The impeding effect of minute particles over the crystal blank on the vibration of the crystal blank is smaller while the particles are just in contact with the blank than while they adhere to it. This is supposedly because the standing wave owing to the vibration of the crystal is unlikely to propagate to the particles. When the minute particles adhere to the surface of the crystal blank, part of the standing wave owing to the vibrational crystal propagates to the particles as shown in Fig. 9, disturbing the rest of the standing wave and increasing the CI value. That is, the CI value increase owing to the adhesion of the particles to the crystal blank facilitates the separation of the particle-adhered products in the screening process by their characteristics. This principle is applied to the high-temperature treatment used in the inspection process of quartz crystals. The high-temperature treatment is effective in the treatment of organic particles. In a high-temperature environment, the organic particles denature and adhere to the surface of the crystal blank, whereas inorganic particles are unlikely to adhere to the crystal surface. The adhesion of the inorganic particles requires another technique.
Fig. 9. Propagation of the Standing Wave by Particle Adhesion
4. Solution to the Adhesion of Inorganic Particles
Because the surface of the inorganic particles contains a trace of organic material, we investigated the possibility of adhering the inorganic particles to a crystal blank using moisture (water molecules) as a mediator as illustrated in Fig. 10.
The products that incorporated a crystal blank, to whose surface particles were deliberately adhered, were left under a high-temperature and high-humidity environment, and the variation of the characteristics was investigated. The result indicated the effect of the inorganic particles on the CI value increase under the high-temperature and high-humidity environment. Figure 11 presents the CI variation under the following three different high-temperature and high-humidity environments: (a) at 125°C and a relative humidity (RH) of 40% for 24 hours, (b) at 180°C and 20% RH for 24 hours, and (c) at 85°C and 85% RH for 24 hours. The particles derived from the human body such as skin (organic substance A) are denoted by the symbol ◆, those derived from a cured adhesive substance in a form of minute fragments (organic substance B) are denoted by ■, and the inorganic particles derived from the powder of metal such as stainless steel (inorganic substance) are denoted by X. The plot under the (a) conditions indicates the CI variation was less than 50% in all cases. In particular, the blank to which the cured-adhesive-derived fragments (organic substance B) adhered and the blank to which the metal powder (inorganic substance) adhered exhibited a small CI variation. The plot under the (b) conditions shows the CI variation of the product to which the cured-adhesive-derived fragments (organic substance B) adhered was larger than that under (a), whereas the CI variation of the product to which metal powder (inorganic substance) adhered was not very large. In contrast, the plot under the (c) conditions shows that the CI variation was significantly large in all of the products to which the human-body-derived particles (organic substance A), the cured-adhesive-derived fragments (organic substance B), or the metal powder (inorganic substance) adhered.
The above result tells us that the high-temperature environment affects the CI characteristics of the organic particles, whereas the same environment affects those of the inorganic particles only slightly. Our humidity treatment deposits the products under a high-temperature and high-humidity environment to enhance the adhesiveness of the inorganic particles, thereby separating the particle-adhered products from the rest.
HCR has a resin-sealed structure, which allows the permeation of water vapor. The product deliberately allows water vapor to enter the package and lets moisture (water molecules) adsorb on the surface of the inorganic particles, which enhances the adhesiveness of the inorganic particles to the crystal blank. When an excess volume of water vapor enters the package, the load of the mass of moisture promotes the deterioration of the characteristics; therefore, the volume of water vapor entering the package must be controlled. The volume of water vapor that a package can hold, given by Equation 1, is dependent on the internal volume of the package. Figure 12 shows the relationship between the water content entering the package and the CI variation when the internal volume of the package was deliberately changed. For example, designing the internal volume of the HCR XRCHA series to make the water content no more than 220 ng allows you to verify no impact of the water content on the CI characteristics. To summarize the above, the introduction of the humidity treatment will ensure more secure screening of particles than the conventional high-temperature treatment.
Fig. 10. The Mechanism of Moisture-Mediated Particle Adhesion
Fig. 11. The Effect of the Conditions of Aging on the CI Characteristics
Equation 1. Water Content in a Package
Fig. 12. The Effect of Water Content in the Product Package on the CI Characteristics
The new particle screening technology we proposed here is a patented technology of Murata (Patent No. 4998620), which has been implemented in the production process of HCR. Volume production of HCR started in 2009 and since the introduction of the humidity treatment, there has been no defective product owing to particle adhesion recorded in shipped products. Expanding versatility of mobile devices and automotive components is expected to further boost the expansion of the timing device market in the future. We will continue to strive to provide stable quality products for our customers.