Consequences of not following reflow profile

Question

I found many questions about how to perform reflow soldering, but my main concern is

1. What are the consequences of not following the recommended reflow profile.
2. Would there be any risk going the approach I plan to take.

So, I have a board about 9 inch^2 and a hot air gun and Kester solder paste. When doing my first assembly I noticed that if I got the lead partially melted and then slightly cooled down, it is more difficult to re-melt it again. Also, it takes a long time to heat the board with the hot air gun. I did not want to use oven, being afraid that I might overheat the components, whereas using a hot air gun would let me control local heating just to the melting temperature, simply by observing it. What I plan to do is to get a cheap oven at the store and preheat the board to say 180-200C. Then, use the hot air gun to finish the job.

I realize that the peak temperature must not exceed the maximum allowable temperature for components. I can easily ensure that. But how about the profile curve? The heating rates?

Regardless, whether using only the air gun, only the oven, or the combination of the two, I will definitely not be able to perform the task within 250 seconds as suggested by the profile. Neither, I will be able staying close to the suggested profile. So, again, what are my risks on not following the profile and is it worth combining the two techniques, or use exclusively one?

Answer 1

If you don't follow the reflow profile properly, there are a few things that can happen

1. If your board contains large copper pours, and are not warmed up properly, during reflow, your components can tombstone which is when a resistor or something will literally stand up and then is soldered in place like that. This can be a pain to fix depending on where the component is.
2. If you heat up too quickly, you can thermally shock your components that are already placed on the board. While there have been measures taken to reduce this during the design of chips, it is still a problem if procedures aren't followed properly. Thermally shocked components may not function as expected until they are brought to regular conditions although sometimes they will flat-out break.
3. Another more obvious problem is that some of your components might not be soldered. Its true that solder paste is metallic although solder paste alone is not a great connection. If you don't reflow the board properly, some components might just not get soldered and will eventually fall off or cause other problems.

With those said, if your board is not the most dense board, like it doesn't have a bunch of decoupling capacitors underneath a processor, I wouldn't worry too much about it. You can easily observe whether something is soldered. It would still be a good idea to get a preheater since the copper pour issue is kind of annoying (You can literally watch it happen while using hot-air rework)

Since you do have a double sided board, then you might want to consider getting thermal adhesive. It is like a little red glue that you put on the components that you reflow first on to the board so that when you flip it over to reflow the other side, the parts don't fall off.

Answer 2

In addition to Shannon's answer, not properly reflowing the solder can prevent the proper creation of intermetallics in the solder joint. This can cause reliability issues later down the line (intermittent connection breaks, parts possibly coming off the board).