跟读练习: What is SMT (Surface Mount Technology)? PCB Assembly (Part 2) - 通过视频学习英语口语

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Welcome to this video where we're going to be looking at what is an SMT

and this is using the surface mount technology to assemble PCBs part 2.

So here we're going to be looking at the PCBA production process.

We're going to be focusing on these next two elements within the process.

So there's reflow soldering process and the cleaning process.

So let's start with the reflow soldering process.

The circuit board assemblies need to be preheated to activate the flux in the solder paste.

Then they need to be heated at a higher temperature to reflow the solder.

A cooling step follows that allows a solder to solidify.

The The preheating, heating and cooling down cycles in the reflow machine are defined by the thermal profile.

We'll have a look at that in more detail in the following slides.

Let's have a look at the cleaning process.

So the PCB cleaning is the process of removing solder flux residues from the PCB after the SMT process.

The flux present in the solder paste reacts with the metal oxide during the reflow process

and prevents further oxidization of the solder metal.

The byproduct of this reaction is the solder flux residue that gets trapped beneath components and near the undersides of solder balls.

Let's have a look at the reflow thermal profile.

We're going to break this down into the separate zones.

So to start with we have the preheat zone.

The highest temperature is 2.5 degrees centigrade per seconds in the preheat area.

Over rising rate will cause solder balling.

In this image we have the recommended reflow soldering temperature profile.

We'll be focusing on this throughout this thermal profiling overview.

The next zone we're going to be focusing on is the soak zone.

Here we need to keep the temperature within a range of 150 to 210 degrees centigrade

and the temperature rise rate under 2.5 degrees centigrade per second during the soak zone

which lasts regularly from 60 seconds to 90 seconds and again we can see that here in this temperature profile.

The next zone is the reflow zone.

The peak temperature in the reflowing zone changes in the range of 230 to 255 degrees centigrade.

Temperature stays above 217 degrees centigrade for 40 to 70 seconds.

You can see this peak temperature here.

The final zone is the cool down zone.

The temperature dropping rate during cool down zone should be kept under 4 degrees centigrade per second.

Note the curve varies according to the shape of the plate and function of the devices.

The circumstance parameter is suggested to be 25 plus or minus 2 degrees centigrade by temperature,

and 45% and 65% by humidity.

You can see the overall time in this profile is around 270 seconds.

Let's switch over to the importance of cleaning.

Cleaning the PCB.

The solder flux residue, which is trapped between the components of the PCB substrate and the near underside of the solder balls,

is electrically conductive, as it is made up of ions.

As PCBs and modules are subjected to an external electric field,

the diffusing flux residue particles get excited by the momentum transfer of the conducting electrons in the circuit.

This leads to the particles getting displaced from their position.

A problem may arise when these particles cause bridging between two parts of the circuit.

This problem is known as electro migration.

Now the best practice, let's have a look at that.

Now there is a no clean solder paste available.

If possible you should be using this.

So many of the best performing lead free and lead containing solder pastes today are no cleans.

They have been designed to solve assembly problems described previously.

For the vast majority of applications the small amount of residue left by non-clean is not a problem.

However some assemblers want the performance of no cleans

but need to clean the no clean residue as they have extreme reliable or cosmetic requirements.

You can see how clean this no clean solder is in this image here.

So don't forget to check out our other videos in this PCB series

and you can contact us if you need any help with your projects at all in China.

So don't forget to like, share and subscribe to our channel and if you hit this little bell symbol

that will notify you each time we upload new content.

So thanks for listening my name is Paul Adams from I'm SoftEast and I shall see you in the next video.

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FOR YOU

本课的词汇与口语要点

这段视频共有 60 个句子、742 个单词可供跟读。讲话部分时长为 5:34。说话人语速平稳，每分钟约 133 个词，很适合跟读。只有 78% 的单词属于英语最常用的 3,000 词，词汇难度较高。

视频中的重点词汇

视频中 15 个值得学习的单词，附发音和释义：

单词	发音	释义
residue 名词	/ˈɹɛzɪduː/	殘餘 /残余
pasta 名词	/ˈpɑ.stə/	мянфан, 意大利麵 /意大利面
particle 名词	/ˈpɑɹtək(ə)l/	粒子
trap 名词	/tɹæp/	陷阱, 圈套
component 名词	/kəmˈpoʊ.nənt/	元件, 部件
soak 动词	/soʊk/	濕透 /湿透
vast 形容词	/væst/	浩大
mount 动词	/maʊnt/	騎 /骑
element 名词	/ˈɛləmənt/	要素
solve 动词	/sɒlv/	解決 /解决
define 动词	/dɪˈfaɪn/	界定
symbol 名词	/ˈsɪmbəl/	符號 /符号
slide 名词	/ˈslaɪ̯d/	滑梯
reliable 形容词	/ɹɪˈlaɪəbl̩/	可靠, 穩妥 /稳妥
requirement 名词	/ɹɪˈkwaɪɹmənt/	要求, 需求

视频中出现的短语动词

单词	发音	释义
cool down 动词	/kuːl ˈdaʊn/	冷卻 /冷却
check out 动词		退房

需要注意的发音

说话人用了 10 次缩略和弱读形式，例如 we're, don't, we'll。请按听到的简短形式来说。

“th” 音: thermal /ˈθɜːməl/, beneath /bɪˈniːθ/
“sh” 和 “zh” 音: migration /maɪˈɡɹeɪʃ(ə)n/, oxidization /ˌɑksɪdɪˈzeɪʃ(ə)n/
长单词——注意重音位置: momentum /ˌmə(ʊ)ˈmɛntəm/, humidity /hjuːˈmɪd.ə.ti/, parameter /pəˈɹæm.ə.tɚ/, assembler /əˈsɛmb(ə)lɚ/, electrically /ɪˈlɛk.tɹɪk.li/

如何用这段视频练习

先完整听一遍视频，不要开口，记下不认识的单词。
用正常速度逐句跟读，每句重复到你的节奏与说话人一致为止。
录下自己的声音并与原声对比，特别注意 residue, pasta, particle 这类单词。

什么是跟读法？

跟读法 (Shadowing) 是一种有科学依据的语言学习技巧，最初开发用于专业口译员的培训，并由多语言者Alexander Arguelles博士普及。这个方法简单而强大：您在听英语母语原声的同时立即大声重复——就像是一个延迟1-2秒紧跟说话者的影子。与被动听力或语法练习不同，跟读法强迫您的大脑和口腔肌肉同时处理并模仿真实的讲话模式。研究表明它能显着提高发音准确性，语调，节奏，连读，听力理解和口语流利度——使其成为雅思口语备考和真实英语交流最有效的方法之一。

影子跟读法: 阅读完整分步指南 →

跟读练习: What is SMT (Surface Mount Technology)? PCB Assembly (Part 2) - 通过视频学习英语口语

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