New Two-Pad Crystal Uses Seam Sealing for Pb-Free, RoHS-Compliant Devices

By  Jack Chou, Timing Design Manager



Digital electronic circuits commonly use a system clock to enable them to perform tasks at strict periodic intervals. Stable system clocks for this function are created by combining quartz crystals (or resonators) with active electronic oscillator circuits. Designers of quartz-based oscillator circuits need to have a detailed understanding of the characteristics of quartz crystals for their oscillator designs to meet the requirements of target applications.

This blog presents and explains the design specifications of a new RoHS-compliant two-pad crystal family manufactured by Diodes Incorporated (Diodes) using a seam seal, making it fully lead-free, in contrast to equivalent crystals that use a glass-sealed design.

Diodes’ Two-Pad FE/FEQ & FT/FTQ Families

Diodes has developed a quartz crystal with a two-pad design package, like existing two-pad glass-sealed packages but using a seam seal instead (see Figure 1). It consequently has the same footprint as a glass-sealed device but is lead-free (Pb-free).

Figure 1. A glass-sealed device (left) and a seam-sealed device (right)

The two-pad FE/FEQ 5x3.2mm and FT/FTQ 3.2x2.5mm series seam-sealed devices incorporate a sub-miniature AT-cut crystal resonator and are housed in a standard ceramic package. They can be used to replace the legacy glass two-pad package. The FEQ and FTQ are Grade 0 (150ºC) AEC-Q200 qualified, PPAP capable, manufactured in facilities certified to IATF 16949, and designed for automotive applications.

In direct comparison, the two-pad seam-sealed devices are comparable or better across all parameters, as summarized in table 1 below:

Table 1. Two-pad seam-sealed devices performance comparison

The following figure illustrates the long-term stability of both package types, with devices showing slightly better performance over time.

Figure 2. Long-term crystal stability over 1008 hours, at 85°C

Technical Principles of Quartz Crystals

Frequency

The crystal’s fundamental frequency is expressed in kilohertz (kHz) or Megahertz (MHz). Frequencies may be specified to seven significant figures.

Overtones

While the most obvious way to use a quartz crystal is to operate it at its fundamental frequency, operating it in its overtone mode provides some distinct advantages. Overtone frequencies are at multiples of the fundamental frequency and mean that the quartz crystal can have a fundamental frequency much less than its intended frequency of operation, making them easier to manufacture and less fragile. The primary applications for overtone crystals are at frequencies above 50MHz. When using a crystal in its overtone mode, an electronic circuit design must ensure that the quartz crystal operates in its overtone mode only and does not operate at its fundamental frequency. To overcome this, the RF circuit design incorporates a tuned circuit that ensures the feedback in the oscillator occurs at the overtone frequency and rejects the fundamental frequency.

Cut

It is possible to define an infinite number of crystal cuts. However, some have defined properties, and these have specific names like AT, IT, and SC. AT-cut crystals are the most widely used in electronic devices where oscillators operate between 500kHz and 300MHz because this cut exhibits high Q and symmetry in frequency deviation over the specified operating temperature.

Calibration Tolerance (FL)

This is also known as frequency accuracy and is specified in parts per million (ppm), generally at 25°C.

Load Capacitance (CL)

This external capacitance sets a point on the reactance curve at which the crystal resonates. This capacitance is the expected series capacitance presented to the crystal in an application.

Equivalent Series Resistance (ESR)

ESR is the equivalent ohmic resistance of a series resonant crystal for a rated drive level and tuned operating frequency.

Operating Temperature Range (TR)

This is the temperature range in °C, over which the quoted frequency stability is specified. Most crystals will operate over a much wider range, but their stability may not be guaranteed to remain within the specified values. This wider range is referred to as the operable temperature range. The storage temperature range is how the crystal can be stored without suffering degradation in its performance and will resume regular operation once it is restored to its operable temperature range.

Frequency Stability (F/T)

This specifies the frequency deviation in ppm over the entire operating temperature range. The type and angle of cut of the crystal blank affect the way the crystal behaves over temperature. The type of cut (SL, DT, AT, etc.) will be determined mainly by the desired frequency. The specified stability and operating temperature range determine the precise angle of the cut.

Drive Level (DL)

Crystal drive level is the power dissipated in a crystal, usually specified in microwatts (µW) or milliwatts (mW). It can be calculated by measuring the excitation current flowing through the crystal.

Package Seal

Quartz crystals usually sit on ceramic base packages and are hermetically sealed to maintain constant behavior over long durations (typically 15 years or longer). There are two ways to sealed crystal devices: seam sealing and glass sealing. The seam-sealed method uses resistance-generated heat welding of the metal lid with the ceramic base. A glass seal involves melting the glass frit (silica) between the ceramic top and base.

Package Pads

Apart from various package sizes, crystals can also be divided into four-pad and two-pad packages. Two-pad devices have superior soldering capability with higher reliability and are thus widely used in automotive applications. Nowadays, all two-pad crystal devices use the glass-sealed method because it typically has lower manufacturing costs. However, the problem with this sealing method is that the glass frit contains a certain amount of lead (Pb). Therefore, the final products using these crystals may require an RoHS exemption.

Automotive-compliant - AEC qualified, manufactured in IATF 16949 certified sites supporting PPAP documentation.

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