SIR must be designed for, especially in high-frequency boards.

High-frequency technology in HDI assemblies, combined with increased use of Pb-free solders, has initiated closer scrutiny on the flux removal process. Because adequate climatic operating conditions cannot always be assumed, system signal integrity is vulnerable to failure through the “parasitic-type capacitance” of hygroscopic activators. In addition, such contamination, particularly with the new Pb-free solder formulations, is no longer detectable by ion-equivalent measurement alone, as contamination levels are typically below 1.5 µg/cm2   (due to the newer paste formulations). This requires modern cleaning procedures, but also demands an appropriate testing technology for effective monitoring of all assembly line processes.

HDI assembly use, particularly in motor vehicles (i.e., for operating-data acquisition systems) is rapidly increasing. Hence, more of these systems are exposed to widely differing climatic influences, including moisture and harmful gases, which threatens their functional reliability, as well as the products and devices in which they are housed. Moreover, the sensitivity of these circuits to environmental interference is compounded by use of components using high-resistance (ohm). High-frequency circuits that range between 30 MHz and 5 GHZ (a requirement in communications electronics) are highly susceptible to environmental impacts. Thus, to maintain signal integrity, the systems not only require adequate insulation resistance, they also must have stable impedance. For this reason, capacitive surface effects must be taken into account in the circuit design.

Corrosion-induced assembly malfunctions (e.g., electrochemical migration and leakage currents) increasingly are the sources of diminished component reliability and service life. In high-frequency designs, the “parasitic-type capacitance” can distort the “ramp-up” of the signal, thereby disrupting its integrity to the point of causing equipment malfunction. Because guaranteed long-term operational reliability is imperative, an increasing importance is placed on ensuring its respective quality. For high-frequency assemblies, this is primarily determined by circuit surface cleanliness.

Unfortunately, sensitive assemblies are not always stored or operated under specified climatic conditions while being transported or demonstrated. For example, IEC TM 60068-2-48 states that reliability can be impaired significantly by prolonged storage, even at relative humidity rates of less than 80%. Moreover, the diverse climatic conditions under which assemblies may be operated are not always known. Therefore, it has become more common that signal integrity of a circuit design no longer can be ensured in extreme situations. For example, malfunctions in the interconnected assemblies of vehicles may result in responses that are difficult to interpret. Speed sensors for automobile tires are monitored not only by the ABS system, but also by the engine management. A malfunction of a monitored component often causes other assemblies to generate false readings.

Contamination “favors” moisture absorption, and with it comes electrochemical migration and corrosion-induced leakage currents. Studies indicate a higher malfunctioning rate among some Pb-free alloys because of the presence of dendrites, particularly “edge-triggered” circuits. Moreover, the intrinsic conductivity and electro-diffusion effect of most contamination lowers the surface resistance. This is because of the increased surface conductivity resulting from hygroscopic-induced moisture absorption, which is intensified by “hydronium” ions dissociated from the activators; malfunctions and assembly failures are the result. In extreme cases, as the board material becomes overheated along the creepage paths, smoldering or even fires may occur, especially in antenna and power-controlling circuits. Similarly, activator residues can change the impedance of connecting surfaces and through-holes, causing statistically fluctuating virtual enlargements of the pad geometries.

With frequencies higher than 1 GHz, the circuit designer must calculate even the low (but limited) resistance of conductive lines. If residues enlarge pad areas, the electrical layout may be changed, and might lead to malfunctions by causing, for example, a time delay at the air-traffic controller. Additionally, surface insulation resistance might be diminished locally and cause a similar effect by crossing leakage currents. Finally, as well as the prior static effects described, dynamic effects also can be present: Parasitic capacitors will distort the ramp slope. Edge-triggered active components might not recognize the signal if the ramp slope is too flat, and the signal integrity of highly integrated, high-speed or high-frequency circuits primarily is affected.

Reductions in SIR and the capacitive potential that can be built up by activator residues can be shown qualitatively under a scanning electron microscope (SEM). The viewing is possible via a test that responds selectively to carbon acid-based activators of fluxes by a corresponding color reaction. The test not only detects the activator residue from fluxes, but also makes their distribution visible.

Impedance spectroscopy promises to be a direct way to measure electrical values. For example, the “ohmic”-shunt quota under chip capacitors can be determined by this method. In conjunction with a corresponding board-storage climate and temperature, it now is possible to check the aging behavior of assemblies.

The intensified use of high-frequency technology, HDI assemblies and Pb-free solders is giving rise to new aspects in flux removal. As a result, any decision concerning cleaning or no-clean manufacture must be discussed intensively with respect to the needs of quality. In spite of the diversity of efforts to circumvent cleaning as a critical step via new joining techniques, it has become quite clear that cleaning is inseparably associated with electronics manufacturing. Accordingly, the creation of qualified cleaning processes that meet ISO 9001 guidelines also requires provision for optimal testing and monitoring procedures. Cost-optimized solutions that guarantee the highest possible long-term reliability of assemblies only can be realized through a close cooperation between the manufacturers, designers and suppliers of cleaning processes.

Harald Wack, Ph.D., is president of Zestron (zestron.com); h.wack@zestronusa.com.

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