And how do you like it – fluffy, crunchy or liquid?

Drei Erscheinungsformen der Kondensat-Rückstände
Three types of condensate residues

Condensate residues are much more than just a contamination in the soldering machine. Traces of it remain on the PCB and can have a negative impact on its lifetime, e.g. by stimulating surface corrosion. Volatile reactive components from the solder resist, PCB substrate and solder paste, in turn, remain as reaction products in the soldering system and disclose not only the process conditions during soldering but also if contamination or excessive humidity during PCB production have disturbed the cross-linking of the solder resist or PCB substrate. The condensate residue also contains information’s about the solder paste.

Let’s first take a look at three condensate residues from a reflow process that could not have been more different.

Crystalline condensate residues

Kristalliner Kondensat-Rückstand bei 100-facher Vergrößerung
Crystalline condensate residue at 100x magnification

# 1 is a residue that forms crystalline structures. Its source is usually organic acids and other components from the solder resist, PCB substrate and solder paste, the pure form of which has crystalline structures. This residue grows very readily in areas of the soldering machine with little flow, where the needle-like to fluffy structures can form and grow at their ease. It is particularly annoying when they grow in pipes, where they are difficult to remove.

Polymerized and crosslinked condensate residues

Polymerisierter und vernetzter Kondensat-Rückstand bei 400-facher Vergrößerung
Polymerized and cross-linked condensate residue at 400x magnification

# 2 is a polymerized and crosslinked condensate residue. Low-molecular chains from the PCB substrate, the rosin of the solder paste and non-crosslinked acrylate compounds from the solder resist are significantly responsible for its formation. These react with each other to form polymer chains that crosslink with each other. This brings numerous service engineers around the world to despair, because the longer the polymerized residues are allowed to crosslink under the influence of temperature, the more difficult they are to remove. Moreover, like all condensate residues, they cause allergic reactions on the skin, which doesn’t improve the situation.

Liquid condensate residues

Flüssiger Kondensat-Rückstand
Liquid condensate residue

# 3 is a liquid condensate residue, with few crystals in it, but without crosslinking reactions. This residue is the easiest to remove, although it also causes allergic reactions.

Let’s take a look at the molecular spectrum of the residues, and because abstract curves are useless, let’s see what information we can get from them.

FT-IR Spektren der unterschiedlichen Kondensat-Rückstände aus einem Reflow-Prozess
FT-IR spectra of different condensate residues from a reflow process

All 3 residues show clear differences – optically and on molecular level. But they still have things in common. All of them have the same rosin-based flux from the solder paste, which is found here.

Übereinstimmungen im FT-IR Spektrum verschiedener Kondensat-Rückstände
Matches in the FT-IR spectrum of different condensate residues

The crystalline condensate residue shows the lowest amount of rosin, but a strong correspondence with a substance from the solder resist, which is also crystalline in its pure form. And because this is a substance that should actually degrade and initiate a reaction, it is at the same time a warning that not everything went smoothly in the PCB manufacturing process.

In addition to the rosin, the polymerized and crosslinked residue contains a wide range of volatile epoxides from the PCB and acrylic compounds from the solder resist. These polymerize happily in the reflow oven, condense if the chain lengths are too long and finally crosslink, making the cleaning process in mostly still hot machines to hell.

FT-IR Spektrum von flüssigem Kondensat-Rückstand
FT-IR spectrum of liquid condensate residue

The liquid condensate contains a molecular signature of Na2CO3, which at the same time could be a hint that crosslinking has been prevented here. Under the microscope, however, we see that crystalline structures remain quite unimpressed by this and continue to grow blithely, since no crosslinking reactions are necessary for this.

Kristalline Strukturen im flüssigen Kondensat-Rückstand
Crystalline structures in the liquid condensate residue

As you can see, some information can be extracted from the condensate residue about deviations in previous PCB manufacturing process, as well as information’s about the solder paste, and even preventive methods to reduce crosslinking reactions in reflow machine. Some traces are obvious and easy to identify, some require real detective work. And no database in this world, no matter how stuffed with molecular spectra, can replace the human eye, expertise and in-depth research work.