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I am new to pcb reflowing.
I borrowed a friend's DIY oven (which is known to work) and I'm about to try loading my first set of parts on a 2 layer (fairly simple) pcb.
I'd like to try this out in stages and see how well it goes. but I'm not sure how bad it would be if I tried to do 2 or even 3 passes on the board. what I'm thinking of is to solder all the simple parts (caps and resistors) first and see how well that goes. then, try for the smd chips. leave off the expensive smd chips and test the board to see if I can load firmware to it (arduino based using smd atmega 328). if it takes firmware ok, then I can finish the board with the final baking of the more expensive parts.
obviously I will not solder electrolytics and plastic .1" header pins and any other plastic parts (I wonder, how tolerant are smd leds to reflowing?).
can you guys advise me on this? once I get the hang of it and gain confidence, I will likely load the board fully and do the baking just once, but I'd like to take it in stages and if I get it wrong on the first try, I don't want to waste good chips and have to throw the whole thing away, etc.
advice, please?
- Comments(1)
A****min
Dec 12.2019, 18:53:13
Yes, multiple reflow passes even in professionally made ovens with the recommended reflow profiles can harm both the PCB as well as the components.
Read the data sheets, usually in the reflow process related instructions section or document they specify that there shall be a certain maximum peak temperature of X +/- Y degrees C and a certain time limit in that peak temperature range.
For instance in a Cypress data sheet:
Table 13-2. Solder Reflow Peak Temperature
Package, Maximum Peak, Maximum Time at Temperature Peak Temperature
68-pin QFN, 260 °C, 30 seconds
The PCB materials rated for lead free processes also will typically indicate that they're specified for a certain number of reflow passes and certain peak temperatures and times at those temperature ranges. Usually PCB materials made for lead free processes are rated for several reflow cycles.
Components also will usually be specified to be able to withstand three or so reflow cycles but some more fragile components like modules or some BGA or analog type devices may be rated for only one reflow or maybe a couple.
Two soldering passes used to be more common since sometimes THT parts would be wave soldered and then separately SMD parts would be placed and reflowed so there would be some double thermal stress on the parts. And if rework is to be done that is another common case where a second reflow might be done.
PCB materials expand and become more glassy with high temperatures so that is the degradation mechanism. You also run the risk of delamination or soldering problems due to absorbed moisture if you don't bake / preheat the boards properly prior to reflow.
Failure to heat and cool at a fast enough rate to match the recommended reflow profile will also thermally stress parts since you're likely exposing them to temperatures beyond their maximum storage temperature for too long even if it is not fully in the reflow zone.
But the time spent in the reflow zone is the most critical, as well as the peak temperature so you will want to try not to have some areas exceed the peak permitted reflow temperature usually around 260C due to radiative heating or uneven heating in any form.
Sensitive analog parts like precision voltage references usually have shifts of performance that show measurable degradation of performance after every reflow.
