When boards are re-HASL'd, intermetallic formations can become excessive especially when frustrated
attempts are made to effect wetting by "cranking up" solder temperatures or slowing conveyor
speeds. This further exacerbates solder joint formation problems, and long term reliability -
notwithstanding what it does to foil and laminate adhesion characteristics.
The following definitions are known by all:
Soldering is defined as a process in which two metal surfaces are metallurgically joined, using a
specified solder medium (metal filler with a melting point below 800 degrees F.). The process is
effected by "wetting" the surfaces to be joined requiring neither diffusion nor intermetallic
growth. The effect of this process is called a solder joint.
In light of the previous definition (soldering), intermetallics may be formed, though not always
excessively, when heated solder surfaces are brought into contact with solder melted upon them.
Though soldering requires neither diffusion nor intermetallic growth, the latter often occurs as
part of the soldering process. If not immediately so, time effects intermetallic growth as a
continuous process. Also, when soldering operations are repeatedly performed, increased
intermetallic growth is effected.
Someone much more astute than me said, "Intermetallic compounds are a unique class of materials
consisting of ordered alloy phases formed between two or more metallic elements where the different
atomic species occupy specific sites in the crystal lattice."
Intermetallic compounds have much different physical and mechanical properties than the metals
comprising them. Some intermetallics are very brittle and have poor thermal and electrical
conductivity. Also, when exposed to air, they oxidize very rapidly. Therefore, excessive
intermetallics, formed either during the soldering process or over time, cause unreliable solder
joints capable of failing under stressful conditions as thermal, or mechanical shock, and/or
vibration.
Oxidation is the act of burning. When oxygen is present in an atmosphere, it "burns" or oxidizes
all material with which it comes in contact. When this happens, oxides are formed that resist
thermal input as well as solder wetting. Some materials resist oxidation better than others. Most
solder termination areas are copper (oxidizes very rapidly) covered with some protective coating to
prevent further oxidation than that which was present before the coating's application.
In summary, concerning my last HASL, PCB panels often are excessively oxidized before introduction
to what should not be a too difficult to manage solder coating process. Because of this, HASL is
unlike any other soldering operation. If re-processed, boards will suffer quality and reliability
degradation. Also, solder termination areas often are not as solderable as they would be with
tin/lead plated, or alternative coated, surfaces.
One last note: If tin/lead plated boards were HASL'd, they would be very solderable and offer
reasonably flat surfaces (though not always acceptable for fine pitch). The solder mask adhesion
problem would still exist but would be greatly improved compared with it over tin/lead/plated/fused
boards.
Earl Moon
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