Features of led lamps The features of LED lamps
become clear by comparison with tungsten filament incandescent
lamps and discharge tubes in their light emitting mechanisms and
structures.
The following are some qualities of LED lamps:
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1. Long lifetime - The light emitting phenomenon
makes use of the injection light emitted to the P-N junction
instead of thermal radiation, therefore, LEDs are free of
waste and wear and they can be expected to have a long life. |
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2. Excellent drive characteristics - The
LED response time is very fast (a few hundred nanoseconds)
and the forward voltage and current at the practical luminous
intensity levels are very low (i.e. 2V=10mA), which makes
it simpler to design the drive circuits. |
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3. Sturdy mechanical strength - The packages
of LEDs are made of resin, so they have excellent mechanical
strength and can withstand vibration, shock and other abuses. |
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4. Structure - Figure 3-1-3 shows
an example of a LED lamp structure. The main body
that radiates light is the LED chip located at the
center. It is mounted to the top of the cathode lead
frame with solder or conductive paste to apply voltage.
(For GaAlAs and InGaAIP, it is mounted to the top
of the anode lead frame.)
A fine Au wire in diameter of 25 to 30 µm is routed
between the LED chip and anode lead and bonded to
each with a hot press-fitting bonder. Further more,
the LED chip is molded into a transparent plastic
lens to pick up light efficiently. LED lamps of different
appearances can be produced depending on the shape
and material of this lens.
|
|
| Parameter |
Performance |
| Light transmittivity
(Visible ray area) |
80% - 90% |
| Glass transition temperature
(Tg) |
Aprrox. 140º |
| Coefficient of linear
expansion |
Approx 7 X 10
-5 / ºC |
| Elastic modulus of
bend |
Approx. 300kg
weight/mm2 |
| Moisture absorbtion
at boiling (24 hours) |
0.1% |
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|
LED lamps come in many varied profiles including one which
has had its directivity changed by mixing epoxy resin with
light-scattering agent and one which has had its emission
wavelength and on-time/off-time contrast improved by using
dye. Figure 3-1-5 compares directivity characteristics.
Figures 3-1-4 depicts the typical lead frame of an LED
lamp. The Die Attach Post in this diagram corresponds to
the cathode lead in Figure 3-1-3. (This is reversed for
GaAlAs and InGaAIP LED chips.)
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5. Sealing resin - LED lamp packages are
formed with a lead frame on which the LED chip is mounted
and a sealing resin with the lens part (normally transparent
epoxy resin). Table 3-1-2 lists the properties of the typical
epoxy resin employed in high-bright LED chips for outdoor
use.
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Precautions when using leds
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1. Maximum ratings - Maximum ratings refer
to one that cannot be exceeded in any instance under designated
conditions. No product guarantees that two or more parameters
of maximum ratings can be met simultaneously. As a supplement
to maximum ratings, some products list ambient temperature
vs. allowable forward current (or power dissipation) characteristics,
as exemplified in figure 3-2-6. Table 3-1-3 lists an example
of maximum ratings specification. |
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| Figure 3-2-6 |
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|
| Characteristic |
Symbol |
Rating |
Unit |
| Forward current |
IF |
50 |
mA |
| Reverse voltage |
VR |
4 |
V |
| Power dissapation |
PD |
125 |
mW |
| Operating temperature |
TOPR |
-30 ~ 85 |
ºC |
| Storage temperature |
Tmg |
-40 ~ 120 |
ºC |
Table 3-1-3
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|
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2.Soldering - The softening temperature
of the resin of which the LED's packages are made is generally
low; less than about 100¡ãC. The following table lists
the different soldering methods and conditions: |
| |
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| 3. Soldering work conditions - Unless
otherwise specified in the technical documentation,
perform soldering work under the following conditions.
Soldering Temperature: 260¡ã
C or less (when solder dipping) 300¡ã C or
less (when hand soldering) (note)
Working Time: Within 3 seconds
Place to solder: 2mm or more from
the root of the terminal
Note: When using a soldering iron, be sure to use
a soldering iron with capacity of 30W or less and
adjust the supply voltage so that the iron tip temperature
is 300¡ã C or less.
|
|
| Soldering methods |
Conditions |
| Solder iron |
300ºC ¡À5ºC,
within 3 seconds (1.6mm from epoxy body) |
| Solder bath |
260ºC ¡À5ºC,
within 5 seconds (1.6mm from epoxy body) |
| Reflow |
Preheating |
75ºC, within
30 seconds |
| Soldering |
Soldering |
24.5ºC, within 5 seconds
(1.6mm from expoy body) |
|
|
|
As long as soldering work is performed under these conditions,
most problems such as reduction in light amount, opening
or shorting, or mold breakage due to soldering heat can
all be prevented.
