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June 2008

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Subject:
From:
Rex Waygood <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Rex Waygood <[log in to unmask]>
Date:
Tue, 3 Jun 2008 13:16:20 +0100
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text/plain (96 lines)
My worry would be the LED.
Although you say they work when removed, do they continue to work
through mechanical or thermal stress? (hot gas pencil, freezer aerosol?)
Did the tight clinch make the LED wire bond to the lead frame
intermittent?


Rex Waygood
Technical Manager
 
PartnerTech Poole Ltd 
Benson Road
Poole
Dorset BH17 0RY
United Kingdom
 
Tel: +44 (0)1202 674333
Fax: +44 (0)1202 678028
DDI: +44 (0)1202 338222
Mob: +44 (0)7887 997403
 
[log in to unmask]
www.PartnerTech.co.uk

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: 03 June 2008 12:55
To: [log in to unmask]
Subject: [TN] Intermitent contact on LEDs

Dear Technos,

 

I am investigating some returns from the field and can't really figure
out what the problem is.

 

On boards having 200+ TH LEDs we see 4 to 10 malfunctioning ones, but if
pressure is applied on the component it starts functioning. Under my
regular microscope I cannot see any problem with the joints and when
removed, the LEDs work properly. One unpleasant thing though is that the
LEDs were machine inserted and clinched to the board pretty tightly, so
the solder did not wick up on the leads too much, but if the joint was
somehow fractured, applying pressure on the component would only widen
the gap, since the bulk of the joint is on the opposite side of the
pressure point, am I right?

 

The PCB is a regular 0.062" thick 2 layer board, with the regular 1oz of
Cu on each layer, of pretty impressive dimensions, 22"x10". There is no
pattern in the disposition of the malfunctioning LEDs.

 

Besides cracked PTH, which we will investigate since I am preparing to
send out some boards for cross-sectioning, can it be anything else?

 

Thanks,

 

Ioan


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