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June 2008

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Subject:
From:
Guy Ramsey <[log in to unmask]>
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Date:
Tue, 3 Jun 2008 09:24:05 -0400
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I once consulted on  problem like you describe. 

The company was using through hole LEDs for signs. The board design had been
unsupported single side. An EMS suggested that it would be better for them
to provide PTH to better support the leads. 

Shortly after they started using the PTH version of the board they began to
see LED failures. 
A lot of FA was done. The actual failure was internal to the LED. A wire
bond was being pulled inside the LED package. I don't recall if it was the
Cathode or Anode side. Not all but some were intermittent. 

We never identified the mechanism by which the damage was done but returning
to an unsupported design, one that prevented solder from touching the
package, eliminated the LED failures. We did some tests with the LED in a
PTH with a standoff. They did not fail. 


 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Tuesday, June 03, 2008 7:55 AM
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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