If one or more of the above conditions cannot be followed
for reason of available space or relationships with other
components, take caution not to apply stress to the lead
wires during solder dipping work and prevent increases in
temperature from being conveyed to the device ( in places
above the root of the lead).
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4. Surface mount soldering
Reflow Soldering: • It is recommended
to use a reflow furnace with an upper and lower heater.
• The temperature profile as shown in Figure 1 is recommended
for soldering LEDs by the reflow process. • Reflow is
permitted just one time.
Post Solder Cleaning: When cleaning after
soldering, the following conditions must be met and adhered
to. • Cleaning solvents: AK225 or Alcohol. • Temperature:
50¡ã C (122¡ã F) max. for 30 seconds or 30¡ã
C (86¡ã F) max. for 3 minutes max. • Ultrasonic:
300W max.
Precautions for Mounting: • Do Not
apply force to plastic part of LED when LED is under high
temperature. • Avoid contact friction between LED and
other components during the assembly process as this may
damage the plastic portion of the LED.
Recommended Soldering Patterns: TLx1005 Series TLx1002
Series MTSMx35K-xx Series
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5. Washing
When the devices are washed in chemicals for removal of
flux after soldering, the use of unsuitable chemicals may
result in a change in quality and color, and even cracks
in the packages. The recommended chemicals for washing Marktech
visible LED lamps are: Chlorothene, Freon TE or TF, Dai-Fron
Solvent S3 or S3-E. For washing LED Luminators (MTBLx41x-XX),
use water only.
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When the devices are ultrasonically washed in these chemicals,
use an ultrasonic washing machine that has an output power of
less than 300W; do not resonate the devices attached to the board.
It is also strongly recommended that the printed circuit board
does not touch the oscillator and the devices are washed in less
than 30 seconds.
LED LAMP APPLICATION CIRCUITS
Since the optical output of a light-emitting diode depends on
the LED forward current IF, you can easily implement a circuit
to turn the optical output on and off by controlling the forward
current. The following describes the typical methods of lighting
- DC lighting, pulse lighting, and AC lighting - and some precautions
to be observed when designing.
DC LIGHTING
Figure 3-1-7 depicts a basic circuit to light the LED by using
a DC power supply. In this case, IF is expressed by the equation:
IF = (VCC - VF ) / R where
VCC = supply voltage
VF = forward voltage of LED
IF = forward current flowing in LED
Figure 3-1-8 shows a circuit where variations in VF of an LED
is compensated for by a transistor. In this case, IF is expressed
by the equation:
IF = (VB - VBE) / R3 where
VB = base voltage
VBE = voltage between base and emitter
R3 = emitter resistance
With this circuit, the temperature dependency of the optical
output can be reduced by setting VBE and VB appropriately.
 |
| Figure 3-1-7 |
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|
 |
| Figure 3-1-8 |
|
If the radiant power output is insufficient, the problem can
be solved by connecting a diode in series or parallel to the other.
In this case, IF is expressed by the equations:
IF = (VCC - NVF) / R (series connection)
IF = (VCC - VF) / R (parallel connection)
AC LIGHTING
Figure 3-1-10 depicts a basic circuit to light the LED to approximately
a half-wave by using an AC power supply.
Generally, there are two drive methods, (a) and (b). In either
case, a protective diode is used to prevent the LED from being
subjected to a voltage greater than its reverse withstand voltage.
For (a), this protective diode must have a reverse voltage that
corresponds to the supply voltage, VCC. For (b), the protective
diode must have a reverse withstand voltage of approximately twice
the forward voltage of the LED.
 |
| Figure 3-1-9 Increasing
radiant power |
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|
 |
| Figure 3-1-10 AC
lighting circuit |
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Here, the circuit constant. R, must be one that has the appropriate
rated voltage according to the supply voltage VCC. Also, R is
determined so that the forward current of the LED IF is held to
within the rated value at a point where the supply voltage VCC
is maximum.
PULSE LIGHTING
Pulse drive method: This pulse drive method
is designated by using a TTL gate or a combination of CMOS and
transistors.
| The advantage of converting optical signals into pulse-modulated
light by pulse lighting is that if the device is powered by
a battery, the useful life of the battery is extended since
the device's power consumption can be reduced.
Figure 3-1-11 calls for attention to the IOL electrical
characteristics of TTL and CMOS. For IF < IOL to be met,
these circuits do not allow large current to flow.
