Cindy Millsaps
John C. Copeland
Motorola Product Testing Services
With the growing availability of low cost, portable devices, designers need test programs developed and administered quickly to provide accurate information about the performance of the entire system. Although there is constant pressure to bring more products to market faster, it is critical to ensure that the product's performance will meet customer expectations. A well thought out and aggressive test program is one of the most significant techniques to validate the design's integrity and ability to meet established usability requirements. Included here are discussions of general test planning plus considerations spanning the categories of environmental, mechanical and electrical and safety testing.
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| Lab data collection and monitoring area. |
Test
Plan Development
The first step in developing a test plan is to understand the
product's usage profile, which identifies the product's typical use and
potential misuses. Evaluation of the entire system provides insight into
the risks involved with the design and the types of testing appropriate
for validation. Foregoing this step in favor of using pre-existing test
standards is an option. There are many standards in existence for various
product categories. The risk is that these standards will likely not address
the exact usage profile or design features of the product under evaluation.
Customization of the test plan using the usage profile can result in more
pertinent data at a lower cost with less test time involved.
Once the usage profile is established, there are two main approaches to
testing: simulation and stimulation. Simulation is where an actual usage
environment is created to bring out failure mechanisms. It is the traditional
stress testing approach. The down side is it usually involves longer,
more expensive testing, but can sometimes be accelerated. The up side
is that reliability parameters can be estimated. Stimulation refers to
driving the device in a manner that is not consistent with its expected
operating profile, but is certain to expose failure modes for correction.
It is the more modern approach, driven primarily by the need for reducing
time to market and test costs. Statistically, most failure modes found
during stimulation tend to be valid for the device over its expected service
life.
As most product testing eventually leads to a "ship" or "do
not ship" decision, deciding how the data will be used and the criteria
for passing or failing a product is imperative. Determining how much design
margin is sufficient is a difficult balance of risk and resources. It's
easy to identify that a problem exists when a portable product does not
function after a single drop, but not all test results are that obvious.
Careful consideration is needed to pick the test methods and criteria
that ensure a reliable product, without overly restrictive constraints
in the construction or design.
Building on these general test-planning guidelines, the following are some of the many test options appropriate for portable devices that should be considered.
Environmental
Testing
Portable electronic devices are exposed to a wider variety and severity
of environmental stresses than their desktop counterparts. Understanding
and designing for these conditions is critical to customer satisfaction
and must be validated through testing. In this pursuit, there are many
test options that can be optimized to a given product's requirements.
Thermal Shock/Thermal Cycling - Thermal shock is an extreme application
of rapid temperature change. It compares to moving from the desert to
the arctic in a matter of seconds. Improperly formulated or manufactured
materials can have undesirable internal stresses that can lead to latent
failures. Thermal shock provides the stimulus to relieve these stresses
in the form of cracks, seal failures, warping, etc. Thermal cycling is
similar to thermal shock but employs lower temperature gradients. Instead
of exposing undesired stresses, its purpose is to thermally fatigue materials.
As the temperature cycles, the materials expand and contract. This cycling
can expose materialweaknesses over time. Increases in temperature extremes
or ramp rates can be used to accelerate the process. As with the host
device, battery performance is also affected by thermal effects. Depending
on the level of thermal stress and other variables, the available capacity
could be reduced by 50 percent or more.
Humidity or Moisture Exposure - Humidity testing is typically done
at the extremes, e.g. high temperature/high humidity and low temperature/low
humidity. The upper extreme tests to see if moisture can be forced into
components resulting in undesirable parameter variances or corrosion.
The lower extreme tests factors like the vulnerability of seal materials
that might dry out and crack. A related, but separate, category of testing
is water intrusion. Many portable devices will be used outside. An evaluation
of their ability to survive rain exposure may be necessary to determine
the product's viability for field use.
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| Thermal shock chamber. |
Mechanical
Testing
The most technologically advanced electronic device housed in a case where
the battery compartment cover breaks early, or the belt clip never quite
works right will be perceived as inferior. Mechanical testing reduces
the risk of such design weaknesses reaching customers.
Vibration - Every portable device will be exposed to vibration
through its projected life. It will be shipped by truck, rail, ship or
aircraft, all which involve varying vibration profiles. Customers will
set it down on vibrating equipment such as vehicle seats or vibrating
machinery. Designs not robust to vibrational stresses may suffer from
broken or degraded components, leading to loss of customer confidence
and warranty liability. These failures may be out of box or latent depending
on the specifics. Vibration testing can provide an understanding of a
product's sensitivity in this regard early in the design cycle. Some test
options include transportation simulation, resonant frequency determination
(e.g. which frequencies are most likely to damage the product) and comparison
testing with existing designs. Vibration is also an integral component
of highly accelerated stress testing (HAST) used to reduce lengthy test
times while ensuring high field reliability.
Mechanical Shock/Drop - Inevitability, portable devices will often
be dropped. Robustness in both an operational and safety aspect must be
assured. Mechanical shock and drop testing provide quantification of this
margin. Drop testing for small, portable electronic products is typically
done from a height of 2 to 5 feet to surfaces such as steel, wood, tile
or concrete. Some specifications call out precision drops to a combination
of planes, edges and corners. For others, random drops by hand suffice.
