# Please! Need help with choosing the right 3528 SMD White LED

Z

#### z_bled

##### Guest
Dear friends,

I work for a company which wants to produce LED strips based on 3528 SMD white LED. We ordered samples from three different Asian producers. Two from China and one from Taiwan. Taiwan producer has factory in China, so we can say they are all from China. I enumerated them 1,2,3. 3 is a Taiwan-China producer.

My biggest problem is to choose the right LED. I don't have very much time – about 7 days to decide, so I want your advice and help with the answers on my questions.

By saying the "right LED" I mean such LED that will depreciate less than 70% in 40000 hours and won't change its tint significantly (at 20 mA, soldered to one.square inch of FR4-PCB copper, ambient-air temperature will be 25 deg).

Here is the data from datasheets and my observations about these LEDs and some photos.

Number 1:

1) Working current: 20 mA

2) Absolute Maximum current 50 mA

3) Lum. Intensity at 20 mA: 2600 mcd

4) Transparency of the lens: you can almost see the chip.

5) Rigidity of the lens: I have almost broken my syringe needle. It is very-very rigid.

6) The size of the chip: take a look at picture 1_c.jpg

7) The placement of the chip: not in the center – you can see it at 1_b.jpg.

8) The behavior of the LED at 60 mA after one hour: The color shifted towards bluish, it was very noticeable. After I cooled it down the color became white and then again bluish (after it reached some temperature).

Number 2:

1) Working current: 20 mA

2) Absolute Maximum current : 20 mA (Maybe here is the mistake in datasheet, also they say the maximum current can be 50 mA in pulse mode t/T=0.1)

3) Lum. Intensity at 20 mA: 2500 mcd

4) Transparency of the lens: you can not see the chip, the lens are uniformly yellow.

5) Rigidity of the lens: It is soft and elastic. You can take the whole lens (without crumbling the lens) out with syringe needle at once.

6) The size of the chip: take a look at picture 2_c.jpg – the size is the same as Number 1.

7) The placement of the chip: not in the center, but close to the center than Number 1 – you can see it at 2_a.jpg.

8) The behavior of the LED at 60 mA after one hour: The color looked the same (white) as it was at the beginning of the testing.

Number 3:

1) Working current: 20mA

2) Absolute Maximum current: 30 mA

3) Lum. Intensity at 20 mA: 2000 mcd (6lm).

4) Transparency of the lens: you can almost see the chip, but a bit less clearly than in Number 1.

5) Rigidity of the lens: It is softer than Number 2 – like jelly. But it is not elastic. When you want to extract the lens from the LED you will need three-four attempts, because it is a bit crumbly comparing to the Number 2, where the lens material holds like a uniform mass.

6) The size of the chip: take a look at picture 3_d.jpg – the width is the same as Number 1,2, but the length is 1/4'th shorter than Number 1,2.

7) The placement of the chip: almost in the center – you can see it at 3_b.jpg.

8) The behavior of the LED at 60 mA after one hour: The color looked the same (white) as it was at the beginning of the testing.

Ok, now I want to ask questions:

All three LEDs have almost the same price – about 0.05\$. Number two is the most expansive (0.057), others cost 0.045.

Question 1: What can the transparency of the lens tell us?

Question 2: What can the rigidity of the lens tell us?

Question 3: What the size of the chip can tell us?

Question 4: Can I throw away the Number 1 only because of its shift toward bluish (see p.1.8).?

Question 5: How else can I compare the quality of the LEDs if it is only 7 days left?

Question 6: I could not find very much information about SMD 3528 white LEDs depreciation rate and deprecation causes. Mostly what I can see is an information about 5mm and HighPower Leds. The reason for 5mm LEDs was in temperature and in a browning of a case material (epoxy). What are the main reasons of degradation for 3528 LEDs? Can I simply avoid them with, let's say, 15 mA drive current and one sq.inch of copper PCB layer under it.

Here is the photos:

http://bda.110mb.com/1_a.jpg

http://bda.110mb.com/1_b.jpg

http://bda.110mb.com/1_c.jpg

http://bda.110mb.com/2_a.jpg

http://bda.110mb.com/2_b.jpg

http://bda.110mb.com/2_c.jpg

http://bda.110mb.com/3_a.jpg

http://bda.110mb.com/3_b.jpg

http://bda.110mb.com/3_c.jpg

http://bda.110mb.com/3_d.jpg

Thank you.