Second Side Reflow: Holding Heavy Components Through the Oven
In a second side reflow pass the components already soldered are underneath the board, held only by their own joints, while the new components are placed on top. Anything that is too heavy to be held by surface tension has to be glued, and the decision is made from the mass of the part rather than from the size of its pads.
Why the Second Side Is Different
During the first pass every component sits on top of the board and gravity helps the joints form. During the second side reflow the board is inverted, so the first side parts hang from their solder and are supported by the surface tension of molten alloy until the joint freezes again.
That support is real but it is limited, and it scales with the wetted perimeter of the joint rather than with the pad area. A large part with small joints is the worst case, and it is exactly the case that a designer notices last.
The Physics: Surface Tension Against Weight
The force that holds a joint together while the alloy is liquid comes from the surface tension of the solder acting around the wetted perimeter, and for a typical 0603 chip it is far larger than the few milligrams of the part. For a transformer or an inductor of several grams on four small pads the same calculation goes the other way.
The practical guide is a mass per joint rather than a mass per package, and most shops set their own limit from experience with their profile. Where the figure is close to the limit, the part is treated as a heavy component and is glued rather than left to the solder. The calculation is made per joint rather than per package, because the same body can sit on four small pads or two large ones and the holding force follows the wetted perimeter rather than the size of the part.
Adhesive Dots: Chemistry and Cure
The adhesive used for this purpose is a thixotropic epoxy or acrylic that is dispensed as a dot, and its job is to hold the part through the molten phase and then out of the way for the rest of the product’s life. It is dispensed before placement, and it may be cured in a separate step or during the reflow pass itself.
A dot of 1 mm to 2 mm diameter is typical, and the height is set so that the dot touches the component body rather than bridging between two parts. Cure during reflow is convenient, but the adhesive has to expand and contract without pulling the part off its pads, so the material data sheet is read before the shortcut is taken.
Where the Dot Goes and Where It Must Not
The adhesive dot is placed on the solder mask between pads or under the centre of the body, never touching a pad, a via or a test point. Adhesive that reaches a pad interferes with the paste deposit and prevents the joint from forming, and the failure appears as an open circuit rather than as a mechanical problem.
For a part with a large central pad, the dot is placed on the mask outside the pad outline and the part is seated on the dot as well as on the paste. The placement programme adds the dot position as its own step, so the nozzle and the placement head are sequenced correctly. Where two dots are used on a large body, they are placed symmetrically about the centre, since an asymmetric pair lets the part rock as it is placed and settle at an angle that no later inspection can correct.
Solder Paste Volume and Stand-off
The solder paste volume on the second side is the same calculation as on the first, with one addition: too much paste lets the part float during reflow, and a floating part is one that can slide. The paste is sized from the joint requirement rather than from the aperture that happens to be on the stencil.
Stand-off is the other half of the same question, because a component that stands high on a thick joint has more leverage on the adhesive. Where a heavy part is close to the mass limit, the paste volume is reduced to a value that still forms a fillet and leaves the part lower.
Thermal Profile for the Second Pass
The thermal profile of the second pass has to melt the new joints without destroying the ones already made, and the first side joints do remelt during it. A lead free assembly therefore sees the same alloy melt twice, and the second exposure adds intermetallic growth to every joint on the first side.
Where the alloy allows it, the second pass is run with a lower peak and a shorter time above liquidus, and the profile is verified with thermocouples on both sides of the board. A profile that is fine for the new side can be excessive for the old one, and the check has to include the joint that is being remelted.
<img src="https://www.gopcba.com/wp-content/uploads/2026/06/Industrial-5G-Networking-Equipment-PCBA.jpg" alt="Adhesive dot dispensed on a board before component placement” />
Supporting the Board and Carrier
A board that is running its second pass has components on the side that faces down, so it cannot be supported on a flat plate. Support pins are placed between the existing parts, or a carrier with pockets is used, and the pin positions come from the placement data rather than from the operator’s judgement.
The support also affects the profile, because a carrier adds thermal mass and shadows the joints beneath it. Our profile verification notes describe the measurement with the carrier in place, which is the only configuration that describes the production condition.
Inspection and Acceptance
The second side is inspected for shifted and lifted parts, and the check is done after the board has cooled rather than at the oven exit, because a part that has slid may still settle back into its paste. X-ray is used where the joint is hidden, and the criteria are the same as for the first pass.
The first side is inspected again after the second pass, since a joint that has been remelted can shift or form a fillet that no longer meets the drawing. A part that has moved by a fraction of a pad width is a defect even when the joint is electrically sound, and the placement accuracy budget for the line has to include it, as set out in our placement accuracy notes.
Process Control and Records
The parameters that belong in the record are the dot size and its cure, the paste volume, the profile with both sides instrumented, and the placement force used to seat the part into the adhesive. A change to any of them is a process change. Dot size is verified by weighing a known number of dots or by measuring a sample under a microscope, which is quick enough to run at the start of a shift on a valve that has been sitting idle.
The part list should also carry the mass of each heavy component, because that is the input to the decision to glue it. Without that figure, the next engineer cannot tell whether the adhesive on a part is a requirement or a habit, and parts get glued or left loose on that basis.
FAQ
Can adhesive be used on the first side as well? It can, and it is sometimes done to hold parts during handling, but on the first side gravity and the paste are usually enough. Adding adhesive where it is not needed creates a cleaning problem and a place for flux to be trapped.
Does the second pass damage the first side joints? It remelts them, so it adds a thermal cycle to every joint on that side. The effect is a thicker intermetallic layer and a slight change in the fillet, which is why the second pass is kept as short as the paste allows.
How is a component judged too heavy to hold itself? By comparing its mass with the wetted perimeter of its joints, and in practice by the shop’s own limit for the profile in use. The figure is written down, because the same part can be acceptable on one profile and not on another with a longer molten time.
