Posted  by Jefriz: Tue Apr 25, 2006 7:11 pm    Post subject: Destructions of IR Reflow

Hello Everybody,
1) Can anybody give me more explanation why the IR Reflow increases or induce the die crack phenomenon? Also package crack?
2) What are the other phenomena that the IR Reflow induce?
3) Will convection Reflow will also induce the same phenomena? Other reflow types (if there are) will also do the same?
4) In the usual SHRT (Precon), 3x IR Reflow is commonly used. Why need to be 3X? Is there an industry standard that limits an IC to be processed at reflow(SMT, rework, SMT)?
5) Is the heat from oven will induce the same destructive strength as with Reflow?
6) Is 3X Reflow (IR) destructive to molded IC? Can this be used in production screening for Die/Package crack and still can be ship to customers (electrical pass)?


Thanks in advance for your time.

Jefriz (Pinoy in Taiwan) ~_~

Posted by FARel Engr: Wed Apr 26, 2006 10:15 pm    Post subject:

Hi Jefriz,

1) IR Reflow vaporizes whatever internal moisture the package has, so the water vapor exerts tremendous thermomechanical stresses on the die and package as it tries to escape;

2) Delamination is also a primary mechanism induced by IR reflow - delam can lead to corrosion, package cracking, neck breaks, and even bond lifting;

3) yes, as long as the correct temp profile is achieved;

4) this is to simulate the worst-case scenario wherein a device is subjected to the board mounting process 3 times during board assembly (either to multiple pass requirements or rework);

5) yes, as long as the correct temp profile is achieved; in reality, however, IR Reflow has a more stable and direct stressing effect on the package than, say, vapor reflow;

6) it is not recommended to use 3X IR reflow as a production screen; it definitely weakens the parts and can result in marginal parts reaching the customer; performing 3X IR Reflow on a good number of samples and assessing the main population's reliability based on this is a better alternative (wherein the stressed samples are just scrapped).

Posted by Jefriz: Thu Apr 27, 2006 6:05 pm    Post subject:

FARel Engr.,
Thanks for the fast and educative response.
I have some additional questions (if you may)
continue item #4 = if the package is subjected to more than 3 times reflow, will the number of defects increase?

New items:
7) Is there a maximum time gap in between reflow pass? What if the third pass was done 24 hours after the second, will the result be affected?
8) Is it needed to do moisture soak or prolong floor life exposure before doing the (sampling) Reflow screening (die/package crack suspected lot)?

Thanks again for your time.

Jefriz ~_~

Posted by FARel Engr: Fri May 05, 2006 10:54 pm    Post subject:

Jefriz

BTW, IR reflow is not a stand-alone screen - it is a part of the SHRT, which is the standard rel test. SHRT must always consist of a standard bake, a standard soak, and 3X shock (e.g., IR Reflow).

FARel Engr