[SI-LIST] Re: SDRAM Blowup!

["Andrew Seddon" <andrew.seddon@xxxxxxxxxxxx>]

Robert,

You are well and truly on my Christmas card list! I've never heard of this
type of problem before, but it looks to be exactly right. The problem is on
a through hole connector fitted by hand after reflow, presumably the CEM
used flux and didn't clean up properly, I'm going to have a shout at them
tomorrow!

Doug,

Thanks for the tip's, I did manage to get hold of a high speed DSO and the
signal looks OK, not perfect but inside acceptable boundary's so luckily
it's not a SI issue after all.


P.S I'm serious Robert! send me your address off list as you've saved me a
whole load of hassle!


Cheers, 



-----Original Message-----
From: Robert Clarkson [mailto:clarkson@xxxxxxxx] 
Sent: 02 December 2004 15:55
To: andrew.seddon@xxxxxxxxxxxx
Subject: Re: [SI-LIST] SDRAM Blowup!

Andrew,

I don't know how long a "few hours" is, but I've experienced similar
problems after a week of operation on a large batch of boards.   Problem
turned out to be dendrites, or metal migration.  The board assembly shop
left a residue of flux under a BGA which provided the path for a very small
current.  The lead from the solder joints migrated though the contaminant
and formed thin threads of metal through the flux and shorted out various
signals.   Removing the "bad" part fixed both the board and the part.   You
can't see the dendrites, but you can measure them.   You can test for this
by physically cleaning between every place the two shorted signals come
close to one another and measuring resistance again.   It is worth taking a
look at.

Regards,

Robert Clarkson


----- Original Message ----- 
From: "Andrew Seddon" <andrew.seddon@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, November 30, 2004 12:08 PM
Subject: [SI-LIST] SDRAM Blowup!


> Ok, slight exageration, but an interesting problem non the less!
>
> I have a board with a uP connected to a 133Mhz SDRAM and a connector. The
> connector brings out the address/data bus which is then passed on to
another
> board. Total trace lengths across both boards for the address/data bus
> (including connector) are about 4 inches. The connector is a simple 1.27mm
> spacing type. All specific SDRAM signals stay on the first board(CLK,
> CAS,RAS etc). Both boards are 50ohm controlled impedance but no traces are
> terminated and the connectors could have any impedance??
>
> Now we've had a few back that have been working nicely with no SI problems
> what so ever. However the latest lot are showing a really weird problem.
> After running fine reading from SDRAM for a couple of hours data lines
> become shorted together (about 7 ohms, eg D7+D9 etc) removing the SDRAM
> cures the short. However there is no short on the bare removed SDRAM.
> Secondly forceably driving the data lines to different values takes the
> short resistance upto about 200ohms, and the SDRAM then works fine for
> another couple of hours.
>
> This problem occurs even at low 50mhz bus freq's.
>
> So my question is, has anybody ever seen anything like this before/have
any
> suggestions? I'm trying to get hold of a high speed scope at the minute to
> see what's going on but any tips would be really usefull. My current
theory
> is some sort of ringing effect that is overdriving the pins and causing an
> internal brake-down in the SDRAM's output drivers.
>
> The confusing thing is another batch of supposidly identical boards runs
> fine for day's on end.
>
> Cheers guys, any help much appreciated!
>
>
>
>
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Other related posts:

[SI-LIST] SDRAM Blowup!

[SI-LIST] Re: SDRAM Blowup!

[SI-LIST] Re: SDRAM Blowup!