Absolute 100% sterilization is impossible. There's always a tiny chance that some microbe lands on a component during manufacturing, gets entombed inside the packaging, and therefore is unreachable by sterilization. Getting such a microbe out to the environment would require breaking the spacecraft in a very peculiar way, without incinerating or damaging the microbe itself. Most crash landings will either not break open the electronics, or will incinerate the electronics.
The gold standard for sterilizing electronics is ethylene oxide gas. It is used to sterilize implanted electronic medical devices, such as pacemakers, which also require strict sterilization to prevent putting microbes into the human body. (Autoclaving is also used for non-electronic implants, but the heat will destroy electronics.)
The NASA Contamination Control Handbook, pp. VI-21 to 23, describes the properties of ethylene oxide:
- "Most versatile gas for sterilizing purposes"
- Strong, rapid penetrating ability.
- Easily penetrates a variety of materials which can be used as "sterile packaging", maintaining a device's sterility until it is used.
- Moderately microbicidal. Effectiveness is increased by longer exposure times; 24 hours at high concentrations is as good as any other form of sterilization.
- Absorbed into rubber and plastics; blisters rubber and attacks plasticizers; considerable aeration time required after sterilization for these materials.
- Aeration after sterilization is effective at dispersing the gas. No residue.
Other sterilization methods listed in the Contamination Control Handbook (p. VI-17) are not suitable for electronics:
- Wet heat (autoclaving) and dry heat will damage electronics by excessive heat.
- Formaldehyde (steam or dunkbath) and beta propiolactone are in water solutions that can short out electronics. They are surface disinfectants with poor penetration, and do not penetrate sterile packaging. They will also outgas. Formaldehyde can also polymerize, leaving a residue.
- Peracetic acid and sodium hypochlorite (bleach) corrode metals, are in water solutions, damage sterile packaging, and can outgas.
- Ultraviolet radiation has low penetration.