Soldering components on PCBs is still the most popular method for the following reasons:

The stress-coupling ability of solder, the economy and reliability of the process and the ease of manufacturing, repair and inspection.

The quality of the soldered joint is a major consideration as PCB technology develops.

Therefore the following aspects of the process:

Determine the end-product reliability and cost price.

The basic rules for quantitative measurement of electronic component Solderability using the wetting balance method are now well-known, applied and approved by European (French) and international (MIL - DIN - JIL - IPC) standards and we will consequently not detail them.

However, we believe that to obtain objective, repeatable results, a measurement can be made only if based upon proven physical or physicochemical laws.

The scientific principles used in our Wetting Balance are physical laws that date from the nineteenth century when two physicists, Thomas YOUNG, and Pierre-Simon LAPLACE, evidenced the so-called « surface tension » phenomenon which occurs when the following

are brought into contact.

A soldering alloy pellet put onto the surface of a metal plate previously fluxed* and heated to a temperature at least equal to the melting temperature of the alloy deposit becomes liquid and more especially spreads out as the solid is wettable.

* Flux is absolutely necessary to prevent the metal plate from getting oxidized under effect of the heat and to reduce oxides which might be present in the metal plate.

* The point O represents the junction between the solid, liquid and flux surface.

* The liquid phase is represented by the molten alloy.

* The solid phase corresponds to the components.

* The vapor phase corresponds to flux evaporation.

F = capillary forces

v = volume of the component part immersed in the molten alloy

alloy

P = component wettable perimeter

The above statement proves physically and mathematically that the value of the angle q is fully representative of the quality of wetting ( or solderability).

Many inter-laboratory measurements followed by in-shop soldering tests have enabled the following table to be worked out as can be seen in US and European official documents (standards, specifications . . .)

This measurement unit q allows an easy, accurate exchange not only among all departments within the same company but also on a worldwide level between companies.

We clearly see from the above described physical laws that if representative capillary forces are to be reckoned, one must know g LV which is the surface tension between L (Liquid: molten Sn/Pb alloy) and V (Vapor: flux).

The basic equation for reckoning g LV therefore is :

The force is measured using a calibrated WETTING BALANCE .

The wetting balance is named « SOLDERABILITY TESTER » in cases where the liquid used is a molten solder.

As the measurement only rests upon the analysis of the formation of a meniscus, the instrument is also named « MENISCOGRAPHE ».

- high sensitivity,

- ease of use whatever the component size,

- ease of calibration,

- simple efficient operating and computation software tool fit for use whichever the standard.

BLOCK  DIAGRAM OF THE WETTING BALANCE:

F = g .cosq .c - g.p.v

c = specimen perimeter ( mm )

g = gravitational acceleration ( 9.81 m/s² )

p = solder density ( 8 mg/mm³ )

v = specimen immersed volume ( mm³ )

a) Sample reaches the surface of the Solder Bath

b) Sample at end of Immersion depth - Archimedean push

c) Forces at Equilibrium

d) Maximum Wetting force

e) Sample lift out of the solder bath

f) Sample is out of the solder bath

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