Before vacuum sealing electronic devices, it is useful to leak check the device. Aside from saving a bit of time finishing off a device that is bound for failure, it lets you know exactly where the problems are, such that they may either be repaired if possible, or otherwise resolved when future devices are made.
Tesla coil leak detectors
A simple method to check for leaks in glass to glass seals is with a Tesla coil leak detector. Tesla coil leak detectors produce high voltage AC at low current. When the Tesla coil tip is brought near the device to be leak detected (under vacuum), the tip will spark around the glass, and ionize the low pressure gas remaining inside the device. Any small leaks in glass to glass seals will result in the sparks concentrating to the point where the leak is located when the high voltage tip is brought close by.
Below is an image of the leak checker operating near a glass seal that is good. It helps to dim the lights when using a Tesla coil leak checker, but if you look closely at the image it can be seen that the sparks merely spread out on the glass surface, and have no good point of entry (and therefore the glow of ionized gas in the vacuum is fairly weak).
A Tesla coil will not work near glass to metal seals, as the sparks will concentrate to the metal feedthroughs. In this case acetone may be dripped around the seals to be checked. If a leak is present, the pressure in the vacuum system will shoot up. If a low pressure gauge is not available, a Tesla coil may be used to ionize the gas within the device. At moderately low pressures, air will ionize purple/pinkish to pale white, while acetone results in a pale blue color. By monitoring the color of the glow before and during application of acetone, the location of a leak may be determined. The Tesla coil should be wired directly to one of the metal feedthroughs so no external sparking occurs, as this may result in igniting the acetone outside the device. If a Tesla coil is not available, a piezoelectric grill ignitor may work, it just requires some manual effort to continuously press the button.
A vacuum device with a leaky seal is shown below, attached to the inlet port of a refrigeration compressor to act as a vacuum pump for the purposes of leak checking (only a marginal vacuum is produced, see Refrigeration Compressor Vacuum Pump). When the Tesla coil is attached to the lead wires of the device, the residual air ionizes to a bright purplish glow. Upon applying acetone to the seal, the glow quickly fades to a dim pale blue. A video of the application of acetone can be seen embedded below, or downloaded here: acetone_leak.mov (4.1MB).
Helium leak detectors
The most sophisticated and expensive way to leak check (but most thorough, and it works with all types of seals) is to use a mass spectrometer to detect helium. As the device under test is pumped by the helium leak detector, a flow of helium gas is brought near the seals to be checked. If the helium leak rate jumps up every time the helium gas is brought near a seal, then you have found a problem.