Soaking the devices at temperature extremes prior to drop can further
explore worst-case conditions, and repetitive
drops to failure can be used to compare design margins.
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| Example of vibration test equipment. |
Electrical
and Safety Testing
Most of the electrical and safety testing that could be performed on a
portable device will be at the energy system level due to the nature of
the products. Most of the power and available energy is in the battery
and charging system of the product. As consumers demand more functionality
from portable devices, such as the integration
of cellular and paging devices along with the integration of Internet
capabilities, the energy needs are increasing. But at the same time the
product size and weight requirements have declined steadily. This means
that most applications will use Li-Ion solutions. To meet the needs of
consumers, cells need to have higher energy density. This means that there
is more stored energy in the battery, and therefore a higher potential
for failure. Additionally, the higher current drain can adversely affect
the cycle life and performance of the battery.
Short Circuit - A spare battery carried in a pocket or purse with
keys, loose change or other conductive objects could potentially short
the battery resulting in personal injury or property damage. Application
of both hard shorts (~0.10 Ohm) and soft shorts (~2 Ohms) is prudent for
portable energy sources that may be transported in this fashion. These
tests can be done at room or elevated temperatures. A soft short may simulate
a more hazardous condition since it involves a fault that may not cause
enough current flow to trip the device's safety circuit, yet may still
generate significant heat.
Electrostatic Discharge (ESD) - A condition that can affect any
electrical product from battery to host device to charger or power supply,
is ESD. Everyone has at one time witnessed this effect. Getting out of
a car on a cold winter day may produce an electrical arc. If a consumer
is holding a PDA in their hand as this arc happens, the resulting discharge
to the device could be tens of thousands of volts. This exposure could
cause failure or loss of functionality. ESD sensitivity testing can provide
evidence of where a portable device is vulnerable and how much margin
is inherent in the design. Most ESD test standards require discharge testing
to all user accessible ports on the product at voltages up to 15 kV, but
testing may be expanded to levels as high as 25 kV in the interest of
quantifying design margin.
Overcharge - Consideration of the entire system to be used with
a product, should include what could happen if there is a fault in one
portion of the system. If a charger experiences a fault that allows a
higher current than anticipated to charge the battery, there is a risk
that the battery would be able to charge too fast or above the safe limits
of the product. This could cause personal injury or property damage to
the consumer. A determination of the maximum single fault output of the
charging system can provide test parameters to test for this potential
hazard in the design phase.
In the end, a test program that is carefully planned and addresses the
usage profile of a product will provide the best information regarding
product robustness. Proper test planning may also net an initial saving
in test expenses and a reduction in time to market. Additionally, a properly
validated design runs less risk of customer dissatisfaction and warranty
replacement expenses. To meet this need, there are many test options available
as well as competent service providers to help optimize and execute test
plans.
| Test Examples | |
| Consideration | Potential Testing |
| Outdoor use | Humidity/moisture, temperature exposure (storage & operating range), temperature cycling, temperature shock, rain, weather- ing, corrosion/salt-fog, sunlight exposure, sand or dust sensitivity |
| Hand held | Drop, mechanical shock, electrostatic discharge and some outdoor testing |
| Commonly transported by automobile or aircraft | Altitude simulation, temperature exposure, temperature cycling, temperature shock, vibration |
| Aftermarket accessories | Overcharge, reverse charge |
| Li-Ion batteries used | Nail penetration (polymer), overcharge, short circuit, heating, crush, reverse charge, cycle life, temperature storage |
| Use of switches, keypads, hinges or other mechanical devices | Cycling tests (at room, elevated or lowered temperatures), temperature shock, temperature cycling |
| Usage
Profile
Is the product
going to be carried in a carrying case, on a belt clip, or in a
purse/briefcase? Will it spend time in a vehicle being transported during use or while being stored? How will the product be used in a typical day? Who is the typical user of the product? What accessories are intended for use with the product? And what accessories might be used that are not necessarily intended? How will the product receive and maintain power? Will there be different suppliers for the accessories and charging systems and what is the variation within these products between suppliers? How might the product be misused? What are the expected results of misuse? Recommend Failure Mode and Effects Analysis (FMEA). Availability of test samples and product launch schedule? Is the calculation of reliability parameters required for the product? What is the typical operating and temperature range for the product? |
At Motorola Product Testing Services, an organization within Motorola Energy Systems Group, Cindy Millsaps is senior staff compliance engineer and John C. Copeland is senior staff reliability engineer. In her position, Ms. Millsaps is responsible for regulatory compliance for power supplies, chargers and battery packs. She holds a BS in electrical engineering and prior to joining Motorola, worked at Underwriters Laboratories. In his current position, Mr. Copeland is responsible for development and execution of environmental, mechanical and electrical stress testing plans. He holds a BS in electrical engineering, a masters degree in quality assurance and professional certifications as both a Certified Reliability Engineer and a Certified Quality Engineer. Combined, they have more than sixteen years of experience in working with the development of test programs for product performance and reliability.
Contact
Motorola at www.motorola.com/producttesting.
