Double-Sided Assembly: Second Side Reflow, Support and Weight
A board with parts on both sides passes through the reflow oven twice. The second pass heats an assembly that already contains molten joints on the other side, and that changes almost every process decision.
What the Second Pass Does to the First Side
The joints formed in the first pass are reflowed again in the second. Their solder melts, and the components on that side are held in place only by the surface tension of the molten joints and by the flux residue around them.
A heavy component on the first side will fall off unless it is held. Small chip components are held adequately by surface tension, and large parts, connectors and any component with a high mass to pad area ratio are not.
The second pass also adds another thermal cycle to every component that was placed in the first pass, which matters for parts with a limited number of reflows and for parts with a moisture sensitivity limit.
Holding the First Side Down
The classical solution is a pallet or carrier that supports the board and holds the components on the first side with clamps or with a soft material that presses them against the board.
An alternative is to glue the heavy components with a dot of adhesive before the second reflow, which is the same technique used in a wave soldered assembly. The adhesive must survive the second reflow and must be compatible with the paste.
A third approach places the heavy parts on the second side so that they are never reflowed twice. This is a layout decision and it is often the cheapest answer where the mechanical design permits it. Our design release checklist notes where the side assignment is recorded.
Pallet Design and Thermal Effects
A pallet adds thermal mass and it shields the board from the convection and from the infrared energy of the oven. Both effects change the profile, and the profile must be developed with the pallet in place rather than on the bare board.
The pallet also constrains the board, which changes the way it expands during heating. A board held tightly in a pallet can bow or can be stressed at the clamping points.
The material of the pallet matters for the same reasons. A composite pallet has a different thermal behaviour from an aluminium one, and the profile that works for one does not transfer to the other without verification.
Paste and Stencil for the Second Side
The second side printing is performed after the first side has been reflowed, so the board is no longer flat in the way it was originally. The reflowed side has solder, and the paste printed on the second side must not be disturbed by the support underneath.
The stencil and the printing parameters are the same as for the first side, and the board support must accommodate components on the reflowed side without pressing them into the board or lifting the board away from the stencil.
Vias that were open on the first side may have been filled with solder, which changes the surface the paste is printed onto. Our paste inspection notes describe the measurement that confirms the print.
Defects Specific to the Second Pass
A component that has moved slightly during the second reflow produces a joint that is displaced, which may pass inspection and fail later. The movement is driven by the surface tension of the joint rather than by the machine.
Solder that was left as a ball on the first side can re-melt and roll, producing a loose ball inside the finished product. Loose conductive material inside an enclosure is a latent failure that is very expensive to find.
A component that falls off lands inside the oven and can damage the belt or later boards. Our solder defects notes describe how these appear at inspection.
Inspection After the Second Pass
Inspection must cover both sides, and it must distinguish the defects from the first pass from those of the second. An automated optical inspection program is usually run once per side and the results merged into the unit record.
Where a component on the first side has moved, the evidence is in the second side inspection because the board is turned over. The inspection program should therefore check the joints of the first side components as well as their presence.
Our AOI notes describe how the two programs are combined and how the false call rate is controlled.
When the Second Pass Can Be Avoided
Where the design allows, placing all of the through-hole and heavy parts on one side and all surface mount parts on the other reduces the problem. Selective soldering of the through-hole parts can then replace a second reflow entirely.
A design that uses one reflow and one selective soldering operation has fewer thermal cycles, fewer fixtures and fewer opportunities for a component to fall. The saving is often larger than the cost of the selective process.
Where the second pass is unavoidable, the design should consider it as a constraint rather than as a detail. The side assignment of every heavy component and the location of the fiducials for both sides are decided in the layout, not in the process.
Verification and Records
The records should include the pallet used, the profile measured with the pallet, and the assembly operations for the second side. Where adhesive is used to hold components, the amount and the cure should be recorded.
The first article for a double sided assembly should be inspected on both sides after the second pass, and the orientation of polarised parts on both sides verified. Our quality notes describe the inspection sequence.
Where the same product is built on both a pallet and a bare board line, the profiles are not interchangeable and the records should say which was used.
Process Control and Verification
On a design of this kind, adhesive is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, adhesive is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
FAQ
Will small chip components fall off during the second reflow? They are held by the surface tension of the joint and rarely do, provided the paste volume is correct and the profile does not shock the assembly.
Does a pallet always change the profile? Yes, by adding thermal mass and shielding the board. The profile must be measured on the board in the pallet.
What does gopcb provide for double-sided assemblies? We provide pallet design that supports the first side without stressing it, profiles measured with the pallet, adhesive selection where components must be held, and inspection programs for both sides with merged records. Where a second reflow can be avoided by changing the side assignment, we propose it.