To increase the drive current, it is necessary to use a
buffer IC with a greater output current capacity or connect
an external transistor as shown in Figure 3-1-12.
|
|
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| Figure 3-1-11 IC-based
lighting circuit |
|
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| Figure 3-1-12 Lighting
circuit using IC & buffer transistor |
|
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The following lists the IOL and VOL characteristics of
typical TTL, CMOS, and buffer ICs for your reference.
Other pulse lighting circuits: Various
pulse generator circuits may be considered as the pulse
lighting circuits. In most cases, however, the pulse lighting
circuits can be configured easily by using multivibrators
or UJTs.
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Pulse allowable forward current of led lamps
| The pulse allowable forward current IFP when an LED lamp
is driven by applying a periodic square pulse like the one
shown below is listed in Table 3-1-4.
Note, however, that the maximum allowable value must be
consulted to obtain the correct value.
• Applied Pulse
IFP = pulse allowable forward current
PW = pulse width
T = repetition period
DR = duty ratio (PW / T)
|
|
| Classification |
VOL |
IOL |
Product name |
| TTL |
0.4V |
16mA |
SN7xx Series |
| CMOS |
0.4V |
3.2mA VDO = 5V
Ta = 25ºC |
TC4009BP, TC4010BP,
TC4049BP, TC4050BP |
| Buffer IC |
1.3V Ta = 25ºC |
200mA |
TD62000P, TD62003P, TD62004P |
|
If the ambient temperature (Ta) exceeds 25¡ã C, derating
of IFP for Ta (shown in Figure 3-1-13) is required.
|
PULSE DATA
Pulse Driving - With the exception of GaP red, optical characteristics
in the high-power zone are excellent, permitting effective
pulse driving. Since permissible pulse forward current varies
depending on driving conditions, refer to the characteristic
diagrams as follows. Also, during DC driving, derating is
similarly required against ambient temperature.
Mounting precautions
1. Precautions on Mounting - Mounting on printed circuit
boards (PC boards): Printed circuit boards are the typical
method of mounting used for optical semiconductor devices.
The following describes the recommended methods for mounting
each device classified by type and several precautions to
be observed when mounting these devices.
2. Precautions to be taken for specific type
For plastic type: When mounting this type of device on
a 2.54mm-pitch standard PC board, solder the device to the
PC board 2 mm or more apart from the root.
|
 |
| |
| Type |
DC forward current
IF max. (mA) |
Pulse allowable forward
current IFP max (mA)* |
Fig. No. |
| GaP (red) |
25 |
100 |
1 |
| 25 |
2 |
| GaP(Green) |
40 |
160 |
3 |
| GaAIAs (red) InGaAIP (orange,
yellow) |
50 |
200 |
4 |
| Types other
than above |
25 |
120 |
5 |
| 30 |
6 |
| |
|
*PW = 100µs, DR = 10-1
Also, take care not to forcibly press the device against the
board. Even when mounting on a pitch-compatible board, solder
the device 1 mm or more apart. In the case of through-hole boards,
however, the device must be fitted 2 mm or more above the board
surface.
For subminiature axial type: Unlike the standard stand-alone
type, there are two methods of mounting for the double-end subminiature
type: a method of mounting from the surface of the circuit board
( Figure 3-1-14) and a method of mounting from the reverse side
(Figure 3-1-15).
 |
| Figure 3.1-14 |
|
|
In the former case, a good method of mounting is if possible,
open a hole in the PC board to fit the plastic bottom part
and position the device there, so a deviation in the optical
axis can be minimized. However, there is a precaution to
be observed. Never solder the device after forcibly pushing
it into the hole or while being subjected to mechanical
stress (Figure 3-1-16).
|
Also note that this type of device is very small and therefore
has its resin temperature rapidly increased when soldered. To
prevent this problem, nip the lead wire with nippers or tweezers
to radiate heat during soldering work.
 |
| Figure 3.1-15 |
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|
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| Figure 3.1-16 |
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3. Handling Precautions
Wear Resistance: Molded devices use a plastic of relatively low
hardness since they require a clarity of lens. Therefore, friction
with metal or your finger nail must be avoided.
Heat Resistance: The plastic section may be
discolored if subjected to heat for a long time. Therefore, make
sure that the device is not exposed to temperature environments
where their storage temperature is higher than the rated value.
