I am looking at reflow ovens. A lot of them have a metal conveyor belt the boards sit on. On a double sided board, when you put the board on the metal belt, won't the metal belt knock components off as they are only held on by paste?
In the case that the side that sits on the metal is already assembled, when the solder melts, won't any components sitting on the belt get pushed out of place by the weight of the board?
CNC-machined carriers (pallets nb thanks to Aaron for the term) are generally used. The panel or board goes in the carrier and the carrier goes on the conveyor belt. They're made from a black high temperature composite of some kind.
Small parts stay on fine during a second run through due to surface tension. You can find a rule of thumb and references in this previous question and answer.
Similar carriers are used for single-sided assembly of flex circuits.
Glue is mostly used if wave soldering is required for THT parts.
Dots of glue are used where components may fall off. If components are on the bottom side of the board, then all are glued. Boards with bottom side components must be raised on stand-offs to provide clearance from the conveyor.
Our assembly house uses two different solders. First, they use a solder with a higher melt temperature, and solder the components on the top side. They flip the board, then use a solder with a lower melt temperature for components on the bottom side. I think the difference in the melt temperatures is only 20 °C, but don’t quote me on it. You need a reflow oven with good temperature control to do this.
I’ve also seen assembly houses that solder one side, and then glue components. For the assembly houses we deal with, this is much less common.
First, the boards don’t sit directly on the conveyor belt. They are in carriers, and so their surfaces make no direct contact with the belt or anything else.
Now, what holds the backside components in place? In a single-pass reflow, two different things:
After pick-place but before reflow: the flux in the solder paste.
During reflow: solder surface tension
The key to making this work is the ratio of the component weight vs. its soldering surface area. If there isn’t enough surface area the part will fall off. That’s why in some datasheets you see a weight specification for the packaged part: you need this information to validate that ratio.
Further, some packages (LCCs in particular) add extra soldering pad area, not just for better grounding and thermal, but also to meet this weight/area ratio for backside reflow.
Knowledgeable PCB layout designers will keep this in mind, and if possible place larger and heavier components on the topside, reserving the back for smaller (lighter) stuff that's less likely to fall off.
If you instead choose a 2-pass reflow, then there isn't an issue. First pass (usually top) uses a higher melt solder; second pass (usually backside) uses a lower melt. This requires careful process control at the reflow stage.
