Solder Paste Flux Activation In Reflow
Solder paste is a mixture of metal powder, flux and a solvent, and the flux is there to remove the oxide that covers both the powder and the surfaces being joined. The chemicals that do that work are the activators, and they only function within a narrow temperature range during reflow. Understanding that range is what separates a profile that is copied from a data sheet from one that is set for the paste actually in the printer.
This article explains what the activator does, which part of the profile performs the work, and what the assembly looks like when activation is incomplete.
What The Activator Has To Remove
Every metal surface in the joint carries an oxide layer at room temperature. On the copper pad and the component termination the layer is thin, but on the powder inside the paste it is a significant fraction of the particle surface, and the powder has far more area than the joint. The activator drives oxide removal by reacting with these oxides and converting them into compounds that the flux can dissolve and carry away, so that clean metal meets clean metal when the alloy melts.
Until that happens, molten solder will not wet. Instead of spreading into a fillet it beads up and sits on top of the oxide, and the joint that forms has a small contact area and a high resistance. This is why a joint can look bright and still be unreliable, and why the appearance of a fillet is a poor guide to whether the activator finished its work. The alloy system involved changes the temperature at which all of this happens, as described under lead free versus leaded solder.
Where Activation Happens In The Profile
Activation begins as the paste warms and the flux becomes fluid, and it accelerates through the soak region, typically between one hundred and fifty and two hundred degrees Celsius. This is where the activator has the time to act, and where the solvent is supposed to evaporate without boiling violently. Reaching the liquidus without passing through a proper soak means the activator is still working when the alloy melts, and much of its effect is lost.
The peak temperature then finishes the job and determines whether the activator residues decompose. A peak that is too low leaves active chemicals in the residue, which is a reliability problem rather than a cosmetic one. A peak that is too high burns the flux, and the burnt residue is difficult to remove and can itself trap oxide against the joint. The window between the two is set by the paste specification and is generally quoted as a minimum and a maximum for both peak and time above liquidus. The residue that remains after that window is described under solder mask ink thixotropy in the context of the mask surface it sits on.
Activator Chemistry And Its Consequences
Activators are classified by strength and by whether they are designed to be cleaned. A mild rosin based activator leaves a benign residue and is suitable for no clean processes; a strong halide activated flux removes oxide aggressively and must be cleaned because the residue is corrosive and conductive. The classification is not a comment on quality, only on the conditions the assembly can tolerate.
Stronger chemistry is not always better for a given joint. A very active flux that is left in place attacks the copper it was meant to protect, and a flux that is specified for a dusty, oxidised surface will be too aggressive for a freshly plated board and leave more residue than the process needs. The paste is chosen for the surface condition the process actually delivers, which is a function of the storage, the plating and the time between fabrication and assembly, and not for the worst surface that could be imagined.
What Incomplete Activation Looks Like
Incomplete activation shows in several ways. The fillet does not spread to the full pad, leaving a narrow wetting angle or a distinct boundary where the solder stopped. The joint surface is dull or grainy rather than smooth, because particles merged but did not fully coalesce. Small balls sit on the mask beside the pad, where a thin film of paste reflowed without joining the main fillet. Solder climbs the component termination only part way, leaving a step at the point where the oxide was not removed.
The same signature appears if the powder itself was oxidised before printing, for example because the paste was beyond its working life or had been exposed to air. In that case no profile will fix the joint, because the oxide is inside the deposit rather than on the surfaces the flux can reach. The distinction matters when the process is being corrected: a profile change helps one case and not the other. The deposit itself is a printing result, and the way it sits on the pad is treated under placement order and pad positioning.
Setting The Window For A Real Process
The profile window comes from the paste supplier, but the profile that the assembly actually sees is measured. A thermocouple attached to a pad on a representative board, with the board loaded into the oven exactly as in production, gives a curve that includes the thermal mass of the assembly and the effect of the fixture. A profile taken with a bare coupon tells you what the oven does, not what the board does.
The measurement should be taken at the slowest point on the assembly, which is usually the largest thermal mass, and at the fastest, which is a small board at the edge of the conveyor. If both are inside the window, the process is controlled; if the window is too narrow for the spread between them, the paste is wrong for the assembly rather than the profile. Paste that has been left on the stencil for hours has already begun to lose solvent, and its activation window shifts, which is why the interval between printing and reflow belongs in the process specification alongside the temperatures.
Common Faults Attributed To The Wrong Cause
Poor wetting is frequently blamed on the flux when the real cause is the surface. A pad that was contaminated by handling, a component that has been stored in packaging that outgassed onto its terminations, or a mask that was applied over a dirty copper surface will all resist wetting however active the paste is. Cleaning the board before assembly, or simply handling it less, often resolves a wetting problem that no change of paste could fix.
Equally, a joint that wets well can still be weak if the thermal profile was so slow that the alloy spent too long at temperature. In that case the activator did its job and the intermetallic layer grew thicker than it should, which embrittles the joint without any change in its appearance. The measurement that catches this is a pull or shear test on a sample, not a visual inspection, and the test conditions should match the process window rather than an ideal that the line never achieves.
FAQ
How long can paste sit on the stencil? The paste supplier states a working life, and it is usually shorter in a dry room than in a humid one, because the solvent leaves either way. The practical limit is set by the point at which printed deposits start to look dry or ragged.
Can a higher peak compensate for a weak activator? Only within the window the paste allows. Above the maximum peak the flux burns, which produces more residue and less wetting, so raising the temperature past the limit makes the joint worse rather than better.
Does a no clean paste need activation at all? It does. A no clean paste still removes oxide during reflow; the difference is that its activators are designed to decompose during the peak so that the residue left behind is no longer chemically active.
