ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: What is SMT (Surface Mount Technology)? PCB Assembly (Part 2)

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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ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: What is SMT (Surface Mount Technology)? PCB Assembly (Part 2)

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