Recommended Electrode Trace Connection Orientation
XECO supply of high frequency fundamental and overtone crystals and goal to supply 100% tested base-plated crystals creates a unique set of challenges.
Crystal Blanks Piezoelectric crystal frequencies vary for a variety of reasons associated with handling and processing of blanks, and tester variability. The airgap test method is commonly used. Setting up an airgap to read blanks well and with good repeatability is difficult. At higher frequencies, the effects of blank placement, stress, element size, and the tester setup may create an unacceptably high repeatability variability which renders the tester incapable of use as a screening device. XECO has demonstrated testing capability which yields the narrowest frequency distribution after base-plating as compared to other blank airgap test systems; we invite you to read our study posted on the web regarding this subject.
High Frequency Plated Crystal Blanks have a different and more complex dimension to them as there are multiple parameters of the crystal to consider. Frequency is the easy part! Testing to a set of test limits may control specification distribution; however, absolute accuracy is still a major issue as there are no true reference standards or common platforms for measurement. Test sockets, the ability to make reliable contact, hardware set up calibration, scanning widths and number of points scanned, crystal model complexity, hardware model complexity, software and assumptions used add a new dimension of variance to add to an already long list of variables. These include stresses from mounting in a package due to package construction and it expansion coefficients, adhesive hardness, thickness, placement, and curing. And also include parasitic capacitance, parasitic inductance, and parasitic resistance associated with the packaging of crystals and subsequent measurement of the crystal parameters.
At lower frequencies the differences imputed by these effects are much smaller, but as frequencies extend above 100MHz to 1GHz and higher, all of these items must be considered. Commonly, software compensates for parasitic capacitances which exist in tester hardware, but may not consider parasitic inductance or resistance. This may not be important at lower frequencies, but at higher frequencies this results in tester errors as these parasitic components affect results. And test socket hardware inductive parasitic elements induce resonance at certain frequencies which will result in grossly varied results. A new tester may not address effects induced by test socket hardware or cabling, and lead to incorrect conclusions about crystals being tested. A new test socket with lower parasitic elements is needed and if not practical, at least a test socket with different resonance and improved software is needed. One of the problems with these types of occurrences is that without a reference to measure against, it can be difficult to discover and quantify these affects.
Tester methodology IEC444 (IEC60444 part 5) acknowledges variances among its recommended test methods, in addition to test hardware and software as stated above, however, XECO bases its test methodology on IEC444 as it is the most comprehensive "standard" to date. XECO test systems test plated blanks and when mounted in package or using a different test socket, the motional parameters will vary. Our studies have shown differences between commercially available crystal testers at various frequencies. And as frequencies increase to the tester upper limits, sometimes markedly different results are noticed.
XECO current testing matches more consistently to theoretical values than to off-the-shelf testers; at very high frequencies, comparing to theoretical may be the only viable option as there are no off-the-shelf testers or standards to compare!
Considering tester variances which are shown to be significant, this may create difficulty in communicating value changes between XECO and you, unless correlation for that parameter has already been established. Traditional correlation test limit setting may not initially yield satisfactory results until all factors leading to testing differences are comprehended. In some cases methodology difference may not allow for a fixed correlation to be determined. Mutual cooperation and making adjustments until desired results are achieved is important for such situation. At times, recommending an incremental change ( example: 5 ohms lower) may be easiest approach.
XECO provides products based on its test methodology and requires customer accept these results as the final screen for acceptance. XECO will provide 15-20 data-logged correlation units on initial sample orders, and as requested for production orders as needed. Correlation to packaged unit measured parameters or yield variances in product may reveal a need to negotiate a change in a XECO test limit. Changes which significantly alter yields may affect pricing. At the time of a specification change, XECO will issue a new internal part number for that product so that XECO can clearly delineate the point of any such specification change.