For the complete machine manufacturers, especially the large-scale terminal product manufacturers (such as Huawei), there are corresponding "selection specifications" for the selection of electronic components. The reason is very simple, take the mobile phone for example, a mobile phone needs thousands of components, if a material problem will affect the entire production line, so the specification seems indispensable.

But as for the company's "selection specification", someone inside Huawei said:

When I joined Huawei, the whole company was engaged in a "standardization" movement. Everything was standardized, everyone was standardized, tenure, performance, and technical level were all based on standards (big companies use KPIs to guide them, which is easy to turn into a "movement"). At that time, according to the type of device, many people wrote a variety of device selection specifications. So when it comes to schematic reviews, one of the most common phrases you hear is "this is how the specification is written."

Possible questions about "selection specification"

First, are the people who wrote the specification expert? Is the writing meticulous? If there is a mistake that is more harmful.

Secondly, specifications sometimes inhibit the developer's thinking. Everything according to the specification, not necessarily suitable for the actual design scenario. For example, if I need a low-cost design, but the specification emphasizes high quality, it may not be applicable.

Third, having specifications can also cause some developers to not think. For example, when the crystal oscillator is above 50MHz, the PF capacitor is put for power filtering, while the one below 50MHz is not used. No one wants or knows why; Another example of network port transformer protection, indoor and outdoor, in accordance with the design requirements of various EMC standards, directly according to the picture can be, but few people want to why, do not know how the test results, such as the actual encounter difficulties on blind.

It is true that norms sometimes improve work efficiency and product quality, but the more developed the tools, the more people may be degraded.

Finally, due to the rapid development, part of the device selection is not suitable for writing specifications, because there is strong hysteresis.

Although there are disadvantages, it is undeniable that norms are a very useful means, which is the essence of a large number of theoretical analysis + experience accumulation + practical data. The specification I read most at that time was the "Drawdown Specification for Device Selection", which was based on a lot of tests and practical cases.

For example, when the aluminum electrolytic capacitor is selected, the steady-state working voltage is less than 90% of the rated voltage. For tantalum capacitors, the steady-state derating requirement is 50%; And ceramic capacitor, steady state reduction requirements in 85%; This is because the actual mode of the device, the worst environment (high temperature, low temperature, maximum power consumption), the difference between steady state power and transient power and other factors are taken into account.

In Huawei's PDM system, the devices have a preferred level of "preferred", "non-preferred", "forbidden selection", "terminal exclusive" and other categories. Engineers can intuitively feel whether the device is optimized or not according to this preference level. So what are the factors that are taken into consideration for the device preference rating?

1. Availability

In particular, manufacturers like Huawei have a large number of products shipped. Carefully select devices in declining life cycle, and prohibit the use of discontinued devices. I once designed a circuit in 2005. When I designed the circuit, I copied the circuit of others. However, when I processed the circuit, I found that the components could not be bought at all.

For critical components, there are at least two brands of models that can be substituted with each other, and some of them need to consider solution level replacement. This is very important. If it is an exclusive product, there are layers of reporting, decision making and risk assessment.

2. The reliability

Heat dissipation: RJA with small thermal resistance and TJ with larger junction temperature are preferred for power devices. Processor selection, in the performance to meet the situation, as far as possible to choose smaller power devices. But if you have a monopoly like Intel, you have to live with it, add radiators, add fans.

ESD: The antistatic capacity of the selected components should be at least 250V. For special devices such as: RF devices, the ESD resistance capacity is at least 100V, and the design is required to do anti-static measures (Note: Huawei is strict requirements, prohibit naked hand holding board. I didn't understand it either. Later, when I led the team, I found out that my brothers spent a lot of time repairing the veneer. Our team is very strict with this point, it may seem to reduce the efficiency, but in fact it can improve the efficiency. At least there is no constant suspicion that the device was broken by static electricity). Selected components are considered to have higher humidity sensitivity levels.

Safety: the materials used should meet the requirements of anti-static, flame retardant, anti-rust, anti-oxidation and safety regulations.

Failure rate: avoid devices with high failure rate, such as labeled dialing switches. Try not to choose bare DIE devices, easy to crack. Do not choose glass-enclosed devices. Do not choose ceramic capacitors in large packages.

Failure mode: it is necessary to consider whether the failure mode of some devices is open or closed, and what the consequences will be, all need to be evaluated. This is also an important reason for the careful selection of tantalum capacitors.

3. Productivity

Devices with package size smaller than 0402 are not selected. As far as possible to choose the table stick device, do only one reflow soldering to complete the welding, do not need to carry out wave soldering. If some plug-in devices are inevitably selected, it is necessary to consider whether the through-hole reflow welding process can be used to complete the welding, so as to reduce the welding process and cost.

4. Environmental

As a large number of Huawei products are shipped to Europe, so the environmental protection requirements are relatively strict. As the European Union put forward the lead free requirements, once the whole company almost all hardware engineers are doing lead free rectification.

5. Consider normalization

For example, a product has chosen this device, and in a large number of shipments, often sometimes the selection of the device is not very suitable, will choose, because not only can through the number of increased to renegotiate the cost, but also because after a large number of verification can be assured to choose. This is also the reason why we tend to choose the device in the mature period and carefully select the introduction and decline period.

6. Industry management

For certain categories, such as power supply, clock, processor, memory, Flash and so on, there are specialized people who plan and coordinate the use of the whole company, conduct market research, analysis and write specifications in advance. They will be involved in the selection of new devices.

7. Devices Department

Specialized colleagues in the device department will analyze the failure cause of the device, analyze the reliability of the device, take the X-ray of the device, evaluate the life of the device and so on.

The cost of 8.

If none of the above is fatal -- and none of the above is fatal -- keep an eye on No. 8.