A Brief Discussion on Reliability Design
The traditional automotive industry has developed in Europe and America for over a century, establishing a complete set of product technical standards and management standards.
The TS16949 quality management system represents the automotive industry’s comprehensive management requirements for the entire supply chain.
Specifically regarding management details, the two most difficult aspects of TS16949 are:
(1) Consistency, and (2) Reliability.
TS16949 specific requirements for quality consistency are as follows:
For quality parameters affecting assembly, Cpk (Process Capability Index) must be 1.33.
For quality parameters affecting function or performance indicators, Cpk must be 1.67.
For quality parameters affecting safety, Cpk must be 2.0.
Regarding reliability, the most important content in TS16949 is undoubtedly FMEA (Failure Mode and Effects Analysis). FMEA is essentially a Q&A process:
Q: What function is this design intended to achieve?
A: …
Q: If a problem occurs in a specific link of this design, will the function be degraded or lost?
A: …
Q: If a problem occurs in a specific link of this design, will negative effects occur? How severe are these effects?
A: …
Q: Will the function degrade or disappear, or will negative effects occur when this design operates long-term, at high frequency, or under extreme operating environments?
A: …
Q: Is there a way to prevent problems in the aforementioned links?
A: …
Q: If prevention is impossible, can the problem be detected before it occurs? How?
A: …
Therefore, the “links” and “problems” are critical. We divide the design into two stages: product design and manufacturing process design, leading to DFMEA (Design FMEA) and PFMEA (Process FMEA). The APQP standard provides relevant explanations on which specific links need consideration.
For DFMEA:
It involves the structural tree and functional analysis, followed by interfaces between structures (physical connection/material exchange/energy transfer/data exchange/human-machine interaction, etc.), and noise factors affecting functions (component variation, time effects, unreasonable customer usage, external environment, system interactions, etc.).
For PFMEA:
It involves the process flow chart, process functions, and analysis of process steps and elements (such as 4M—Man, Machine, Material, Method).
There are many DFMEA examples in the design of the Three-Electric Systems (Battery, Motor, Electronic Control) for New Energy Vehicles (NEVs), which we will not list here but can discuss separately later. If you encounter reliability issues in products or processes, the author can analyze them with you and solve them 100%.
Here we clarify a question:
Is there a relationship between Consistency and Reliability?
Of course!
Example: The range of an NEV is a concern for everyone. Design engineers must not only maximize the range but also ensure the range indicator decays stably and slowly. However, in reality, it is difficult to achieve consistent internal resistance (DC-IR) in lithium batteries, especially after multiple charge-discharge cycles.
Before leaving the factory, we can screen and group lithium batteries with consistent capacity and internal resistance into a PACK. However, these seemingly highly consistent batteries will show significant divergence in internal resistance after 1-2 years. This is particularly true for LFP (Lithium Iron Phosphate) batteries, which have poorer internal resistance consistency. This leads to accelerated decay of the total capacity of the entire PACK, causing the vehicle’s range to decay significantly. This is equivalent to the degradation of the “range function,” representing a classic example of consistency affecting reliability.