Process to measure the wetting forces between a liquid and a solid body

A process to measure the moisture permeability between a liquid and a solid body is described, which is of importance for determining the suitability of material combinations. In the course of the process, the oscillation behavior of an oscillating system is changed when a solid body or test object is immersed in a liquid by the interaction of surface tension forces between the surface of the solid body or test object and the fluid. The change in the oscillation behavior or the transfer of oscillation energy between the solid object and the fluid is then used as an indicator of the strength of the surface tension force.

The invention concerns a process to measure the wetting forces between a 
liquid and a solid body. 
The hydrophilic force between a liquid and a solid medium plays a 
significant role in the determination of the suitability of combinations 
of materials, in particular for the formation of a joint or adhesive layer 
between various materials. Areas of application include, for example, the 
insertion of reinforcing fibers during the manufacture of composite 
materials. In surface bonds, the moisture absorbing characteristics 
between the individual fibers is also often very important for the quality 
of the bond. Measurement of the surface tension and thus the wetting 
forces plays an extremely vital role in the determination of 
solderability, in particular in the areas of electronics and 
microelectronics. 
The reliability of parts and components in the fields of electronic 
technology and electronics is to a great degree dependent on the faultless 
production of the soldered connection, in particular between the 
individual components and the printed circuit board. Soft soldering is the 
single most important process during large scale manufacture of 
electrically conductive connections. With today's production methods, 
those soldering processes which permit a high degree of automation have 
taken on particular significance. These processes include wave soldering, 
drag soldering, reflow soldering and combinations of these processes or 
soldering methods which are derived from them. 
In order to ensure an unvarying product quality which can be monitored, and 
thus the required level of reliability, it is essential that the 
solderability can be quantitatively and reproducibly measurable. 
Solderability is directly tied to the hydrophilic characteristics between 
the solder and the substrate. The wettability process consists of a 
reaction which takes place at the three-phase border of three materials: 
the solder, the solid substrate and the surrounding medium. Thus, the 
wettability is dependent on the type and the surface condition of the 
substrate material, the temperature, the composition and condition of the 
solder, as well as the flux. Various processes are employed to determine 
the wettability. The following represent the most important methods: 
1. Optical test; 
2. Diffusion measurement in accordance with DIN 8516 
3. Stroke immersion test in accordance with DIN 32506, Parts 2, 3; 
4. Hydrophilic attraction test in accordance with DIN 32506, Part 4; 
5. Solder sphere test in accordance with DIN IEC 68, Part 2 to 20; 
6. Rotation immersion test in accordance with DIN 40803, Part 1; 
7. Bouyancy height test 
The measurement of the forces dependent on time when the test object is 
submerged in the solder bath plays a significant role in determining the 
solderability, since this method can be employed to obtain a series of 
reproducible and meaningful numerical values. The procession of the forces 
over a period of time, which are to be measured by means of an hydrophilic 
scale during the immersion step result from the fact that, when a test 
object to be soldered is submerged in the solder, the surface tension and 
the frictional forces arising from the viscosity of the solder and from 
the penetration of the oxide surface (FIG. 1a) must first be overcome. As 
the wetting process begins to take effect, the surface tension forces 
become active allowing either the solder to rise along the surface or 
pulling the test object into the solder. Finally, the buoyancy of the test 
object also acts to counter the surface tension forces (FIG. 1b). 
Illustrative plots are shown in FIGS. 2a-c. The wettability curve is the 
result of the progression of the forces during the period of time over 
which the test object is submerged in the solder bath, and provides a 
reproducible means of quantitatively depicting solderability. The 
force/time graph developed with the aid of the hydrophilic scale reflects 
the progression of the processes which take place upon immersion (FIG. 3). 
Measurements of surface tension forces by means of the hydrophilic scale 
have been subject to variations, in particular with respect to more 
precisely determining the start of the wetting process and/or the 
separation of the solder when the test object is removed from the solder 
bath. Deviations from standard, vertical immersion by the application of 
various angles of immersion have also been employed. 
The variations in the processes for measuring the wetting forces with the 
aid of the hydrophilic scale arise in part from the difficulty in 
determinating the solderability of miniaturized components, in particular 
for components used in SMD technology. Because of the small amount of 
expansion of the surfaces to be metal-coated, a complete solder meniscus 
cannot form on SMD components. Thus, the ascent of the meniscus when 
components such as SMDs are submerged is limited by the metallic coating 
already present, and is therefore no longer characteristic for the 
wettability. This is reflected in moisture permeability curves by the fact 
that the maximum wettability of geometrically equal parts always ends at 
the same level, regardless of the test parameters. Thus, under these 
preconditions, the only differentiation which can be made is between 
wettability or no wettability while the quantitative measurement of the 
wettability cannot be made in the manner of standard, wettability curves 
developed for components without a geometrically dependent limitation to 
the meniscus (FIG. 3). 
It is the object of the invention to develop a process to measure the 
surface tension forces between a liquid and a solid, which will permit a 
quantitative measurement of the surface tension forces.

