Q and A

 

Pernicka Q and A

What does Pernicka do?

The Pernicka Corporation designs and sells ultra-high vacuum systems for high-reliability electronics test applications.

Integrated circuits, and many other electronic components, don't tolerate moisture or other impurities very well. To meet the highest reliability standards they must be placed in packages that prevent contaminants from leaking in, either quickly or slowly. In some applications the packages must also prevent material inside the component from leaking out. Such packages are called hermetic packages. For most high-reliability applications, metal and ceramic materials work best.

The Pernicka Corporation has developed a technique called Cumulative Helium Leak Detection, or CHLD, for testing the package integrity of electronic devices. This single-step process replaces both gross-leak and fine-leak testing. Furthermore, it is able to detect far smaller leaks than previous methods.

The Pernicka Corporation has developed the Model 700G, the first commercially available test machine which uses the new CHLD technique. It significantly improves on the state-of-the-art in testing of small electronic devices to make sure they don't leak.

What is CHLD?

CHLD, or Cumulative Helium Leak Detection, is the process that the Pernicka Model 700G uses to detect leaks in small hermetic packages. The process combines an ultra-high vacuum chamber with a programmable mass spectrometer to find, analyze and identify gases that escape from the device under test. CHLD is better at finding very tiny leaks than previous methods. To understand why, it's helpful to review the previous leak detection method.

How has leak detection been performed in the past?

Historically, packages have been tested for leaks using a two-step method. Leaks can be very small; even submicroscopic cracks or other assembly defects can allow contaminants to slowly enter a package over a period of months. To test for these very fine leaks, the package is subjected to high pressure helium. This will force helium into the package if there is a leak. The package is then placed in a test chamber. Almost all of the air in the chamber is pumped out; as the very last bit is removed, it is analyzed to see if any helium is present. If it is, the device has a leak.

Note that most of the gas in the test chamber is pumped out to the atmosphere without being analyzed. If the package had a large leak, all of the helium will leak out before analysis begins. To deal with this possibility, a second test is performed.

In the gross leak test, the device is subjected to high pressure fluorocarbon liquid that evaporates easily. The high pressure will force the liquid in if there is a package hole. The device is then submerged in a different fluorocarbon liquid, which is heated. The heat will cause any of the test liquid to boil. If there is a leak, a bubble will appear as the fluorocarbon vapor leaks back out of the package.

The gross leak and fine leak test methods have served the semiconductor industry well in the past, and still give good results in many applications today. However, several factors have led the industry to seek better techniques for some applications.

How does CHLD work?

The key difference between CHLD and the traditional method is that CHLD is a closed system; it accumulates all gases in the test and analyzes them. This cumulative technique significantly enhances the sensitivity of the test. CHLD is about 1,000 times more sensitive that the traditional leak detection method.

CHLD testing begins by first exposing the component to high-pressure helium, just as was done with the old test method. The package is them placed in the CHLD test chamber, and the chamber is purged with pure nitrogen. A specially-designed cryopump is used to create a vacuum by solidifying – freezing, in effect – the nitrogen in the system. The solidified gases remain in the system. If no helium has leaked out of the device under test, the pressure will fall to essentially zero. At this point, the package is known not to have a large, or ‘gross' leak.

For the next phase of the test, a mass spectrometer is activated between the test chamber and the pump. The cryopump will not solidify helium. If any helium is leaking out, its concentration will slowly but steadily increase within the system, and the mass spectrometer will detect it. As an additional benefit, the mass spectrometer can detect any other material in the test chamber. This is particularly useful for certain types of implantable medical devices and some components used in space applications.

What size of leaks are we looking for?

Very small ones. Leak rates are typically measured in atmospheric cubic centimeters (CCs, or milliliters) per second, usually abbreviated as atm-cc/sec. Hermetic packages must have leak rates that are one-billionth to one-trillionth of an atm-cc/sec. This is a leak rate so small it's hard to put in human terms. A typical car tire with a leak rate of one-billionth of an atm-cc/sec would go flat after about 500,000 years.

What's the smallest leak you can detect?

The old fine-leak test had a limit of detection of about 5 x 10-10 atm-cc/sec. The CHLD process has a limit of leak detection more than 1000 times better than the old fine-leak method. It can detect leaks as small as 3 x 10-13 atm-cc/sec.

 

Has CHLD been accepted in the industry?

CHLD is currently in use in applications for NASA and the US Food and Drug Administration (FDA). The CHLD method has been approved by DESC ( Defense Electronics Supply Center ) as MIL-STD 750 Method 1071 CH1 and CH2.

What sorts of devices are tested by Pernicka's CHLD machine?

The CHLD process and the Pernicka Model 700G machine can be used to test objects of almost any shape, size, or material to see if they leak in or out. The only restriction is that the object must tolerate vacuum.

The following three categories represent the most common applications for CHLD testing:

1) Implantable medical devices:

Implantable medical devices are exposed to more moisture than most other electronic devices. They also require very high reliability: it is critical to insure that the assembled implantable device will not allow moisture in, or any toxic compound out.

2) Electronic modules used in satellites and similar applications:

Communications satellites are being designed for longer operational life. This is largely in response to the high cost of launch, which provides a strong financial incentive to get the most out of each satellite. Satellites operate in the vacuum of space, so moisture is not a problem once the satellite is launched. However, the satellite electronics must be able to survive the manufacturing and testing process here on earth prior to launch. In addition, some high voltage electronic components are gas-pressurized internally to avoid arc-over or high voltage breakdown. Leakage of this gas must be avoided.

