In a recent blog, we mentioned using projections as “energy directors” to achieve weld joints at specific, pre-defined locations. Ring projections - also known as annular projections - are commonly utilized in the electronic packaging industry to achieve hermetically sealed electronic packages, for transistor outline (TO) packages and, more recently, rectangular packages. Following are some tips for successful design of these ring projections and possible solutions to help you overcome less-than-perfect designs.
Ring projections are usually coined into the drawn cap material, or, in some cases, machined into the header of the package:
Ring projections must be on either the cap or the header only; never on both because these projections direct the heat and you want it directed to one predetermined location only. The typical height for projections used in electronic packaging applications ranges from 0.006”-0.010”.Furthermore, the projection height must be co-planar within 0.002 or you will experience uneven heat distribution between the 2 mating parts which will result in a non-hermetic seal due to a “cold weld” or material expulsion in some sections of the weld joint. Refer to Fig 1:
Compensating for design flaws
The quality and flatness of the projection ring is a key factor in achieving a hermetic seal. Typical causes of failure include bowed cap flanges and uneven projection height. How do you overcome those issues?
- Increase weld force
- Increase weld energy
To some degree, increasing the weld force can compensate for cap flatness and uneven projection ring height issues. Be careful, however, not to use excessive force as it can flatten the projection or dig into the mating part which will eliminate the effectiveness of the projection you’ve worked so hard to create. Furthermore, remember that when you increase the force, you decrease the contact resistance between the cap and the header. Under this condition, the weld energy needs to be increased to achieve a successful weld.
Visual indicators and reliability of a hermetic seal
Welded packages generally undergo Mil-STD gross and fine leak tests to guarantee hermetic seals, however, in most cases, visual confirmation of a continuous nickel fillet along the perimeter of the cap can be used as a good indicator of a hermetic seal. In some cases where the projection is far from the outer edge, however, a fillet may not be present. In those cases, you may consider visual monitoring of the projection collapse or performing a destructive test by mechanically separating the cap from the header. Weld checkers or weld monitors can also be installed on the weld head to measure actual current, resistance, and projection collapse during welding; these are very useful tools for process development and production monitoring.
Don’t forget to check for Post weld defects
You’ve successfully designed your projection and achieved a hermetic seal . Success! You’re done! Right? Not so fast... There are a couple of potential post-weld defects which need to be investigated: Particle Impact Noise Detection and Glass Feed-through Cracking
Particle Impact Noise Detection (PIND) failure generally occurs due to material expulsion trapped inside the package during the projection welding process (see figure 4). In most cases, this material expulsion can be eliminated by either increasing the weld force or decreasing the weld energy.
Another post-weld defect which should be investigated if fine leak failure is being encountered during weld schedule development is glass feed-through cracking. All electronic packages like TO packages, oscillators, modules, and photonics have feedthroughs surrounded by glass which can be cracked during the welding process (Figure 5) compromising the seals. These defects are attributed to excessive heat or sudden impact as the weld head connects with the parts and can be avoided by adjusting the clamping speed of the weld head and decreasing the weld pulse duration.