Back in October 2023, our Technical Director (previously Engineering Director) Nick Amey, here at IQD Frequency Products sat down with Jack Portley and Justyna Matuszak from KnowHow by Distrelec for an interview on all things Quartz Crystals.
We’ve shared some of our favourite excerpts from ‘Quartz Crystals: An Interview with Nick Amey, Engineering Director at IQD’ below, but you can find the full interview available as a blog post on knowhow.distrelec.com or as a podcast on Spotify...
Ever wondered how your watch keeps time so accurately? The answer lies in quartz crystals. Despite their small size, quartz crystals play a massive role in modern technology. They’re the overlooked heroes behind the precision of our clocks, reliability of our devices, and much more.
What makes quartz crystals unique and what is their role in consumer electronics?
All the products that we offer have a piece of quartz inside. That’s the fundamental thing that makes the timing effect. And the reason that it’s used is because it has this effect called the piezoelectric effect. This is because quartz is made of silicon oxide, and its atomic structure behaves in a way that when you apply physical pressure, it moves negative ions in one direction and positive ions in the opposite direction. And conversely, if an electrical charge is put across a lump of quartz, it will pull positive ions in one direction, negative ions in another direction, and that will cause a physical change of shape. When I say physical, we’re talking on the atomic level here. So it’s teeny weeny distances. But that coupling between electrical and mechanical motion is quite rare. It was first discovered in about 1880, I think, by the Curie brothers. They were the first people that studied the piezoelectric effect. There are a few different things that have that effect but quartz crystals are the main one that is solid enough to be produced into something. The experiments happened on different kinds of salts but they crumble a little bit and it’s not possible to make long-term electronic devices out of it.
How do you pick the perfect crystal for a specific appliance? Are there multiple types of crystal that could be used for certain things?
So, the piezoelectric effect in quartz is produced in different planes through the crystallographic axis and the secret is to try and destroy the vibration mode in all the planes, except one. So, our job is to try and design a shape where there’s just one mode of vibration that’s really clear. A lot of the research on this was done in the ‘70s and ‘80s, there’s loads of white papers on it from a really interesting time. But then when I came in, it was pretty much established that for 1 MHz, through to about 30 MHz and now pushing up to 60 or 80 MHz, we use the AT-cut. So that’s 35°15′ cut from the crystallographic axis. Then it’s a bulk acoustic wave, so we’re plating on either side of that. Finally, you end up with a single mode of vibration and all the other modes are nicely suppressed.
Do these devices get worse or age over time?
I’ve been there for 18 years and I’ve never had a part fail because it’s just too old. It doesn’t happen, they keep working forever. There is an ageing effect which is what we need to talk about here. So crystals will change frequency a small amount over their lifetime. Predominantly that’s a logarithmic decay curve and you see a lot more ageing in the first year than you do over the lifetime. The amount of ageing reduces over the time span and that’s caused by two factors. There’s the outgassing of the epoxy – so when we mount the crystal inside the package, we use a silver loaded epoxy glue and epoxy, as you know, when you’ve used them yourself at home they have a smell so they’re outgassing, they’ve given out gas. That gas pollutes the purity of the cavity where the crystal is stored. So the silver electrodes begin to tarnish a little bit as it reacts with the gases that are coming off. It’s very small and obviously in the manufacturing stage, we use things like vacuum ovens to try and extrapolate as much of that as possible but there’s always a residue.
The other effect is stress relaxation so the quartz itself is a lump of rock and then we plate it with an electrode on either side. When we plate it, we hold it in a mask. Sometimes that mask can grab the quartz in a little tiny way and put a bit of flexure on it, then when you plate the silver on it and take it out of the mask, the silver ends up with the flexure as the flexure on the quartz relaxes and then over time that inconsistency and in the stress is begins to relax a little bit and that will cause a small change in frequency. I mean microscopic amounts here. This is a minimal effect but that’s the cause of ageing. Meaning that maybe on a standard quartz crystal, perhaps in one, two, three PPM ageing. And then it will diminish down to something that’s almost inconceivable.
One of the offers we do for our customers that have experienced real critical problems is to decrease the ageing by putting it in an oven in an elevated temperature that can be run for 30 days. We take that first bit of the logarithmic decay curve away before it is given to the customer.
What would you say is the most important in choosing the right crystal?
So the first thing we ask customers is:
With those things in mind we can then move into the technical discussion, such as the stability, frequency and the temperature range the person wants to work over, etc. And then the base frequency that one wants to use. We tell customers: “if you run it at lower frequency, normally you’re executing some piece of code, where you need the time to be synchronising the code events that are happening inside your processor”. If a customer runs it at a lower frequency, their code can’t execute so quickly, but the CMOS switching is a lot slower. So therefore, overall, their device will draw a lot less current and in a product like a microwave or washing machine that’s fine because they probably don’t need to execute complex bits of code but in a product like a smartphone, for example, they do need to execute code extremely fast. So they probably need to run a higher frequency for that.
Another thing that needs to be considered here is how far away from that base frequency do you need to stay within so what’s the overall frequency stability that you’re looking for. Again, if you’re looking at something like a washing machine or a microwave where there’s no communication going on, you’re simply executing a piece of code, actually, you can be pretty far away from that base frequency, before it’s going to stop working.
