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SMT electronics assembly manufacturing forum.


Cleanroom Basics

#35117

Cleanroom Basics | 23 June, 2005

Hi all, we will be moving our pcb production to a different (newer) part of the plant this summer in an effort to reduce dust and better control the temp/humidity. What can I install to keep the dust down? Are there air exchangers/filters for a room this size? The room is 100' x 60'

Thanks in advance

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#35119

Cleanroom Basics | 23 June, 2005

We asked a similar question here on SMTnet a month or so ago. Major sources of dust and dirt are: * People * Packing materials * In-bound materials

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#35120

Cleanroom Basics | 23 June, 2005

I think the single most important thing will be to maintain positive pressure in the room. All the air filtering will not be able to keep up if dirt is blown into the room everytime a door is opened. The next would be to control as much as possible, sources of dust. I have not seen it myself (except at homes) but I would think something as simple as having special mats outside entrances would help cut down on dirt and dust tracked in on shoes.

Stephen

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#35124

Cleanroom Basics | 23 June, 2005

Your HVAC contractor is probably the guy to talk to (they can hook you up with a variety of air handling and filtration systems depending on the level of cleanliness you want).

Another thing you can do is reduce the amount of dust you bring in and/or create in the room. Unpacking our boards outside the room helped us out a great deal.

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#35125

Cleanroom Basics | 23 June, 2005

As Dave states:

Don't let any people in the room Don't unpack or pack anything in the room Don't let any material enter the room

This should keep the room clean.

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#35127

Cleanroom Basics | 23 June, 2005

Thanks a lot guys, is there a way to measure air quality? Like ppm of airborne particulates? Thanks again

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#35128

Cleanroom Basics | 23 June, 2005

#35129

Cleanroom Basics | 23 June, 2005

That was a great article. Luckily for us we dont build at the wafer level. Only PCB assembly through surface mount and wave. I like the positive pressure idea. Im going to contact the HVAC guy and see what he thinks. I think that just being in a closed room (not in the middle of the factory) with its own temp and humidity and only one entrance/exit will be a major improvement for us.

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#35130

Cleanroom Basics | 23 June, 2005

Don't forget about the exhaust. To keep the room at a positive pressure you must add more air than the exhaust from ovens and waves removes.

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#35154

Cleanroom Basics | 24 June, 2005

Good point. I am thinking of adding a small "mudroom" before you enter the main assembly room. There the workers can put heel straps on, smocks, hang coats, things like that.

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Rob

#35156

Cleanroom Basics | 24 June, 2005

You used to be able to get tear-off sticky mats that came on pads, I remember seeing the mobile phone boys using them (when the UK still used to build them!).

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#35157

Cleanroom Basics | 24 June, 2005

Hi,

There are many aspects to consider.

1. Firstly the room needs to be designed so it's sealed, so linoleum floors than curve where the wall meets the floor, so dirt does not get trapped in the corners, and mops get all dirt and pick it up. Floors and walls need to be sealed, so are smooth, and can be wiped down. The roof needs to be sealed and strong, and lights should be recessed and sealed with glass below the lights. The roof should be strong enough so maintenance people can get up and lift the lights out of the roof to replace lights. Even with the lights removed the glass will provide a seal.

2. You need an air handling unit installed with HEPA filters in the roof. HEPA stands for high efficiency particle air filter. Also, air handling units need to consider humidity and temperature. We used to overcool the air, to remove moisture, then reheat to the desired temperature, and then add the required humidity with humidifiers.

3. HEPA filters should be placed over areas you need the cleaners, so they effectually blow clean air over the product, and then the air flows through the room to the air recovery vents. It's generally a closed loop via the air handing unit, with a percentage of filtered outside air admitted.

4. The room should be run a positive pressure so clean air leaks out, and not dirty air leaking in.

4. Unpack products in the clean room is ok down stream from the HEPA filters, however paper should be eliminated if possible, as it emits particles. Cut plastic wrappers, and don't tear plastic warping, as it emits quite a lot of particles.

5. Try and use intelligent methods to transport materials in and out without using trolleys. The trolly wheels will track in dust, so cannot be freely wheeled in and out. We had draws that rolled in and out with a front and back, and as it moved in and out of the room on a track through a long square box structure, the front or the back of the draw always blocked the air from escaping. The draw can be cleaned and nothing is allowed to roll direct into the room. We had an anti room and to avoid people moving trolleys in, we put in a room wide bench seat to stop it. It could be removed it required. You might be able to do small anti-rooms that have a conveyer through them, and as a product moves through access panels open.

6. Staff need to wear clean room garments to avoid particles from clothing, and skin. That's a long blue overalls, as well as clean room shoe covers, and hair nets. People with beards or facial hear need to weak face masks, when in a class 100 or lower clean room. People taking smoking breaks also need to weak face masks. Clean room garments are generally laundered via special clean room suppliers, and supplied back to you packed clean in plastic sealed bags. Make sure you have hangers for garments when not in use, as they don't need to be cleaned after each use, and you can use them a few times.

7. Make sure there is clean room mats installed in every entrance to clean peoples feet. After putting on the garments, as the staff walk into the room they need to walk over a sticky matt to remove particles from feet, and there are two types available. Sticky rubber mats that need to be cleaned, or the easiest is the removable mats that have sheets of sticky plastic layered. After they get dirty, you just tear off the layer, and the layer underneath is clean.

8. Design the room with no horizontal surfaces, and if you need to have something like that, angle it, so no dust collects. Try to eliminate as many flat work surfaces as possible.

9. Install a vacuum cleaning system that uses the wall mounted vacuum hose connection, so dirt is sucked outside the clean room. You cannot use a conventional vacuum cleaner for this. Also, the vacuum hose and sucker thing should be left in the room so it's always clean, and not brining in stuff with it.

10. Keep as few things in the room as possible, and don't move things between the room and outside. It's a clean room, so as little movement from the clean to the outside environment as possible is important.

11. Put is some large windows on the front of the room, as a clean room looks really high tech, and impress people visiting. Paint it light blue so it looks clean, and it's easy to see particles and dirt. Make sure if you use electronic locks on the anti-room to ensure both doors cannot be opened, that they both unlock in case of power failure and other emergencies.

That's about it. Clean rooms will provide amazing process control if built and operated correctly. There are various grades of clean room, from class 1000 to class 100, and class 10. Class 10 is amazingly clean, while class 1000 is typical for most use. At once company we had class 1000, however it really ran at class 100, because the HEPA filters were so good.

Testing is done by a qualified clean room engineer, and they test by using special equpment that measures the particle count. I think it uses a laser while a known quantity of air is passed through a tube. That's how they know the class of the room, and the class number is the number of particles of a certain size per some cubic size of air. I cannot really remember the spec exactly. HEPA filter condition is tested by emitting a known quality of particles (smoke) into the air handing unit, and measuring the particle count.

You test every year or so depending on your requirements. Clean room technology is quite common, as it's used in every hospital operating room, so you can find specialist suppliers everywhere that will be able to provide advice and supplies.

I hope it helps.

Regards,

Grant Petty Blackmagic Design

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