Hermetic windows in electronic package solutions

Every piece of electronics in the modern world and the larger systems that they make up relies on millions of moving parts – literally and figuratively – all working in concert. The failure of any one piece of the puzzle can potentially lead to much bigger problems, depending on what the initial error is, and as a result, engineers strive to always be cognizant of how every part in a particular machine or system is doing. This attention to detail should apply to everything from a massive server bank to a tiny semiconductor or printed circuit board.

When dealing with devices that rely on delicate microelectronic components to power complex and mission–critical tasks, such as infrared or thermal imaging displays, it sometimes becomes necessary for an engineer or other technicians to closely observe the inner workings of such precision parts. Optical windows, within an electronic package lid or incorporated elsewhere into the cover assembly, allow these users to do exactly that. But they serve a purpose far more critical than vision as well – the windows' material must provide the same level of hermetically sealed protection for the vulnerable components within that the rest of the package's cover structure and lid can offer. As a result, it's important to have a strong understanding of what hermetic windows can offer and the sealing standards they are required to uphold. 

Essential characteristics of hermetic windows 

In the most basic sense, a hermetic window is exactly what its name would suggest – a small, transparent (or translucent) piece of glass built into the cover assembly or lid of a hermetically sealed electronic package. The window is tiny, like so many of the components at this most granular level of electronics: usually just a fraction of a centimeter across. Sometimes, the surface of this glass is integrated with the package without being altered in any way, but more often than not it will be outfitted with an anti–reflective coating.

Also, the window material isn't always glass – sometimes it is a small sliver of sapphire that has been treated with a special type of AR coating. According to Laser Focus World, sapphire transmits light at a particular and highly useful wavelength band (between 0.2 and 5.5 micrometers), and it also has an enviable level of thermal conductivity. Because the material is vulnerable to damage from scratches stemming from wear and tear despite its considerable hardness, the AR coating affixed to the sapphire that is used in windows for hermetic lids and related components is a special, hafnium–based blend. It significantly mitigates any chances of the sapphire taking more damage than it can handle in the ordinary course of its use.

Last but not least, windows allow for the passage of certain elements into the package without compromising the airtight boundaries of the hermetic seal. Specifically, heat and light can get through the small aperture of the sapphire, glass or other material of the window within the lid or other area of the cover assemblies used in these devices. In fact, in a number of commercial or industrial use cases, the free flow of these elements from their sources to the interior of the package is directly or partially essential to proper, full–fledged operation of the components within the package. 

The vital importance of hermeticity

Electronic components don't operate in a vacuum –  at least not in the most literal sense. But it's still extremely important to insure that as few impurities or environmental stimuli as possible come into contact with an integrated circuit, semiconductor or any of the other elements contained within a particular electronic package. Anything ranging from unfiltered air and other gases to moisture, dust, tiny debris and other materials can be dangerous and possibly terminally fatal to electronic parts, even in small amounts such as a few hundred parts per million. Hermetic seals are what keep all of those substances and any others that may emerge at bay, and they've been viewed as essential to the field of electronic component manufacturing for approximately half a century.

Strictly speaking, the most prominent industry standard associated with the determination of appropriately hermetic conditions in electronic packages doesn't come from a direct "industry" source: We're talking about MIL–STD–883, of course, which was first established in 1969 by the Department of Defense for the purpose of creating a standard procedure for electronic devices that would be used within the extremely sensitive context of military aerospace operations. This set of criteria related to package conditions, exists in harmony with related standards from groups like ANSI and ASTM, but neither the military standard nor its civilian counterparts supersede or counteract one another.

While it has gone through a considerable number of revisions since then (the version linked above is an update from the 1990s, and changes have occurred as recently as 2015), many of MIL–STD–883's principal tenets and requirements are either identical or extremely similar to their 1969 iterations. Some notable guidelines that directly or indirectly pertain to optical windows, lids and other cover assembly elements are as follows:

• For example, it's still true that any condensation within a microelectronic environment should typically never exceed 500 parts per million, with anything over 1,000 ppm effectively representing the point of no return and something like zero to 200 ppm being the best moisture measurement. The hazard of unwanted electrical conductivity means that water vapor represents one of the most pressing dangers to any components within an electronic package or similar device. 

• Corrosion of any kind that affects more than 5% of "the area of the finish or base metal of any package element," is unacceptable under MIL–STD–883, as is any corrosive damage that "completely crosses the element when viewed with a magnification of 10X to 20X."

• Broken, partially separated or missing leads anywhere in a package or similar device are also unacceptable for the maintenance of proper hermetic standards.

• Packages must undergo electrical endpoint and insulation resistance tests before being put into use to be considered truly hermetic. (It can be good practice to conduct these tests from time to time after they have been implemented, as well.)

Establishing a hermetic seal with a windowed lid

Creating an appropriately hermetic seal using a package lid or cover with a window is not that much different from doing the same thing with a non–optical lid. The biggest difference between the two is that engineers who fabricate electronic packages typically opt to weld a standard lid or cover to the rest of the device through seam welding or laser-based welding, and then the product is complete. (This is due to the superior strength of welded bonds as opposed to those that are either brazed or soldered.)

With an optical lid, the window first has to be hermetically soldered in place so it holds fast to the lid before the finished item can be put to use as part of a package. Once the lid has been properly modified, it would technically be possible to weld it to a package, but some engineers may opt for soldering so as not to risk damaging the material that makes up the window. Beyond that, though, both optical and non-optical lids offer the same level of protection against the unwelcome presence of gases, moisture, unclean air, dust and other random elements that may be present in these devices' working environments, as well as general wear and tear.