Understanding Solder Paste Flux
Welcome back to WISDOM WEDNESDAY! Today we’ll follow last week’s post about different solders and flux with a deeper dive into the components of solder paste, and especially the non-metallic components. Wisdom Wednesday is brought to you by Micron Corporation that produces PCB assemblies for their customers with Micron precision.
Soldering
Soldering is at the heart of the PCB assembly process. Individual electronic components are soldered to a printed circuit board. This produces the solder alloy interconnects necessary for the electronic functioning of PCBAs. It is that electronic functioning that distinguishes the electronic collaboration taking place throughout a PCBA from the grounded electrical contacts of a building light switch. The difference between electrical connections and electronic interconnects is the information inputted, manipulated, and outputted in the latter. Mechanical connections are predominant for electrical joining (though much soldering does take place). Soldering is by far the most utilized method of producing electronic connections (though other adhesive/conductive methods are utilized). When we asked MAX, the AI agent for the professional organization SMTA, for the “most utilized method of creating electronic interconnects” it responded with the following bulleted summary.
- Solder has been the primary method for interconnecting electronic assemblies for well over half a century.
- Solder attachment and electrical connection characterizes the dominant paradigm of electronics manufacturing.
- Solder embodies the chief method for attaching components to a printed wiring board during manufacturing of electronic assemblies.
Fluxing
In order to solder optimally there must be flux present on the PCB during soldering. When using bar solder in a wave soldering machine flux is usually applied to the PCBA before it enters the wave solder pot. Wire solder contains flux in its core and is therefore delivered as the solder melts; often supplemented with even more external flux. Solder paste also contains flux. Its chemistry is the most complex of the three solder vehicles (bar, wire, and paste). By the term “flux” we are referring to all the components of solder paste other than the metal alloy. Technically this is not correct. Flux is only one part (with many of its own molecular components) of the mixtures that make up the nonmetallic parts of different solder pastes. Solder paste is a complex mixture of highly designed components.
When we asked MAX what was present in flux it listed 47 individual compounds and classes of compounds and then listed 36 more chemical components. That’s a lot of chemistry. Let’s sort some of it out. There are five major ingredient groups in most solder flux mediums. These are: resin, activators, solvents, carrier agents, and other additives. Resins used are mostly rosins which are colophonium derivatives: and that description is worthy of another one-thousand-word post. Let’s just say rosin is a natural resin obtained from pine trees. It’s the sticky stuff that comes out of their trunks and branches. Activators are a mixture of chemical compounds that set the flux chemistry in reactive motion within a certain temperature range. Solvents are the liquids within which the chemical mixture is transported and hopefully preserved until required. Carrier agents are what gives the flux its rheoplastic properties which are thixotropic. Think peanut butter that spreads under the pressure of a butter knife and then firms up on a slice of bread when the pressure is relieved; unlike the jelly in that sandwich. As for the additives, well, let’s just say in true FDA fashion “they preserve freshness.” Okay, actually, they are plasticizers, antioxidants, and preservatives. Freshness – right. Flux is a whole bunch of chemistry that can be categorized into five ingredient groups as resin, activators, solvents, carriers, and additives.
Labelling Flux
Rosin and resin are not the only flux materials available. IPC J-STD-004 identifies the composition as rosin (RO), resin (RE), organic (OR), or inorganic (IN). The next character indicates low (L), moderate (M), or high (H) activity, and the final digit indicates the halide designation. A “0” designation corresponds to less than 0.05% halide by weight in the flux solids; it does not necessarily mean absolutely zero halides. A “1” designation does not universally mean 0.5% or more: the permitted range depends on the activity classification. ROL0 therefore identifies a rosin-based, low-activity flux with less than 0.05% halide by weight in its solids. AIM’s H10 no-clean solder paste data sheet provides an example of a ROL0 classification.
That brings us to the end of this Wisdom Wednesday. We look forward to sharing much more about solder, flux, and cleaning PCB assemblies. You can share your industry knowledge with MICRON CORPORATION by contacting info@microncorp.com.