The computer tries to interpret the noise as a data bit, which can cause errors in addressing or processing program code.
In these early devices, ceramic chip packaging materials contained small amounts of radioactive contaminants.
Very low decay rates are needed to avoid excess soft errors, and chip companies have occasionally suffered problems with contamination ever since.
Controlling alpha particle emission rates for critical packaging materials to less than a level of 0.001 counts per hour per cm2 (cph/cm2) is required for reliable performance of most circuits.
A 2011 Black Hat paper discusses the real-life security implications of such bit-flips in the Internet's Domain Name System.
At the Earth's surface approximately 95% of the particles capable of causing soft errors are energetic neutrons with the remainder composed of protons and pions.
This process may result in the production of charged secondaries, such as alpha particles and oxygen nuclei, which can then cause soft errors.
For the common reference location of 40.7° N, 74° W at sea level (New York City, NY, USA), the flux is approximately 14 neutrons/cm2/hour.
Computers operated on top of mountains experience an order of magnitude higher rate of soft errors compared to sea level.
Further, the increase in the solar flux during an active sun period does have the effect of reshaping the Earth's magnetic field providing some additional shielding against higher energy cosmic rays, resulting in a decrease in the number of particles creating showers.
When the same test setup was moved to an underground vault, shielded by over 50 feet (15 m) of rock that effectively eliminated all cosmic rays, zero soft errors were recorded.
Energetic neutrons produced by cosmic rays may lose most of their kinetic energy and reach thermal equilibrium with their surroundings as they are scattered by materials.
Thermal neutrons are also produced by environmental radiation sources, including the decay of naturally occurring radioactive elements such as uranium and thorium.
The thermal neutron flux from sources other than cosmic-ray showers may still be noticeable in an underground location and an important contributor to soft errors for some circuits.
Neutrons that have lost kinetic energy until they are in thermal equilibrium with their surroundings are an important cause of soft errors for some circuits.
Either of the charged particles (alpha or 7Li) may cause a soft error if produced in very close proximity, approximately 5 μm, to a critical circuit node.
Soft errors are caused by the high level of 10B in this critical lower layer of some older integrated circuit processes.
[8][9] Soft errors can also be caused by random noise or signal integrity problems, such as inductive or capacitive crosstalk.
However, in general, these sources represent a small contribution to the overall soft error rate when compared to radiation effects.
Since the area and power overhead of radiation hardening can be restrictive to design, the technique is often applied selectively to nodes which are predicted to have the highest probability of resulting in soft errors if struck.
Tools and models that can predict which nodes are most vulnerable are the subject of past and current research in the area of soft errors.
Software-based soft error tolerant schemes, on the other hand, are flexible and can be applied on commercial off-the-shelf microprocessors.
These often include the use of redundant circuitry or computation of data, and typically come at the cost of circuit area, decreased performance, and/or higher power consumption.
The concept of triple modular redundancy (TMR) can be employed to ensure very high soft-error reliability in logic circuits.
In this technique, three identical copies of a circuit compute on the same data in parallel and outputs are fed into majority voting logic, returning the value that occurred in at least two of three cases.
In practice, however, few designers can afford the greater than 200% circuit area and power overhead required, so it is usually only selectively applied.
Modern DRAMs have much smaller feature sizes, so the deposition of a similar amount of charge could easily cause many more bits to flip.
The unit adopted for quantifying failures in time is called FIT, which is equivalent to one error per billion hours of device operation.
While many electronic systems have an MTBF that exceeds the expected lifetime of the circuit, the SER may still be unacceptable to the manufacturer or customer.
Therefore, it is advantageous to design for low SER when manufacturing a system in high-volume or requiring extremely high reliability.