SIR must be designed for, especially in high-frequency boards.
High-frequency technology in HDI assemblies, combined with increased
use of lead-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 lead-free
solder formulations, is no longer detectable by ion-equivalent
measurement alone, as contamination levels are typically below 1.5
µg/cm
2 (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 threaten 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. To maintain signal integrity, the systems not
only require adequate insulation resistance, but 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.
Contamination favors moisture absorption, and with it comes
electrochemical migration and corrosion-induced leakage currents.
Studies indicate a higher malfunctioning rate among some lead-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. 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 are
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
lead-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 provisions 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.
PCD&F