The process in accordance with the invention is based on the supposition 
that the wettability is a result of the driving forces of surface 
diffusion, that is, that it follows the attempts of a surface to saturate 
itself with atoms from the surrounding medium to a point where minimum 
energy results. In contrast to the hydrophilic scale, which measures 
static forces over a period of time for this process, the process in 
accordance with the invention determines the forces active during a 
dynamic process such as is represented by an oscillation. The effects of 
the forces active during the wetting process cause the oscillating system 
- of which the test object is a part - to experience a change, for example 
by a dampening of the frequency shift resulting from an energy transfer 
between the solid and the liquid, which results in the entire solid/liquid 
system is affected by the oscillation of one component. The measurement of 
the damping effect or frequency shift is not limited by the expansion of 
the metal-coated surfaces, and can thus be performed even when very 
limited metal-coated surfaces are present. In addition, an oscillating 
system reacts much more sensitively to changes than is the case with 
static measurements. 
In order to perform the measurements in accordance with the invention, a 
system with a test object is preferred which can be subjected to 
longitudinal or torsional oscillation; transverse oscillation and surface 
oscillation can also be reasonably employed. A single frequency such as, 
for example, the resonance frequency of the system (pick-up and probe) can 
be selected. On the other hand, all frequencies within a specific range 
can be employed in order to produce a variation in the system. In the case 
of a variation, the influence of the surface tension forces as dependent 
on the frequency setting can be observed. In addition to measuring the 
dampening and/or shift of the frequency or frequencies, the oscillation 
energy transferred across the liquid/solid interface, which also supplies 
information concerning the surface tension forces, can also be employed. 
Measurement of the energy arising from the transfer across the 
liquid/solid interface can also be used by itself. In order to measure the 
transferred energy, a sonic sensor which receives the transmitted energy 
at another point (in the bath or on the solid body) is required in 
addition to the system to be energized. 
In order to measure the surface tension forces and their influence on the 
oscillating test object, the amplitude can be selected in such a manner 
that at the minimum of the wave (its lowest point), the body is barely in 
contact with the bath so that, as the wave proceeds to its maximum, the 
bath is drawn upward by the surface tension forces. By varying the height 
of the amplitude, the maximum point at which the meniscus is torn off can 
be determined. Measurement of the dampening of the oscillation as a result 
of the forces between the bath and the solid body surface (surface tension 
forces) can, however, also be made with a fully submerged test object. 
Both the amplitude as well as the frequency can either remain constant or 
can be varied during the test procedure. 
An example of the embodiment of the invention shows the influence of the 
surface tension forces on the frequency. The component to be tested is 
securely connected to a piezoelectric plate, for example by adhesion. The 
application of an alternating current causes the piezoelectric/component 
system to oscillate in, for example, sinusoidal waves, thus displacing the 
component parallel to its longitudinal axis (FIG. 4). In this case, the 
frequency is selected in such a way that no self-oscillation of the test 
object occurs. If the oscillating system is now immersed in the solder 
bath, two effects occur as a result of surface tension: 
1. The systems characteristic frequency is shifted by the surface tension 
forces. 
2. The oscillation is dampened in direct proportion to the degree of 
moisturization taking place. 
In order to measure the change in the resonance frequency, the oscillating 
quartz can, for example, be employed as the frequency-determining link in 
an oscillator circuit (FIG. 5). The change in frequency over time will 
then reflect the course of the wetting. The dampening which arises as the 
result of surface tension forces causes an increase in the resistance 
R.sub.L in the equivalent circuit of the oscillating quartz and can 
therefore be measured via the admittance as resonance (FIG. 6). 
MELF (Metal Electrode Face Bonding) type SMD resistors measuring 
5.9.times.2.2 mm as well as a Valvo piezo-oxide oscillator membrane (12.5 
mm diameter, 12 kHz resonance frequency, 6 nF capacitance) were employed. 
The admittance of the piezoelectric part was measured. In order to measure 
the wettability the function generator was set at the frequency at which 
the admittance displayed its maximum value. The metering system is shown 
in FIG. 7. If the test object id immersed in a solder bath and the 
frequency of the function generator remains constant, the admittance 
changes due to the following factors: 
1. The characteristic frequency of the piezoelectric part is shifted 
because of the arising, time-dependent forces, so that the maximum 
admittance occurs at a point other than the set frequency. 
2. The system is dampened by the permeation of the solder, which produces a 
reduction of the admittance. 
Temperature changes which can influence the frequency must either be 
prevented or compensated for. The curves plotted from the data obtained 
from the test set up (FIG. 8 and FIG. 9) depict the admittance progression 
for both wetting and no wetting. FIG. 9 clearly shows that additional 
forces caused by the wetting process appear in the curve as well as how 
the dampening effect of wetting manifests itself. 
Interpretation of the progression of the admittance (FIG. 9) permits 
differential deductions concerning the surface tension forces and thus the 
solderability of components to be made. These deductions go far beyond 
those of previously employed wettability tests. 
Many modifications of the above described embodiments are possible without 
departing from the spirit and scope of the invention. For example, a solid 
body may be brought into contact with a moisturizing medium while the 
medium is still in its solid state, whereby, as the moisturizing medium is 
liquified, the change in the dampening and/or frequency of the oscillating 
body brought about by the rising surface tension forces may then be 
measured. In addition, a measurement may be carried out with a flowing 
medium rather than with a medium which is at rest. Furthermore, the test 
object, rather than being directly connected to the oscillator, can be 
connected by negative pressure or magnetic force to a suitable holder or 
clamp which is, in turn, connected to the oscillator. This holder may be 
used to hold several test objects in such a way that they may be either 
simultaneously immersed in the moisturizing liquid or be immersed 
individually. The test object may also be immersed at a desired angle 
rather than vertically. Therefore, the scope of the invention is measured 
not by the disclosed embodiments, but by the appended claims.