Hi Bob,
Cleaning of the PCBAs is not an optimal solution because its better to
leave the flux on board to hold the balls rather than cleaning the flux and
having PCBAs with loos balls.
Dry brush with antistatic brush is preferred option if you need to remove
balls.
But first you have to check the process / solder paste in order to overcome
this failure.
The criteria of IPC is good enough to approve or not the workmanship.
Reuven
On Thu, Dec 1, 2011 at 5:38 AM, Forrester, Michael (H USA) <
[log in to unmask]> wrote:
> I have a PCA 6"X4" with a 144 pin LQFP on it. The lead spacing is 0.5
> mm which leaves about 0.2 mm space in-between the pins. The board goes
> through SMT reflow and a selective wave. We are seeing various degrees
> of solder balls/spheres. The balls we see are basically two sizes
> 40-45um and 70-80um. The solder paste is a size mesh 3, which would
> explain the smaller balls. At this time I am not sure where the larger
> ones are coming from? We see around the LQFP any where from 2-5 balls
> and literally hundreds of balls. The solder paste is a no-clean and
> then the board is cleaned. The reason for using no-clean is because of
> the process wait time from reflow to cleaning. The vendor does not want
> to leave an aqueous flux on the board for an extended period of time.
>
> My basic question is: Is there an IPC specification(or general rule of
> thumb) on the acceptability of solder balls? I understand that the
> presence of solder balls is a process indicator and that IPC-610 states:
>
> Solder balls are not entrapped, encapsulated or attached or can become
> dislodged in the normal service environment.
>
> Does this really mean that if I find a single ball that is not
> entrapped/attached, this is a reason for rejection? Is this practicable
> or is there a spec./industry standard that says something like if you
> have X balls smaller than X in an area X, this is acceptable? In the
> real world, are companies holding the zero tolerance on solder balls?
> Thank you in advance.
>
> Best Regards,
>
> Michael Forrester
> Sr. Product Engineer
> CSMTPE
>
> Siemens Healthcare Diagnostics
> 101 Silvermine Rd.
> Brookfield, CT 06804
> PH: (203) 740-6452
>
>
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