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August 2001

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Subject:
From:
Grant Emandien <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 30 Aug 2001 18:03:16 +0200
Content-Type:
text/plain
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text/plain (209 lines)
Oops, forgot to mention the (partial) solution.

As a CM, the problem was manifest in 2 areas:

1. PCB layout - some components resided on pads of non-uniform size i.e. one
pad was connected to the groundplane (causing thermal imbalances from pad to
pad). As the customer was reluctant to change the artwork, we offset the
placement slightly (for those persistant ones always moving in the same
direction(but ensuring solder contact). For reasonably thermally balanced
pads, I can only presume (all things being equal) that the one end of the
component may not be in contact (or insufficient contact) with the paste to
begin with to allow the self-centering force of the solder to do perform its
function.

2. Screen aperture/PCB pad alignment to together with placement accuracy was
improved. Are the solder bricks relatively uniform in volume?

The problem was vastly reduced, rather than eliminated due to obvious PCB
constraints.

BTW, has there been any change in type/ manufacturer of the no-clean being
used? PCB supplier?

Grant

-----Original Message-----
From: Tempea, Ioan [mailto:[log in to unmask]]
Sent: Thursday, August 30, 2001 5:17 PM
To: [log in to unmask]
Subject: Re: [TN] Component skew after reflow


Thanks Grant,

it's also with 0805s that I'm fighting.

What did you do about it?

Ioan

> -----Original Message-----
> From: Grant Emandien [SMTP:[log in to unmask]]
> Sent: Thursday, August 30, 2001 11:02 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Component skew after reflow
>
> Ioan,
>
> You're explaining a similar problem (on 0805's) we had recently (also on
> ENIG boards) - what we noticed was that the applied paste at times was
> offset to one end and further exacerbated by the component being placed
> off-centre to the other end, hence the apparant randomness. Inspection
> prior
> to reflow indicated fractional or no contact between the solder and the
> end.
> This condition resulted in 2 types of defects after reflow i.e. not
> soldered
> at one end (as you mentioned), as well as the more obvious tombstones when
> the tension really gets angry.
>
> Regards
> Grant
>
> -----Original Message-----
> From: Tempea, Ioan [mailto:[log in to unmask]]
> Sent: Thursday, August 30, 2001 4:51 PM
> To: [log in to unmask]
> Subject: [TN] Component skew after reflow
>
>
> Hi Technos,
>
> I have an intriguing one.
>
> I find passive components that are not soldered at one end after reflow.
> These components show very good joints at one extremity while the other
> end
> is not soldered at all. The not soldered end is usually outside of the
> pad,
> at the mercy of the surface tension, but sometimes it touches the
> respective
> pad, being untouched by the solder, that forms nice bumps on the pad.
>
> We're on no-clean paste, the board finish is ENIG. The assembly has being
> running for a long time, we built tenths of thousands without this
> problem.
> But all of a sudden... The problem appears randomly, not always on the
> same
> component, not always at the same place. The frequency is let's say one
> occurence every 50 boards. The assembly is not so populated, barely about
> 50
> parts.
>
> I checked the insertion, it is good. I even offset manually some parts
> before the reflow, but got good soldering on them. The reflow profile is
> the
> same, I even ran the profiler. The paste is OK, good date code, same
> printing conditions, paste height OK. The land patterns are adequate, they
> haven't changed.
>
> First, my instinct told me it's the parts, maybe they're too old. I
> checked
> the reels and the manufacturer's date codes (not the broker's) are between
> mid 2000 until june 2001. Not that old, are they?
>
> What could it be? The board? What and how to check, since the pads are
> very
> well wet? The parts? Anything else?
>
> Thanks for any input,
> Ioan
>
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