Hi,

More puzzles :)

Ceramic chips will be hurt by mechanical stress which may be introduced during or after your assembly process. Supporting pin allocation,  fixture design mistakes, spelled board to single one, heatsink assembly, etc.



B.R.
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Li Yi
H3C R&D DFM Engineer
312 Room, Oriental Electronics Bldg., Shangdi Information
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Fax:8610 82774214
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----- Original Message -----
From: "Robert Kondner" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, July 04, 2010 12:48 AM
Subject: Re: [TN] Cracked 2208 smt resistor end joint


> Hi,
>
> I operated a wave machine for several years, when a PCB comes off it is
> very hot and very soft. Flexing while in this state could cause problems.
>
> Bob Kondner
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
> Sent: Friday, July 02, 2010 9:30 PM
> To: [log in to unmask]
> Subject: Re: [TN] Cracked 2208 smt resistor end joint
>
> Hi Eldon,
> I am surprised you ave not seen failures like these earlier.
>
> Also, did you perhaps switch to Pb-free solders?
>
>
>
>
> My present theory is the crack occurs when the assembly is wave soldered
> causing excessive tension on the resistor due to CTE mis-match, and some of
> these crack.
>
>
>
> Sorry, but this is wrong. On heating the PCB expands more than the CC-no
> cracking.
>
>
> But when the board comes back to ambient it seems to be
> longer, and shows a gap.
>
> On cooling, the PCB shrinks more, putting the CC, solder joints and all in
> tension-causes cracking and gaps.
>
>
> After a board goes through 3 heat cycles at a CM,
> wouldn't it be stable in CTE?
>
> No, there is no 'stabilization' of CTE's.
>
>
> And why would the other resistor not have the
> same issue?  Is CTE different in X and Y?
>
> There are differences in CTE in x and y-by sometimes more than 2 ppm/C. The
> loading is also different if running parallel or perpendicular to the wave
>
> Finally, would an error or change in
> the raw fab process cause a change in the CTE and cause this issue to occur
> after many years?
>
>
> Obviously, something has changed-you need to find out what.
>
> Werner
>
>
>
>
>
>
>
>
>
> -----Original Message-----
> From: Eldon Sanders <[log in to unmask]>
> To: [log in to unmask]
> Sent: Fri, Jul 2, 2010 7:31 pm
> Subject: [TN] Cracked 2208 smt resistor end joint
>
>
> We are experiencing fractures in the end metallization or solder joint on
> one of
>
>
>
> two larger high voltage resistors, size 2208.  They are mounted on FR-4.
>
> Only one location is failing.  The other location is an inch away and
> oriented
>
> at
>
> 90 degrees.  The failure rate is variable between assembly lots.  Some have
>
> no failures, and no observed cracks.  The most recent lot has a 2% failure
>
> rate, and another 2% that exhibit visible cracks in the ceramic but the
> solder
>
> joint is connecting the pad to the metallization and making electrical
>
> connection.  I believe the root cause is the poor choice of the FR-4 board
>
> material with such a large ceramic resistor.  However, about 80,000 boards
>
> have been built over 7 years without this failure mode.  So, I am searching
> for
>
> the root cause of this specific timeframe.  I have verified the process at
> the
>
> CM has not changed - the resistor is mounted on the 2nd pass SMT, and the
>
> board goes through wave soldering of TH components using a fixture.  The
>
> same date code resistor is used for both good assembly lots and bad.  There
>
> is some correlation to the date code of the bare fab.  On cracked boards
>
> there is a gap observable between the solder joint and the ceramic resistor
>
> body.
>
>
>
> My present theory is the crack occurs when the assembly is wave soldered
>
> causing excessive tension on the resistor due to CTE mis-match, and some of
>
> these crack.  But when the board comes back to ambient it seems to be
>
> longer, and shows a gap.  After a board goes through 3 heat cycles at a CM,
>
> wouldn't it be stable in CTE?  And why would the other resistor not have the
>
>
> same issue?  Is CTE different in X and Y?  Finally, would an error or change
> in
>
> the raw fab process cause a change in the CTE and cause this issue to occur
>
> after many years?
>
>
>
> Thanks to all the incredible minds if TechNet.
>
>
>
> Eldon Sanders
>
> Sr. Manufacturing Engineer
>
> Welch Allyn Monitoring
>
> 8500 SW Creekside Place
>
> Beaverton, OR 97008-7107
>
> phone: (503) 530-7596         fax: (503) 526-4300
>
> [log in to unmask]
>
>
>
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>
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