The term clean room has many implications. There are levels of cleanliness which are associated with classes of clean rooms.  Class 1 is associated with integrated circuit manufacturing and Class 2 is where many disk drive manufacturing plants operate. Classes 3-5 are associated with assembly operations.  Most standard semiconductor packaging occurs in a class 5 environment. However, a class 3 or 4 cleanroom area can exist within a class 5 environment by design.  But what defines whether a room is classified as clean?

The first definition of a clean room came from the United States Government. Federal standard 209 states the following:

“A Clean Room is an enclosed area employing control over the particulate matter in air with temperature, humidity and pressure control as required. To meet the requirements of a ‘Clean Room’ as defined by this standard, all Clean Rooms must not exceed a particulate count as specified in the air cleanliness class.”

This standard, first issued in 1963, was revised in 1966 (A), 1973 (B), 1987 (C), 1988 (D), 1992(E), and was withdrawn in 2001. In 1992, The American National Standards Institute (ANSI) petitioned ISO to form a committee to develop international standards for facilities, equipment, operational methods, and establish limits on contamination. Formally commissioned in May 1993 as ISO TC 209, this committee established working groups (1-9) that developed the specifications for cleanroom particle contamination standards (1), testing standards (2), construction standards (4) and operation standards (5). Today ISO 14644-X (where x=1 to 9) contain these standards. Table X-1 shows the ISO classification and the number of allowable particles of a particular size.

The Federal and ISO regulations do not indicate what environmental, product, or facility parameters to control. The regulations only provide the standards a company must meet after it decides to build products in the cleanroom.

Examination of this issue has revealed an interesting challenge.  A cleanroom is defined by a specification written to control specific parameters, such as air quality, temperature, or humidity. The term “monitored” requires a measurement of the controlled parameter on a regular basis, and “certification” is a process to verify that the parameters are under control.

By contrast, a controlled environment is defined as a working area that primarily controls physical, chemical, or biological variables.  Although a controlled environment is similar to a cleanroom in that it is controlled and monitored, a controlled environment is not certified. Moreover, it is not subject to ISO cleanroom requirements for construction and operation. Given that a controlled environment is a cleanroom, but a cleanroom is not a controlled environment why would anyone build a cleanroom?

The answer is not always straightforward, and companies must weigh the decision carefully before deciding what path to follow. Although there are no hard and fast rules for what class of cleanroom is needed to package electronics, a generally good business practice, is to produce the products in the same class as your competition.   Most standard packaging is performed in class 100,000 clean rooms, and by all accounts this works very well. However, for some devices such as very fine pitch flip chips and charge coupled devices (CCDs), a class 10,000 or 1000 room may be a better choice.

Within any class of cleanroom there can exist “cleaner areas” though use of laminar flow hoods, or by adding more HEPA filters to create islands of higher quality where specific operations take place.

The following guidelines and work practices can help limit the spread of particulate contamination, and can have a dramatic effect on particulate generation.

Things you should not bring into a cleanroom:

• Non-cleanroom paper
  
• Non-cleanroom wipes such as paper towels
  
• Pencils or non-cleanroom pens
  
• Makeup (should be removed)
  
• Jewelry (hazardous for electrical, equipment, and  bath contamination)

Things you should not do in a cleanroom:

• Remove anything from under your smock or cleanroom suit such as a wallet, money, or a cell phone
  
• Run or move fast ( just walking in a cleanroom, creates or stirs up almost 5,000,000 particles (> 0.3um) a minute)

People are the largest contributor to particle contamination in any cleanroom. They account for about 46% of all particle contamination. Modern semiconductor fabrication cleanrooms have through-the-wall construction and clean tunnel operations that separate the person from the machines doing the work. This has dramatically reduced the particle contamination and improved yields. As the industry moves to tight-pitch flip chip packaging, the day will come (and has already for some products), when the packaging line will have to move into a cleaner environment. Overall, the packaging industry has been driven to an ever-cleaner environment.  In the future, older semiconductor fabrication plants may become the packaging lab for the next generation of devices.