximately.*

## Time Integration Scheme

- Explicit integration is required for impact – the “knee jerk” response
- Explicit method:
- High-quality, structured mesh
- Critical timestep dependent upon element size
- Tiny features and complex geometry drive both modeling and solution time
- Tetrahedrons decrease timestep by ~3.8X
- Widespread mass scaling not valid option
- Successful, but takes too long ( modeling time )

## Time Integration Scheme

- Implicit method:
- Tetrahedrons and small elements do not affect timestep size
- High order elements
- Longer solution, but less modeling ( ~3 weeks )
- Increased debug time

## Problem Description

- Typical, contemporary mobile phone

## Event Characterization

- Contemporary phones typically maintain contact with the floor for two to three milliseconds and resonate with significant amplitude for up to three milliseconds during rebound.  *Total event duration is estimated at six milliseconds.*
- For component-level tests, industry-standard shocks are half-sine, up to 2900 multiples of gravity (G), and as short as 0.3 milliseconds in duration.
- Shock waves on the order of 1.7kHz (0.6 msec period) can be anticipated in the circuit board during an assembly-level drop test.

## Problem Description

*Problem Description*

- 19 solid bodies
- Bond via shared nodes wherever possible, e.g.  screw joints
- Quasi-rigid links (beams) where needed
- 55 contact interfaces

## Modeling

- Battery Cover

## Modeling

- Front Housing

## Modeling

- Front Housing

## Modeling

- Frame

## Modeling

- Keypad

## Mesh Specifics

*Mesh Specifics*

| Element Type | Description | Quantity |
| --- | --- | --- |
| SOLID95 | 20-node hexahedron | 5365 |
| SOLID95 | 13-node pyramid | 901 |
| SOLID92 | 10-node tetrahedron | 14,557 |
| SHELL93 | 8-node shell | 505 |
| COMBIN14 | 2-node spring-damper link | 64 |
| TARGE170 | 3D surface target | 1535 |
| CONTA174 | 3D surface contact | 2252 |

| Total Elements: | 25,179 |
| --- | --- |
| Total Nodes: | 70,270 |
| Total DOF: | 210,810 |

## Material Properties

- All materials modeled as linear elastic
- Deflection is the primary quantity of interest
- No ‘plastic hinges’ expected
- Gross deflections not seriously affected by this assumption
- Rate dependency for thermoplastic resins roughly accounted for
- Stiffness-proportional damping
- Material-dependent
*( Proprietary )*

## Solution Steps

- Quasi-static Initialization
- Grommet compression, etc.
- Contact initiation
- Equilibrium at rest
- Initial Conditions
- Impact Event
- Critical portion of solution
- Rebound
- Some resonation
- Little damage

## Convergence Criteria

- Quasi-static steps controlled by the traditional L2 force residual
- Deflection is the stated goal;  impact and rebound steps controlled by L2 displacement norm only
- Fidelity of contact forces/stresses not required
- Maximum of 0.03mm contact penetration deemed acceptable
- If highly accurate stress calculations were required, the convergence criteria would need to be reevaluated.  For the stated goal of this analysis, the chosen convergence criteria more than suffice.

## Time Step

- Newmark integration scheme with γ=0.5050 and β=0.2525 (very nearly equal to the trapezoidal integration rule, or *average acceleration* scheme)
- Automatic timestepping with a maximum timestep of Δt<0.05 milliseconds (corresponds to 20pts/cycle at 1kHz)
- Period error calculated to be less than 1% at 1kHz;  amplitude error is identically zero for the average acceleration scheme.
- Rayleigh quotient was monitored.  Observed minimum of  40 points per response cycle.

## Results

- Battery Cover Disengagement

## Results

- Physical test showed damage to vibrator contacts situated just inside the top housing of the phone.
- The damage was well predicted by the simulations.
- Simulations showed that the housing would impact and deform the circuit board in the region of the vib connector.
-   Prediction of these types of interactions are enough to allow designers to relocate or better support delicate components, such as connectors, that would otherwise risk being hit by other components.

## Notes

- Total rebound height was also compared to tests.  Depends a great deal on orientation.  Data showed scatter, but in general agreement within 10-15% was achieved.
- Total turn-around time ~6 weeks
- 3 weeks mesh generation
- 3 weeks model debug, tweak, and solve
- Explicit typically takes 8 weeks due to additional modeling time
- HP J6700 dual 750MHz PA-RISC processors with 2GB RAM completed in _36 hours_

## Conclusions

- Develop a technique that can provide accurate predictive data in a timeframe consistent with the needs of the rapid development cycle
- Highly accurate stress calculations not required; only an accurate representation of deformed shape versus time
- Implicit method is a valid option