3) Extremely small IC packages:

New small packages, such as the ‘UB' package, can't be tested reliably using the old test methods. Today, packages of this size are a very small portion of current semiconductor industry packaging, but with the strong history of miniaturization in that industry, the number of very small packages is expected to grow.

What are the benefits of the CHLD process Pernicka offers?

CHLD uses no CFCs or any other hazardous substance. It uses only helium and nitrogen. Thus, there are no environmental or hazardous substance risks to manage, nor any regulatory issues with by-products or wastes.

For Medical Applications:

Improved leak detection eliminates two significant causes of premature device failure while inside the body. Early failure of implanted devices can lead to expensive and inconvenient replacement procedures at best, and even worse can be harmful to patient health. CHLD tests the device to insure that water cannot leak in and cause device failure, but because CHLD analyzes all gases as the part undergoes test, it can do more. For example, some implantable medical devices such as insulin pumps contain a small pressurized gas chamber which acts as a power source. CHLD can detect outward leakage of the gas used in this chamber, thus screening out devices that would fail early in service. In general, CHLD insures that nothing in the device can leak out and cause possible health problems.

CHLD is the only method which can safely test a fully-assembled medical device. An implantable device is usually coated with a silicone rubber covering. This makes gross leak testing using CFCs impossible. Pressurizing the parts in a helium bomb for conventional fine-leak testing is also difficult. The CHLD method can detect leaks through the silicon rubber, thus facilitating 100% leak testing of the final assembly.

For Space and Satellite Applications:

Improved leak detection removes a factor which limits the practical life of electronics in space. This can extend the service life of space vehicles by several years. Given the high cost of putting a satellite in orbit, extending the service life has a very significant positive economic effect.

For Commercial Small-package Semiconductor Applications:

CHLD reduces the cost of package testing by combining the separate steps of gross- and fine-leak detection used in prior techniques into a single-pass test of notably higher accuracy. In the case of the smallest of new packages, the greater precision of CHLD allows for leak-detection testing where prior techniques were ineffective.

Why isn't this test useful for all electronics, in all applications?

Leak detection is normally done only for high-reliability hermetic packages. Most electronic systems for consumer applications use components in plastic packages, rather than hermetically sealed packages. Plastic packages are not expected to be leak-proof, and so no leak detection testing is applied.

Why is Helium Used in Leak Detection?

The Pernicka Model 700G system uses a mass spectrometer to profile all of the contents of the vacuum chamber as it is evacuated. Mass spectrometers can detect an extremely wide variety of materials, in any combination, with very high accuracy. Making use of this capability of the Pernicka Model 700G, the CHLD technique uses helium gas to detect leaks. Helium is chosen as the test gas due to several important characteristics:

- Helium gas is relatively inert and non-toxic. It is safe to handle, store, use, and vent.

- Helium is commonly available and inexpensive.

- Helium is among the smallest of molecules. It can readily pass through materials that other molecules cannot. This makes it useful for detecting very small leaks.

Who would typically buy the Pernicka Model 700G machine?

The decision makers who will purchase the Pernicka Model 700G CHLD machine will typically be in the quality assurance (QA) department of companies that make implantable medical devices or electronic modules in satellites, and would choose the Pernicka machine to verify and extend the expected life of their companies' products. Semiconductor packaging and test facilities dealing with hermetic packaged parts or very small packages will also be interested.

Who are Pernicka's competitors?

No one else offers a CHLD test machine today. Pernicka is the first. Competition can come from either alternative approaches to leak-detection, or by the likelihood that additional manufacturers of CHLD machines will appear in the future.

The Krypton 85 leak test method is an existing alternative to CHLD. This method requires manufacturers to insert a minute radioactive adsorber into the cavity prior to hermetically sealing their component. This method is used extensively in the manufacture of air bag sensors for the automotive industry. However, the radioactive adsorber, minute as it is, is not compatible with most semiconductors, so this method is poorly suited for any device which contains semiconductors.

There are companies that make a combination of high vacuum handling equipment and high-precision scientific instrumentation. Some of these companies could pursue their own CHLD development efforts, and so could become competitors in time. Varian Inc. and Vacuum Instrument Co. are examples of such companies.

What is RGA?

RGA, or Residual Gas Analysis, is an analytical technique used primarily for quality assurance and failure analysis purposes. In RGA, the test device is placed in a sealed chamber and punctured. The interior gases are sucked out and analyzed. This analysis provides the basic data needed to determine if, and why, a package leaked. It can also be used to determine if the package was properly assembled in the first place.

RGA requires precise, well-calibrated equipment and careful technique. In capable hands, it is a useful analytical tool. RGA is usually considered a destructive test process, since it involves puncturing the package. In some cases, it can be performed in a way that allows package repair after test.

What's a Cryopump?

A cryopump is a type of vacuum pump that operates using intense cold. The heart of the pump is an extremely cold pump chamber, wherein most gases such as nitrogen and oxygen will freeze solid. Unlike a typical pump, a cryopump has no discharge outlet. It draws gases in and freezes them; the frozen material remains in the pump. (Yes, periodically the pump must be cleaned out.) Cryopumps cannot freeze helium or hydrogen. These gases will be drawn into the pump chamber, but will remain gaseous.

What's a Mass Spectrometer?

A mass spectrometer (MS) is a device for analyzing materials to determine their composition. Its chief advantage is that it is very sensitive and can detect very small amounts of matter. It operates on an electromagnetic principle, namely that a magnet will tend to deflect an electric current. The MS ionizes the test material and applies a magnetic field. The material is deflected; a detector indicates how much. Analysis of the amount of deflection determines the type of material.