[hpsdr] OZY reflow blows up the board

[Graham Haddock]

Horst:

In addition to an improperly cured PCB, and/or a board that contained
a lot of moisture, as the others have already discussed, I suspect that
you are using a oven where the thermal heating coils are directly
exposed to the PCB.

This adds another variable of infrared absorption to the surface heating
of the PCB.

In other words, if the infrared absorption of the particular materials and
dyes used in this board were higher than average, the surface temperatures
soldering this board could have been much higher than your other boards,
and all the oven settings were still the same.  Your oven probably
measures air temperature, not the actual surface temperature of the board.

I suspect that the solder mask and possibly the screen printed legend
absorbed a lot more infrared energy than the boards you normally
solder, and contributed to much higher local temperatures than normal. 
If the main heat transfer method is hot air, and the PCB is blocked from
"seeing" the heating coils then the surface temperatures would be a lot
more uniform.

It could have still been an improperly cured PCB and/or excess
moisture in the PCB, also.

--- Graham / KE9H

==

H. Gruchow wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi all,
> thanks very much for your input regarding the reflow failure of OZY.
>
> One thing is for sure: I can trust my reflow oven. OZY was the 32nd board which I treated 
> in the furnace. Mostly I use it for de-soldering boards in order to get the parts. But 
> also soldering never failed so far.
>
> The temperature profile I used for OZY was:
>
> 1. Preheat: 1°C/s from room temp to 110 °C. Usually takes about 2 min.
> 2. Soak: 90s from 110°C to 150°C
> 3. Dwell: from 150°C to 180°C
> 4. Reflow: from 180°C to 220°C time 60s
> 5. Cooling in the oven with door open from 220°C down to 45°C before removing the board 
> from the oven.
> The max temp my oven can reach is about 270°C because above that it is switched of by a 
> security thermo switch.
> This all is controlled by a thermocouple and microcontroller.
>
> I do not know what happened here. I have blisters all over the OZY board. My guess is 
> moisture. But that does not explain the heavy de-coloration of the silk screen print into 
> almost brownish color.
>
> One thing is for sure: the other boards will be hand-soldered. I will not take another 
> risk of loosing a board.
> John, W9DDD, is there a spare OZY which I could order?
>
> 73
> Horst
> DL6KBF
>
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