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Preventing Contamination-caused Assembly Failure
December 31, 1969 |Estimated reading time: 5 minutes
Suitable cleaning methods with effective monitoring of all processes can stem the tide of residue-induced board malfunctions.
By Dr. Helmut Schweigart and Andreas Muehlbauer
The application of high-frequency technology in high-density interconnect (HDI) assemblies, together with a wider use of lead-free solders, have served to initiate a closer scrutiny on the flux-removal cleaning process. Because adequate climatic operating conditions cannot always be assumed, system signal integrity is vulnerable to failure through the parasitic capacitances of hygroscopic activator residues. Further, such contamination, particularly with the new lead-free solder formulations, is no longer detectable by ion-equivalent measurement alone. This is because the levels detected are below 1.5 μg/cm2 due to the paste formulations, resulting in an assurance of long-term reliability of assemblies per ISO 9000 certified manufacture that not only requires suitable cleaning methods, but also demands an appropriate testing technology for effective monitoring of all assembly line processes.
Susceptibility of High-frequency SystemsHDI assembly use, particularly in motor vehicles for operating-data acquisition in the logistics area and in modern building machines, is increasing rapidly. Hence, more of these systems are exposed to widely differing climatic influences, including moisture and harmful gases, which effectively threaten their functional reliability and that of the products and devices in which they are housed. Moreover, the sensitivity of these circuits to environmental interference is accentuated by the use of high-ohmic components. High-frequency circuits between 30 MHz and 5 GHZ, a requirement in communications electronics are highly susceptible to environmental because of their frequency.
Thus, to maintain signal integrity, the systems not only require an adequate ohmic-insulation resistance, they also must have a stable complex 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 capacitances of contamination can distort the "ramp-up" of the signal, thereby disrupting its integrity to the point of causing equipment malfunction. Because guaranteed reliable operation of a product for extended periods now is imperative, an increasing importance is placed on ensuring its quality. For high-frequency assemblies, this standard primarily is determined by circuit surface cleanness.
Signal-integrity DisruptionsUnfortunately, sensitive assemblies are not always stored or operated under specified climatic conditions while they are being transported or demonstrated. For example, EN 60068-2-48 states that reliability can be impaired significantly by prolonged storage, even at relative humidity rates of less than 80 percent. Moreover, the diverse climatic conditions under which assemblies may be operated are not always known. Therefore, it has become increasingly common that signal integrity of a circuit design no longer can be assured in extreme situations.
Malfunctions in the interconnected assemblies of motor vehicles, for example, may result in responses that are difficult to interpret. Speed sensors for automobile wheels are monitored not only by the ABS system but also by the engine management and other assemblies. A malfunction of a monitored component often causes other assemblies to generate misleading readings.
Figure 1. The intrinsic conductivity of contaminants serves to lower assembly SIR and is intensified by hygroscopic-induced moisture absorption. Smoldering and even fires may result.
Contamination "favors" moisture absorption, and with it electrochemical migration and corrosion-induced leakage currents. Studies indicate a higher malfunction rate among some lead-free alloys because of the presence of dendrites, particularly in edge-triggered circuits.1 Moreover, the intrinsic conductivity and electro-diffusion effects of most contamination lowers the surface resistance (Figure 1). 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 (Figure 2).
Figure 2. The influence of high frequency on complex-resistance boards. Flux activator residues can change the impedance of connecting surfaces and cause enlargements of 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, due to a support capacitor, might lead to malfunctions by causing a time delay at the watchdog of a controller, further resulting in a function-status error at that location. Additionally, surface insulation resistance (SIR) might be diminished locally and cause a similar effect by crossing leakage currents. Finally, as well as the 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 integraded, high-speed or high-frequency circuits primarily are affected.
Figure 3. A SEM charge-contrast representation of an electrically conducting contamination (SIR reduction and residue-prompted capacitive build-up) that is not light optically visible.
Proof of CleanlinessReductions in SIR and the capacitive potential that can be built up by activator residues can be shown qualitatively under a scanning electron microscope (SEM) (Figure 3). The viewing is possible via a test that responds selectively to carbon acid-based activators of fluxes by a corresponding color reaction.2 The test not only detects the activator residues from fluxes but also makes their distribution visible.
Figure 4. Contamination detection under chip capacitors by means of impedance spectroscopy. With a corresponding board storage climate and temperature, it is possible to check the aging behavior of assemblies with the method.
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 (Figure 4). In conjunction with a corresponding board-storage climate and temperature, it now is possible to check the aging behavior of assemblies.
RemediesThe intensified use of high-frequency technology, HDI assemblies and lead-free solders are 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 9000 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 though close cooperation between the manufacturers, designers and suppliers of cleaning processes.
REFERENCES1. Ko Ebata, Tabei Espec Corp.
2. Flux test (Registered to Zestron Corp.).
Dr. Helmut Schweigart and Andreas Muehlbauer may be contacted at Zestron Corp., 21641 Beaumeade Circle, Ashburn, VA 20147; (703) 589-1198; Fax: (703) 821-9248; E-mail: H.Schweigart@zestron.com and A.Muehlbauer@zestron.com; Web site: www.zestron.com.