Crystal Oscillator Package Types

Key Takeaways

• Crystal oscillators have become widely adopted due to their high-quality factor, frequency stability, cost-effectiveness, and tight tolerances.

• Crystal oscillators are mounted on PCBs using two main technologies—Surface Mount Devices (SMD) and Through Hole Devices (THD). The choice between SMD and THD depends on specific application needs, with SMDs providing efficiency and miniaturization and THDs offering easy testability and resilience.

• Crystal oscillators come with various holder types suitable for different mounting technologies. The HC-series crystal holders are the most common packaging option for crystal oscillators, offering shape variations and designations for high mechanical stress and general utility applications.

Crystal oscillators are critical components in timing circuits and widely used in PCBs

A crystal oscillator is a mechanical resonator that creates accurate timing signals in electronic circuits. The crystal is able to produce signals because of its piezoelectric properties.

When a voltage is applied across a crystal, a proportional amount of mechanical stress is applied to the crystal. In all crystal oscillator package types, there is additional circuitry that amplifies and sustains the oscillations to create accurate periodic signals. Achievable frequencies range from several Hz to hundreds of megahertz.

Crystal oscillators are commonly made out of quartz, but there are other piezoelectric materials that are used, like polycrystalline ceramics. Crystal oscillators are often used due to their high-quality factor, frequency stability, cheap cost, and tight tolerances.

Crystal Oscillator Holder Types 

Crystal Oscillator Package Types: Mounting on the PCB

The two main technologies for PCB component mounting are Surface Mount Devices and Through Hole Devices. 

Surface Mount Device (SMD)

This is the most widely available package for crystal oscillators. 

• Package dimensions as small as 0.95 x 0.6 mm
• Common temperature operating range of -40°C to 85°C
• Feature a compact size and ease of soldering onto a board 
• The size allows for high component density, resulting in more miniaturized circuits
• Features a variety of frequencies, crystal aging, and operating temperature ranges that can suit specific applications 

Through Hole Device (THD)

THD crystal oscillators are the second most common form of crystal oscillator package types. 

• Packages as small as 11 x 4.7 mm
• Temperature operating range between -40°C to 85°C
• Predecessor of surface mount technology but still useful in certain scenarios, such as prototyping by socketing them in and out without soldering
• More resilient under mechanical stress (shock and vibration)
• Can be bulky and difficult for automated soldering

Crystal Holders Used in Common Mounting Types 

SMD Crystal Holders

For SMD crystal oscillator types, common packaging options include 2, 3, 4, 6 and 8 pins, with lead, flat-leaded or no-lead options for the 2, 3 and 4 pin alternatives. Generally, the 6 and 8 pin alternatives come with additional ground pins. 

• These different pin configurations usually come in Dual In-Line Packages (DIP), a common component packaging with a rectangular housing and two parallel rows of pins running lengthwise down the device. 

• Small Outline J-Lead Package (SOJ), similar to DIP, has a similar look to DIP components but consumes less space than an equivalent DIP integrated circuit. For SOJ packages, there are 2 and 4-pin configurations with pitches (distance between two adjacent pins on a package) ranging from 2.85mm to 9.6mm. 

• Cylindrical Cans, or radial cases, have an elongated cylindrical shape. The diameter can be as small as 1.8mm, with a height of 3.7mm. Sometimes, for electromagnetic interference purposes, the cylindrical can packages also come with a metal lid on the top face of the device. 

SMD-4, no-lead crystal oscillator

THD Crystal Holders

Most THD components are made with 2 leads, and some are made with three. Usually packaged in a  Transistor Outline (TO) type, THD crystal holders are characterized by their hollowed-out hockey puck-shaped enclosure, specifically packaged in TO-5 and TO-8 packages. 

• Cylindrical can/radial holders also come in a THD version, with diameters as small as 1.8mm and a height of 4.6mm. An example of a radial enclosure is shown above in THD mounting.  

Diagram of a radial/cylindrical can type holder for crystal oscillator

• UM series enclosures have a similar shape to the HC series but are made for more robust environments. There are three variations of the UM holders: UM-1, UM-4 and UM-5, where the difference between each model is the height of each enclosure. 

Example of a UM series holder for a crystal oscillator

HC-Series Crystal Holders

The HC series crystal holders were initially used for military applications but have evolved into the most common packaging for crystal oscillators in SMD and THD mountings. The differentiating factor of this holder is the shape, which resembles a rectangular prism with rounded edges. The holder part number (HC-XX) is characterized by the specific dimensions of the holder. The letter extension after the number is made to specify if the package is suitable for space or general utility applications.

The designation letters are:

•  /S for space-level environmental conditions

•  /U for general utility 

• /US for general utility with space-level high reliability 

For example, one of the most common crystal packages on the market is the HC-49/US. For this package, the holder resembles that in the image below, but with a  size of 11.4 x 4.7 mm and a height of 4.2mm 

HC-49/US model crystal oscillator holder in through-hole mounting

There are many crystal oscillator package types – you will surely find one to suit any application. To assist in your designs with any crystal oscillator package type, tools like  Allegro X Advanced Package Designer offer features to design, characterize, validate, and enhance IC packages with ease and confidence.

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