Mechanical Stress in lead wire: If the lead wire is soldered
while being subjected to stress, or tensile, torsional, or compression
stress is applied between lead wires while hot immediately after
soldering, open circuits may be generated inside the device. Therefore,
be sure to correct the position and direction as necessary, after
sufficiently cooling.
|
About lead forming:
|
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Stand alone type: While holding the
lead near the root with nippers do not put stress on
thead root, bend the lead at its constricted part or
a position 2 mm or more apart from the root. |
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Double-end type: Bend the lead at
its constricted part or a position 2 mm or more apart
from the root. |
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Reliability
It is the purpose of this section to introduce the periodic reliability
reports issued by Marktech on our optoelectronic products.
As new products, processes and test procedures evolve, the applicability
of past data to reliability changes.
Thus, data presented here represents a "snapshot in time" of
data believed applicable to the product made now and in the immediate
anticipated future.
1. Led lamps and led displays
Reliability Tests:
Reliability Tests are conducted to confirm the design margin
and limit levels of devices, or to maintain and confirm the quality
assurance levels of mass produced devices.
Though the test methods and test conditions depend on the purpose
usually, the electrical stress, thermal stress and mechanical
stress during the use of the devices are assumed and their withstand
levels are estimated.
Table 3-1 shows the reliability test method of LED lamps and
displays.
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| |
Test |
Test conditions |
MIL-STD-750 Reference |
Life
test |
Operating life |
Ta = 25ºC
IF Max. rating |
1026.3 |
| High temperature storage |
Ta = Tstg., Max. |
1026.3 |
| Low temperature storage |
Ta = Tstg., Min. |
- |
| High temperature and high humidity
storage |
Ta = 60ºC or 40ºC, R. H.
= 90º |
- |
Enviromental
tests |
Soldering heat |
Immersed for 10 sec. at 260º
up to 2mm from the body |
2031.1 |
| Temperature cycling |
Tstg. Min. ~25ºC~
Tstg. Max. ~25ºC (30 min.) (5 min.) (30 min.)
(5 min.)
 |
1051.1 |
| Thermal shock (except for non-sealing
type) |
100ºC or Tstg. Max.
~0ºC
3 sec. transfer in water |
1051.1 |
| Moisture resistance |
 |
1021.1 |
| Vibration variable frequency |
100~2000~100 Hz 4 cycles each X,
Y, Z at 20G
 |
2056 |
| Shock |
3 blows, 1500G, 0.5 (lamps)
3 blows, 500G, 1 ms (displays) |
2006 |
| Constant acceleration |
1 minute each X, Y, Z at 20,000
(lamps)
1 minute each X, Y, Z at 5000G (displays) |
2006 |
| Lead bending stress |
Weight 250 g, 0º ~ 90º ~0º
bend, 3 times |
2036.3 |
| Solderability |
Immersed for 5 sec. at 230ºC
flux: 75% isopropyl alcohol, 25% WW resin |
2026.2 |
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Reliability test methods of LED lamps and LED displays
|
|
Reliability test methods - life tests
|
| |
Operating Life Test: To confirm the stability during
usual operation, the maximum rating forward current
at room temperature is applied to the LED. |
| |
Storage Life Test: To confirm the stability in storage,
the high and low temperature storage life tests are
conducted under the conditions of the maximum and minimum
storage temperature. Since the devices may be used or
in storage at high temperature and high humidity, therefore
the high temperature and high humidity storage life
test is conducted. |
| |
Environmental Test: Since the estimation of thermal
stress and mechanical stress applies to the rating,
the attaching and using of the devices, the soldering
heat, temperature cycles, thermal shock, vibration and
lead strength etc. are examined. |
|
|
| |
| Measuring terms |
Failure criteria |
| Luminous Intensity(IV) |
Lower standard limit
X 0.5 |
| Forward voltage(VF) |
Upper standard limit
X 1.2 |
| Reverse leakage current
(IR) |
Upper standard limit X 2.0 |
| |
|
*PW = 100µs, DR = 10-1
Reliability test data - The measuring terms and
failure criteria are as follows:
Prediction of Failure Rate Based upon field
experience and our extensive test data, the failure mode when
using LEDs are dominated by accidental failures (open, short,
etc.) rather than the degradation of the luminous intensity. These
accidental failures are considered to result from, carelessness
in the manufacturing process; fatigue due to the thermal stress
and mechanical stress etc.; breakdown due to over voltage (current).
Accordingly if we take into account these accidental failures,
the failure rate can be predicted.
From our field experience and our extensive test data, this accidental
failure rate can be estimated to be about 10 to 50 Fit.
Regarding the luminous intensity (IV) which is the main characteristic
of LEDs, the half-life (time when the luminous intensity has been
reduced to 50% of the initial value) obtained from the accelerated
operating life test, is estimated as shown in Figure 3-2.
 |
| Figure 3-2 Junction temperature |
