Hand Soldering and Rework Practice for Quartz Crystals
A quartz crystal is a mechanical device in a ceramic or metal package, and it is more sensitive to soldering than almost any other passive component. The resonant frequency depends on the geometry of the quartz blank, and heating the package above the specified temperature shifts that geometry permanently. A crystal that is soldered badly may still oscillate, but at a frequency outside the tolerance the circuit was designed for.
The other risk is thermal shock. The package is a sealed enclosure, and a rapid temperature change stresses the seal and the internal mounting of the blank. A crack in the seal allows moisture in, which changes the frequency and eventually stops the oscillation. Handling and soldering practice therefore determine both the initial accuracy and the long term reliability of the part.
Know the Temperature Limit
Every crystal datasheet specifies a maximum soldering temperature and a maximum time at that temperature, and the values differ between package types. A small ceramic package typically tolerates a peak of 260 degrees Celsius for a few seconds, while a plastic moulded part may be limited to a lower temperature or to a shorter time. A tuning fork crystal, used for the thirty two kilohertz reference in a real time clock, is more sensitive still.
The reflow profile of the assembly has to be checked against that limit, not only against the solder alloy. A part that is rated for a lead free profile can be soldered in a reflow oven, while one that is not should be placed after reflow or soldered by hand with a controlled iron. Reading the datasheet limit before deciding the assembly method is the single most effective precaution, and it is often skipped because the part looks like any other two terminal component.
Hand Soldering Technique
Hand soldering a crystal calls for the minimum heat that will form the joint. An iron temperature of around 300 to 320 degrees Celsius is usually high enough to melt the alloy quickly without overheating the part, and the contact time should be a few seconds at most. Soldering one terminal at a time, and allowing the package to cool between the two, keeps the peak temperature lower than soldering both in one pass.
The tip should touch the pad and the termination together rather than the package body, and the solder should be fed into the joint rather than onto the iron. Where the pad is connected to a ground plane, the copper conducts heat away and the joint takes longer to reach temperature, which tempts the operator to raise the iron temperature or to hold it on the joint. Preheating the board, or using a larger tip with better thermal recovery, is a better answer than more heat.
Reflow and Its Interaction With the Profile
In reflow, the crystal sees the whole profile, including the preheat and the soak. The peak is the obvious concern, but the time above the alloy liquidus also matters, because the part is at an elevated temperature for the duration. A profile that is at the upper end of what the solder requires spends more time near the crystal limit, and the margin disappears.
Placement also affects the temperature the crystal reaches. A part next to a large thermal mass, such as a connector or a heat sink, may be cooler than the rest of the board, while one near the edge or under a shadow in the oven may be hotter. Measuring the actual temperature at the crystal package with a thermocouple on a representative assembly is the only reliable way to confirm that the profile is acceptable, and it should be done for each board design rather than assumed from the oven settings.
Rework and Repair
Rework is where most crystals are damaged. Removing a part requires enough heat to melt the joints on both terminals, which usually means heating the package more than it was heated during assembly. The safest approach is to use a hot air tool with a controlled temperature and a small nozzle, to keep the exposure short, and to allow the board to cool naturally rather than with a blast of cold air.
A part that has been removed should not be reused unless its frequency is measured afterwards, because the thermal cycle may have shifted it. Where the rework is a replacement, the new part should be checked for frequency if the circuit is sensitive, and the board should be inspected for lifted pads and for damage to the adjacent components. Treating rework as a controlled operation, with a defined temperature and a defined procedure, is what keeps the frequency error rate from rising after the first repair.
Frequency Measurement After Assembly
Where the application depends on the crystal frequency, measuring it after assembly is worth the time. The measurement is made with a frequency counter or with a spectrum analyser at the oscillator output, and it is compared with the specification at the specified load capacitance and temperature. A part that measures outside the tolerance may have been damaged in assembly, or it may have been placed with the wrong load capacitors, and the two causes are distinguished by measuring a known good board.
The measurement also reveals the pullability of the circuit. A crystal specified for a particular load capacitance will oscillate at a different frequency if the actual capacitance differs, and on a board where the stray capacitance is higher than assumed, the error can be larger than the tolerance of the part. Checking the measured frequency against the calculated value on the first few boards confirms that the assumption was right. The same discipline that governs the rest of a design release applies here, with the additional risk that the error is invisible until the frequency is measured.
Storage and Handling
Crystals are sensitive to mechanical shock as well as to heat. Dropping a tube of parts, or handling them with tweezers that grip the body rather than the terminations, can shift the frequency or damage the internal mounting. Moisture sensitivity is usually not a concern for the sealed metal can types, but a plastic moulded package that has absorbed moisture can crack during reflow, which is why the parts are supplied in sealed bags with a moisture sensitivity level.
Storage conditions and the handling procedure should therefore be part of the process documentation, and the parts should be kept in their original packaging until they are needed. Where a part is supplied on tape and reel, the tape should be inspected for damage before it is loaded, because a deformed pocket can cause a placement error or a cracked package. These are simple controls, and they address the two failure modes that account for most of the frequency errors found in production.
Choice of Solder Alloy
The alloy used for hand soldering affects the risk to the part as much as the iron temperature does. A lead free alloy with a melting point near 217 degrees Celsius requires a higher tip temperature and a longer contact time than a tin lead alloy that melts at 183 degrees, and the extra heat is what the crystal has to tolerate. Where the product permits a leaded alloy, the lower melting point reduces the risk, which is one reason leaded solder survives in repair and prototyping work.
Where lead free is required, a low temperature alloy containing bismuth can reduce the peak temperature, at the cost of a joint that is more brittle and less suitable for a product that will see mechanical stress. The choice therefore follows from the product requirement rather than from preference, and the crystal datasheet limit should be checked against the alloy before it is selected. For a reflow assembly the same logic applies to the paste, since the paste determines the peak and the time above liquidus that the part experiences.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
FAQ
Can a crystal be soldered with a normal iron at 350 degrees? It can, but the risk of a permanent frequency shift rises sharply. A lower temperature with a shorter contact time is safer for the part.
Should a crystal be placed after reflow? Only if the datasheet limits make the reflow profile unsuitable. Where the part is rated for the profile, placing it with the other components is cheaper and more consistent.
Why does a crystal that measures correctly fail in the field? Usually because the seal was damaged and moisture entered. The frequency shift follows the contamination, and the failure is progressive rather than immediate.
