The reflow that works for a week

2026-06-28 · Assorted boards, mostly BGA-adjacent

Every few months someone brings me a board that "was fixed by baking it in the oven and then broke again". The pattern is real and the explanation is boring.

what actually happens

Heating the assembly softens a cracked or fatigued solder joint enough to make momentary contact. It does not reflow properly — oven temperature is uneven, there is no flux where it is needed, and the joint re-solidifies with the same crack plus new oxide. It works, then thermal cycling reopens it. A week is typical. I have seen three months.

what it tells you

A successful bake is diagnostic information, not a repair: it says the fault is mechanical, at a joint, and temperature-sensitive. That narrows the search enormously. It does not mean the part is fine.

doing it properly

flux under the part (no-clean, tacky)
preheat bottom .............. 130-150 C, 3-4 min
top air ..................... 230-245 C for leaded, 250-260 for lead-free
soak ........................ until the part visibly settles
cool ........................ slowly, no fan, no moving the board

If the part is genuinely cracked internally — common with older GPUs — none of this helps and the honest answer to the customer is that the board is scrap.