Читать книгу Design for Excellence in Electronics Manufacturing - Cheryl Tulkoff - Страница 20

The major difference between software and other engineering endeavors is that software is purely design. Hardware unreliability issues are typically related to physical failures of components, which can be traced back to a variety of root causes, such as material, fabrication, or assembly defects; environment or usage overstress; or wearout and design errors. In contrast, when software problems arise, they are traced back to design faults, which come from human errors such as inadequate or incomplete requirements, coding errors, logic errors, improper resource allocation, or the inability of the software to deal with system faults, noise and exception faults.

Therefore, software integrity and dependability are fully dependent on properly implementing quality procedures throughout the specification, design, and coding of the software. This activity should be coupled with meticulous testing and validation activities that can find and correct inadequate requirements, bugs, and discrepancies as early as possible in the design, development, and validation process.

In addition to verifying basic functional operation per the requirements covered in the various industry standards on software development, procedures that also evaluate functional robustness, fault tolerances, and – for devices with operator interfaces – user‐friendliness should also be considered. The following procedures are recommended for use in the specification, creation, development, and validation processes of functional safety of critical software.

Table 2.1 Common quality and reliability issues.

Area of concern	Impacted item	Failure mechanism	Testing method	Inspection technique
Moisture sensitivity	Plastic IC packages, optocouplers, other polymer‐based components	Popcorn delamination at higher reflow temperatures. Heat damage of IC packages	Moisture sensitivity testing	Visual inspection
			J‐STD‐020	C‐SAM
Heat damage	All passive components, circuit boards, connectors	Cracking, dielectric breakdown (capacitors), PCB delamination, warping, or via cracking	Heat resistance, MIL‐STD 202G #210F, decomposition temp., time to delamination, package planarity, JESD22‐B108	Visual inspection, functional verification
Poor wetting	All solder joints	Cold joints or weak joints fracture in the use environment	Solderability, J‐STD‐002, J‐STD‐003	Wetting balance, visual inspection, x‐sectioning, lead pull
Solder fatigue	Solder joints, particularly on high‐CTE components	Cracked solder joints	Thermal cycling, JESD22‐A104, HALT, Vibration	Electrical continuity, visual inspection
Mechanical shock	Solder joints particularly on higher‐mass components	Solder joint failure during shipping or handling	Shock test	Electrical continuity, visual inspection
Sn whiskers	Sn and SnCu‐plated components	Shorting	iNEMI / JEITA procedures	SEM
Surface mount process control	Insufficient process window creates poor solder joints	Solder joint failures in the use environment	Precondition and assembly, JEDEC 22‐A113	X‐ray, X‐section, visual inspection, reliability test
Rework process	Poor solder joints, damaged components	Joint failures or cracked vias in the use environment	Rework components followed by reliability testing	X‐ray, X‐section, visual inspection, reliability test
Wave solder process	Incomplete hole fill, fillet lifting, damage to the board	Failed through hole, cracked vias, weak joints	Thermal cycle, HALT Vibration	Electrical continuity, visual inspection
Electrochemical migration	Board surface with no‐clean paste residue	Shorting between biased traces in a moist environment	Bellcore GR‐78‐CORE, J‐STD‐004 SIR	Visual and resistance after 35 C/85%RH exposure at 50 V