Dried Paste: 5 Rules for Stencil Life on the Line
Dried paste is the change that happens to solder paste after it has been on the stencil for too long, and it is one of the quietest causes of a yield loss on an SMT line. The paste still looks like paste, so nobody reacts, but the deposits become smaller, the corners round off and the number of skips rises through the shift.
The controls are all about time and exposure. How long the paste roll stands open, how often the wipe runs, how the machine is paused and what the abandon time is set to all decide whether the paste stays inside its working window. Stencil life is not a fixed number, it is the result of those settings.
What Happens as Paste Dries
Solder paste is a suspension of alloy powder in flux, and the flux contains solvents that keep it workable. Those solvents evaporate from any exposed surface, so the top of the roll and the paste sitting in the apertures dry first. As the solvent leaves, the viscosity rises and the paste stops flowing into the corners of an aperture.
The change is progressive rather than sudden. Early in the shift the paste rolls freely and fills the apertures from the bottom. Later, the roll becomes stiff and starts to tear instead of rolling, and the apertures fill only partly. By the time the deposits look visibly dry, the process has already been out of its window for some time.
The Paste Roll and Its Surface
The roll is the working surface of the process, and its appearance is the most useful indicator available at the machine. A healthy roll is smooth, slightly glossy and moves with the squeegee without tearing. A roll that looks dull, cracked or crumbly at the edges is telling the operator that the paste has lost solvent.
Roll size matters as well. A very small roll exposes a large surface for its volume, so it dries faster and it also prints less consistently. The roll should be kept to the size the printer manual recommends, and paste should be added before the roll becomes a thin line rather than after the deposits have already started to shrink.
Pause, Abandon and Resume Rules
The largest single source of dried paste is an unplanned pause. A stopped printer leaves the roll standing, and the paste in the apertures begins to set within a few minutes. Short pauses are handled by leaving the machine in a state that covers the paste, while longer ones require the paste to be removed and the stencil cleaned.
The abandon time is the limit after which the paste must be taken off the stencil, and it should be written into the printer recipe rather than left to judgement. When the line resumes inside the window, a wipe and a fresh roll restore the process. When it resumes after the abandon time, the paste should be removed, the stencil cleaned and a new roll started. The stencil cleaner routine is what makes that restart repeatable.
Wipe Frequency and Solvent Balance
The under stencil wipe removes paste that has partly dried in the apertures, and its frequency has to match the paste rather than the machine default. Too infrequent and dried residue builds inside the aperture, reducing every subsequent deposit. Too frequent and the apertures are repeatedly flushed with solvent, which changes the rheology of the next deposit.
A practical approach is to set the wipe frequency from a measured deposit volume rather than from appearance. If the volume falls across the shift while the printer settings are unchanged, the wipe is too infrequent or the paste is being exposed for too long. The paste volume record is the evidence that the frequency is right.
Aperture Clogging and Partial Prints
Dried paste is one of the few materials that can block an aperture completely. The residue adheres to the wall and reduces the effective opening a little with every print, until the deposit is short enough to fail inspection. Unlike a missing deposit, which is obvious, a partly clogged aperture produces a deposit that is present but thin.
The finest apertures clog first, which is why a panel can look acceptable at the coarse end and fail at the fine pitch end of the same board. Where the defect appears only on the smallest features, the paste condition should be checked before the stencil is blamed. The same pattern appears when the paste has lost solvent, because the small apertures have the least margin.
Humidity, Temperature and Airflow
The room conditions set how fast the paste loses solvent. Low humidity and warm air accelerate evaporation, and a printer standing under an air conditioning vent dries its paste faster than one in the middle of the floor. Air movement matters as much as temperature, because moving air replaces the saturated layer above the roll.
The room should be controlled to a defined range and the printer should be positioned away from direct airflow. Where the environment cannot be changed, the paste exposure time has to be shortened to compensate, which means a smaller roll, a shorter abandon time and a more frequent wipe. All three are consequences of the same problem.
Paste Properties That Set the Window
Pastes differ in how long they tolerate an open stencil. The flux chemistry, the solvent system and the powder size all affect the working window, and a paste chosen for fine pitch work often has a shorter open time than a general purpose product. The supplier data sheet gives an open time, but the usable window on the line is usually shorter.
The right way to find the real window is to run a paste through a shift with the deposits measured at intervals. That curve, rather than the data sheet, tells the shop how long the paste can be left on the stencil. Where the window is too short for the product, the paste rather than the printer is the item to change. The way the paste behaves before it reaches the printer is covered in the solder paste thaw routine.
Recovery: Can Dried Paste Be Revived
Paste that has begun to dry on the stencil should not be mixed back into the jar. The dried fraction is not the same material as the bulk, and stirring it in transfers lumps and dried fragments into the next deposit. The correct action is to remove the roll from the stencil and dispose of it.
Paste in a sealed jar that has been stored correctly is a different case, and adding the small amount of thinner the supplier permits is acceptable where the supplier allows it. Adding thinner to paste on a stencil, by contrast, is a guessing game, because the amount of solvent already lost is unknown. If a deposit looks dry, the paste should be replaced rather than adjusted.
Records, First Article and Training
The record should capture the paste lot, the time the roll was started, the pauses taken and the deposits measured during the shift. When a yield loss appears in the afternoon, that record shows whether the paste was approaching its limit or whether something else changed.
Training should include the appearance of a healthy roll, the intervention points for a pause and the reason the abandon time exists. Handing the shift over with a roll that is already past its window is a common way to lose the first hour of production, and a simple rule about who removes the paste at the end of a shift removes that risk. The mechanisms that make the paste itself easier to work with are described in the deaeration notes.
FAQ
How long can paste stay on the stencil? It depends on the paste, the aperture sizes and the room conditions, and the usable window is shorter than the open time on the data sheet. The reliable answer comes from measuring deposit volume at intervals through a shift on the actual product rather than from a generic number.
Can I add thinner to paste that has dried on the stencil? It is not recommended. The amount of solvent lost is unknown, so the quantity added is a guess, and mixing thinners into a partly dried roll usually produces a paste with uneven properties. Removing the roll and starting fresh costs a small amount of paste and removes the uncertainty.
Why do deposits get smaller through the shift when the printer settings do not change? That pattern usually points to paste drying or to aperture clogging, and the two are related. As the paste loses solvent the transfer efficiency falls, and as residue collects in the smallest apertures the deposits they produce fall further behind the rest of the panel.
