Why Clean RF High-Frequency Hardware? Print E-mail
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Written by Mike Bixenman   
Tuesday, 04 September 2012 02:34

Faster signals and higher transmission frequencies increase concerns of hardware failure.


High-frequency devices require a transmission line. For traditional low-frequency hardware, wires connect devices with little to no resistance. For wireless high-frequency transmission, devices and their interactions behave differently. The sinusoidal voltage source with its associated impedance can affect current that flows into and out of the circuit. Factors than can distort the wavelengths at the frequency of interest may affect the expected performance of the device.

Flux residues are identified by flux activity and their potential to ionize. Flux activity is needed for removing metal oxides from alloys and surface finishes. Halide activators perform well, but have been discouraged due to environmental concerns and the corrosive nature of the residue. Halogen-free no-clean solder pastes are hitting the market at a high rate. The goal is to design flux compositions that improve solderability and leave a benign residue that may or may not need to be cleaned, dependent on the end-use environment.

As electronics require faster signal flow rates and higher transmission frequencies, the concern is that hardware failures will become more prevalent. The reliability challenge forces industry to look outside the existing design rules and toward application-specific field simulations to better understand reliability risks.1

Process- and service-related contaminations may accelerate reliability issues due to a range of different failure mechanisms. Three common failure mechanisms that may come into play on RF high-frequency circuits are electrochemical migration (ECM), electromigration (EM), and parasitic capacitance. Explanations of these failure mechanicals will be explained in context with high-frequency hardware.

ECM occurs from the movement of ions under a potential gradient.2 The source of ionic residues can come from the circuit board, components, flux residues, assembly process and handling.  Common ionic bonds are formed through the combination of metals and non-metals. Metals are characterized by an excess of electrons in the valence shell. Metal ions have a tendency to lose these extra electrons in order to attain a stable configuration, resulting in an electro-positive ion. Non-metals, such as halides, are characterized by a valence shell with a few electrons short of a stable configuration, and as such, gain electrons resulting in an electro-negative ion.

An interionic bond forms when a highly electropositive metal ion is combined with a highly electronegative non-metal ion. The extra electron(s) from the non-metal are attracted to the electron-deficient metal ion. In the presence of moisture (humidity), the interionic bond dissolves in water. The negative end of the water dipole is attracted to the positive hydrogen ion in water, and the positive end of the water dipole is attracted to the negative hydroxide ion.

Nano-layers of moisture (water) hydrate and dissolve these ions. The monolayers of moisture form an electrolytic solution. Anions, most notably halides, remove trace metal oxides from the solder joint connections. When the circuit is biased, the positively charged metal ions within the electrolyte solution are attracted to the negative cathode (Figure 1). Over time the metal is plated from the cathode to the anode in a dendrite tree pattern. This level of corrosion eventually leads to a dead short.

Figure 1

Closer spacing increases the electric field (E=V/d, where E is the electric field; V is the applied voltage, and d is the distance between conductors). The electric field is proportional to the force applied on a metallic ion. Electrical forces applied to a charged particle affect the time it takes for the metallic ion to move through an electrolytic solution. Bumiller, Pecht and Hillman found a strong correlation between dendrite formation and the electric field.3 Dendritic growth was found on boards with contamination as low as 0µg/in2 Cl levels on boards with less than 2 mil spacing. At contamination levels of 0 to 2µg/in2 Cl, dendritic growth was found on 6.25 mil comb spacing, with infrequent appearances on 12.5 mil spacing, and no occurrence on 25 mil spacing. At 5 to 20µg/in2 Cl, dendritic growth was found on both 6.25 and 12.5 mil spacing, with infrequent occurrences on 20 mil spacing. The research found a strong correlation between contamination levels and the distance between conductors.

EM is the movement of electrons caused by the gradual movement of ions in a conductor due to momentum transfer between conducting electrons and diffusing metal atoms.5 As technology scales down and power frequency rises, electromigration can render a failure mechanism that affects power distribution and eventually the device performance. The net effect can lead to the eventual loss of one or more connections and/or the intermittent failure of the entire circuit.5

Electromigration can cause an intermittent and/or eventual connection loss.6 Initial symptoms result in random errors that are challenging to diagnose. Highly dense interconnects increase the probability of electromigration due to power density, electrostatic force and momentum exchange with other charge carriers.6 When the momentum of the moving electron is transferred to a nearby activated ion, the ion can diffuse into adjacent metal conductions (Figure 2). This ion movement can create unintended electrical connection known as hillock and/or whisker failures.5

Figure 2

Parasitic capacitance results from unwanted stray capacitance that exists between the parts of an electronic component or circuit due to their proximity to each other.7 Short pulse rise times in components create the potential for unwanted capacitance when high transmission frequencies propagate through narrow conductor widths.8 To prevent such capacitive effects, design rules must be considered and specified.

High-frequency pulses can render electrical interference from closely spaced circuits (Figure 3). Signal integrity can be interrupted due to residues bridging two components. Flux residues under components can distort high-frequency signals due to interfering with wave paths, which may result in disrupted signal integrity. As circuits increase in density, an accurate correlation of assembly residues and their effects on reliability is needed.

Figure 3

The increasing complexity of electronic circuits using faster signal and higher frequencies is an enabler for transferring increasing amounts of data passing through networks. This column points out some of the complications from process residues. If concerned about reliability risks due to voltage swings, high frequencies, leakage currents and high impedance, cleaning process residues from assemblies is one of the drivers that may improve device reliability.

References
1. Turin Networks (n.d.). “High Reliability Challenge of Broadband Equipment,” white paper, turinnetworks.com.
2. A. Mackie, “Electromigration: Our Mutual Friend,” SMTA International Wafer-Level Packaging Conference, October 2009.
3. E. Bumiller, M. Pecht, C. Hillman, “Electrochemical Migration on HASL Plated FR-4 Printed Circuit Boards,” Pan Pac Symposium, February 2004.
4. X. Maxima, J. Die and C. Meicmei,  Signal Electromigration Analysis and Fixing Research in IC Compiler, nVidia Semiconductor Technology, 2010.
5. Electromigration. Retrieved from www.wikipedia.com.
6. K. P. Philpot, “A Guide to Microwave Diode Package Styles and Their Performance,” High Frequency Electronics, vol. 4, no. 2, February 2005.
7. Parasitic capacitance. Retrieved from wikipedia.com.

Mike Bixenman is chief technology officer at Kyzen (kyzen.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Tuesday, 04 September 2012 12:38
 

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