Print to Reflow Hold Time: How Long Printed Paste Can Wait

Hold time is the interval between printing a board and reflowing it, and it is one of the least controlled variables in a surface mount line. It is invisible on the process sheet, it changes with the queue, and it decides whether the paste is still the material the profile was developed for.

While the board waits, the solder paste loses solvent, its tack falls and its powder settles. The change is slow enough to be ignored in a quiet shift and fast enough to matter on a busy one, which is why the limit should be measured rather than assumed.

What Changes While the Board Waits

Three processes run at once. Solvent evaporates from the exposed surface and from the edges of each deposit, the flux continues to react slowly with the powder, and gravity plus surface tension let the deposit spread and flatten.

The first effect dominates in a dry room, the second in a warm one, and the third depends on the paste rheology and the deposit size. All three are accelerated by the same conditions that make the printing room uncomfortable, which is why a hold time that works in winter may not work in summer, and a limit set during one comfortable season is a limit that will be exceeded later.

Slump and the Limits It Sets

Slump is the spreading of the deposit after the stencil lifts, and a small amount is normal and helpful because it increases the contact area with the pad. Too much slump closes the gap between adjacent deposits, and bridging follows at reflow.

Slump is tested by printing a pattern of fine lines and holding the board at an elevated temperature for a defined time, then measuring how far the lines have spread. Fresh paste and aged paste behave differently in that test, and the difference is the evidence for a hold time limit. Drying appears in the same test and is described in the notes on warning signs of drying.

<img src="https://www.gopcba.com/wp-content/uploads/2024/10/55387056_2.webp" alt="Printed boards queued between the printer and the reflow oven” />

Drying, Tack and Component Adhesion

Tack is the stickiness that holds a component in place between placement and reflow. As the deposit dries, the surface skin reduces tack, and a component placed on a dry deposit can shift, tilt or fall off during the transfer into the oven.

The effect is worst on small, light parts and on parts with a high centre of gravity, and it appears as tombstoning, shift or missing components. A board that has waited too long often shows these defects in a pattern that follows the queue rather than the design. Recording the wait against each board is the only way to confirm that pattern.

Humidity and Room Conditions

Room humidity is the strongest environmental lever. Below about 40 percent relative humidity the paste dries quickly and the hold time shortens, while above about 60 percent the stencil becomes prone to paste sticking and to condensation on cold surfaces.

Temperature has a smaller effect on evaporation but a larger one on rheology, since a warm paste is thinner and slumps more. The usual printing room specification of 22 to 26 °C with 40 to 60 percent relative humidity is set for both reasons, and the hold time belongs in the same document. Where the room cannot be held inside that band, the hold time has to be shortened to compensate.

Measuring the Change

The measurement does not need a laboratory. Printing a test board, holding it for the intended time, printing a second board fresh and comparing the two by paste inspection gives the volume change and the spread change in one experiment.

Reflowing both boards and inspecting the joints adds the consequence. Where the difference is small, the hold time is safe; where the aged board shows more bridging, more voids or more shifted parts, the limit is too long or the storage conditions need attention. The measurement principles are the same as those used for working life on the stencil.

Setting a Hold Time for a Product

Hold time should be set per product rather than per line. A board with a fine pitch device, a small chip component and a large thermal mass in the same assembly has a tighter limit than a board with only coarse parts, because the fine pitch features fail first.

A common production figure is one to four hours, with two hours as a working limit for fine pitch work. The number should be supported by the comparison described above, and it should be reviewed whenever the paste or the room specification changes. A new paste lot with a different particle size distribution can change the slump behaviour enough to matter.

Queue Control on the Line

The limit is only useful if the queue can be seen. Printing time stamping, a physical board with the time written on it, or a line control system that displays the age of each board all work, provided the operator can act on the information.

The practical control is a maximum number of boards between the printer and the oven rather than a maximum time, because the count is visible at a glance. Where the line runs a mixed model schedule, that count has to be set for the tightest product on the line.

Second Side and Double Print Cases

A second side reflow exposes the first side to a second thermal cycle, and the paste on the second side has to survive the same hold time before that cycle begins. Where the board is printed twice before one reflow, the second print is on top of a deposit that has already aged.

Those cases should be treated as longer hold times rather than as a separate process, and the interval is measured from the second print rather than the first. The first side is already soldered and therefore less affected, but the paste on the second side is subject to the full interval from its own print to the oven.

Records and Deviations

The record should show the print time, the reflow time and the queue conditions for each lot, so that a defect pattern can be compared with the wait. Where a lot exceeds the limit, the disposition should be recorded rather than the boards quietly reflowed.

Where the product is reworked by removing and reprinting the paste, the hold time restarts, and the record should reflect that. Treating a reprinted board as though it were freshly printed is a common way for a hold time limit to be exceeded without anyone noticing, because a board printed, held and reprinted has been exposed to the room twice, and the same discipline applies to under stencil cleaning and the rest of the print cycle.

FAQ

What is a typical hold time? One to four hours, with two hours as a common working limit for fine pitch work. The figure should be verified on the product rather than copied from another line.

What happens if the limit is exceeded? Tack falls, the deposit spreads or dries, and the joints show more shift, tombstoning and bridging. The board can be reprinted rather than reflowed, which resets the clock.

Do all products need the same limit? No. Fine pitch devices and small chip components fail first, so an assembly carrying them needs a shorter limit than a board with coarse parts only.