McGizmo said:
Hi Newbie,
How are dice typically bonded to the slugs? If epoxy was used here, was it also electrically conductive epoxy and hence the anode connectivity? Would one or could one measure resistance between the anode lead tab and the slug to test connectivity of die to slug?
The dice are typically soldered to an ESD sub-carrier assy, like with the Luxeons and the CREE. The CREE SiC ESD diode is soldered to the substrate. I know this as fact, as I have actually unsoldered a die from an XR-E and re-soldered to a heat pipe.
I've also unsoldered the Luxeon die, shown here:
From the Luxeon ESD diode shown here:
The electrical resistance of even a few points of silver is going to be rather low, and if you look carefully at the Seoul P4, you will notice there is a bond wire that is opposite the three negative bond wires. The electrical resistance of this gold wire might mask anything that could be measured with a sensitive milliohm or micro-ohm meter.
What might work, is to watch the Vf shift from when the die is cold to when it gets hot. Hotter die will rise more than cooler die.
How one would assure long term reliability from some sort of test one could do might be a little tough. Here I'd be inclined to feel that you'd have to build 10,000 or so of them up, and run a test for a few years. The problem with this, is Seoul Semiconductor is constantly fiddling with their processes, so that would only tell you if that one batch held up over time. And you'd want to thermally cycle them at the same time, or at least turn them off until room temp, then back on, until the temperature stabilized.
One could also run the LEDs across several different current levels and capture the shift in tint, but since tint shift can be caused by other things, it isn't a definite thing. But, you might be able to weed out the ones that have obvious issues from the get go. The only reason this might work, is I'm seeing almost a break point where to color takes off blue, and that should be obvious.
McGizmo said:
This part was obvious from the get go to you as being suspect. Do you think other less obvious LED's will have premature failure due to an epoxy bond? Will this failure be due to thermal build up?
Yes, I've worked with Seoul's LEDs before, and have some experiences with their previous parts. Whether the other LEDs that looked better will have failures over time is pretty hard to say, but it is definitely something to suspect. One could do a few hundred or few thousand thermal cycles, or even a little bit of thermal shock (tough to know at what point you are exceeding specifications on weak items like the epoxy), and not exceeding the slower thermal pre-heat ramp they specify for soldering. This would be a better question for a materials scientist, imho.
This part here did not experience thermal build up, shock, or whatever. It was simply thermal epoxied, with more than adequate heatsinking. The very first time it was turned on, it showed a rapid blue shift at around 500mA. I've got other parts that shift above this, and one that holds on clear up to 1500mA (which is exceeding it's rating).
Now, if you are saying thermal build up in a product and causing pre-mature failure, that would be something I'd be thinking about, and addressing any way I could, as that should help lower the stresses that will be present inside the part.
McGizmo said:
Is epoxy not acceptable as an adhesive? I have had no trouble lifting LED's from stars or even from anodized sinks that have been epoxied as the mechanical bond to some of the smooth surfaces is just not that great. There appears to be delamination in this bad part. Was the delamination a result of some trama to the assembly after assembly or do you think it could have been a bond flaw at assembly time?
Not being a materials scientist, it is hard to say for sure. Surface prep is often of utmost importance when using many epoxies/RTV/silicones. What ever they are doing for their silicone bonding process is pretty decent, imho. When I've seen failures with bonds, on something that should handle the forces according to the datasheet on the substrates that were bonded together, it is often poor surface prep, substandard epoxy was supplied, or the designer forgot to consider something like the CTEs of the two materials joined, or in the case of two materials with differing CTEs- the designer didn't specify a minimum bond layer thickness of epoxy, the production process has poor control, or they are not doing it according to the specification. No idea if it is adequate or not, but I am seeing variations in these production parts I purchased from Mouser- and one that could cause what I am seeing is delamination, but the others could be due to something else. I've got one that has blue tint around it's four edges when driven at 700mA, and the one that runs up at 1500mA without excessive shift doesn't show this.
It would be hard to cause trama, it is recessed, but I do not know their process, so anything here would be pure speculation.
McGizmo said:
On this particular LED, what has its history been with your testing? Could it have suffered some thermal abuse prior to sufficient sinking? Was it soldered to a heat sink and if so, could the process of soldering it have had ill effect?
It was thermal epoxied, so no thermal shock or anything. It had great heatsinking from the get go. It showed the blue shift the first time I slowly ramped up the current, right around 500mA. The tape carrier packaging showed no signs of being crushed, bent, or other issues that might indicate something happened after they were packaged at Seoul.
McGizmo said:
In fairness to potential and unwitting users of these LED's should Seoul be contacted and told to cease production of the P4's until they get a better assembly program in place?
Humm- It really isn't my place to say what they should do. They may want to screen portions of production runs, and life screen, if they are not already. The screening may need to be more rigorous. I've seen a video of a Luxeon lookalike part that the only thing that was done was to test them at the binning current, and that is it, the binning was the only screening. Hard to say, but one might want to talk to the designer of this part himself, and not thru the sales/marketing force, and get his take on things, not some toned down marketing speak.
Better yet, if others find this issue in parts they have, contact their engineering, send them back to Seoul, and let them evaluate what is going on, since only they know what is actually used, the process, the intent, and such.
IMHO, the customers using these parts might want to consider doing some testing of their own (in any case), especially on something that is such cutting edge technology and new processes/materials used.
McGizmo said:
In assuming that your personal opinion on these LED's is not a case of over reaction, what do you suppose should be done?
Be cognizant of any failures or wierd behavior, even odd tint shifting, and if one finds them, to be dynamically proactive. Most especially for the consumer of the end product. If I was the consumer, I'd really want to consider the warranty offered on the light, and the past reputation for warranty handling of the company. IMHO, your reputation in this area is high shined Sterling Silver.
I've got more parts inbound from various sources, it will be very interesting to see what happens with those parts. I'm really hoping for the best, these Seoul parts only require heatsink shimming and reflector grinding to retrofit into existing Luxeon flashlights, so they could be useful for a lot of CPF'ers.
Right now, I'm not going to go out and buy a bunch to modify my own lights, but I may wait, and see what the long term experience is on these parts for others.
As I've been writing this, I've been considering making a circuit that would turn the parts on for a period of time, then turn them off for a period of time, to see how this Seoul P4 construction method holds up. One of the questions, is do I heatsink the part excessively, or do I heatsink it such that the die hits 125C, which is 20C lower than it's maximum specification. Or do both...which is where I am leaning....
It is possible, that somehow, I beat all the odds, and ended up with bum parts. It is really hard to say, without buying a few hundred parts from each of multiple runs, and doing a whole test program...
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