Visible LEDs Application Notes
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 LEDs lamps:
- 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.
- Excellent drive characteristics - The LEDs 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.
- 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.
- Structure - Figure 3-1-3 shows an example of a LEDs lamp structure. The main body that radiates light is the LEDs 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 LEDs chip and anode lead and bonded to each with a hot press-fitting bonder. Further more, the LEDs chip is molded into a transparent plastic lens to pick up light efficiently. LEDs lamps of different appearances can be produced depending on the shape and material of this lens.
- LEDs 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 LEDs 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 LEDs chips.)
- Sealing resin - LEDs lamp packages are formed with a lead frame on which the LEDs 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 LEDs chips for outdoor use.
Precautions When Using LEDs
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.
Soldering - The softening temperature of the resin of which the LEDs packages are made is generally low; less than about 100°C. The following table lists the different soldering methods and conditions:
Soldering work conditions - Unless otherwise specified in the technical documentation, perform soldering work under the following conditions.
Soldering Temperature: 260°C or less (when solder dipping) 300°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°C or less.
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).
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°C (122°F) max. for 30 seconds or 30°C (86°F) max. for 3 minutes max
- Ultrasonic: 300W max
Precautions for Mounting: Do Not apply force to plastic part of LEDs when LEDs is under high temperature. Avoid contact friction between LEDs and other components during the assembly process as this may damage the plastic portion of the LEDs.
Recommended Soldering Patterns: TLx1005 Series TLx1002 Series MTSMx35K-xx Series
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 LEDs lamps are: Chlorothene, Freon TE or TF, Dai-Fron Solvent S3 or S3-E. For washing LEDs Luminators (MTBLx41x-XX), use water only.
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 LEDs 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.
Figure 3-1-7 depicts a basic circuit to light the LEDs 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 LEDs
IF = forward current flowing in LEDs
Figure 3-1-8 shows a circuit where variations in VF of an LEDs 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.
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)
Figure 3-1-10 depicts a basic circuit to light the LEDs 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 LEDs 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 LEDs.
Figure 3-1-9 - Increasing radiant power
Figure 3-1-10 - AC lighting circuit
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 LEDs IF is held to within the rated value at a point where the supply voltage VCC is maximum.
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.
The following lists the IOL and VOL characteristics of typical TTL, CMOS, and buffer ICs for your reference.
Figure 3-1-11 - IC-based lighting circuit
VIS IC Lighting Circuit With Buffer
Figure 3-1-12 - Lighting circuit using IC & buffer transistor
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.
Pulse Allowable Forward Current Of LED Lamps
The pulse allowable forward current IFP when an LEDs 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
IFP = pulse allowable forward current
PW = pulse width
T = repetition period
DR = duty ratio (PW / T)
If the ambient temperature (Ta) exceeds 25°C, derating of IFP for Ta (shown in Figure 3-1-13) is required.
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 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.
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. *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).
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.
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 the forming:
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.
Double-end type: Bend the lead at its constricted part or a position 2 mm or more apart from the root.
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 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 LEDs and led displays.
|Type||Test||Test conditions||MIL-STD-750 Reference|
|Life||Operating life||Ta = 25°C IF Max. rating||1026.3|
|Life||High temperature storage||Ta = Tstg., Max.||1026.3|
|Life||Low temperature storage||Ta = Tstg., Min.||-|
|Life||High temperature and high humidity storage||Ta = 60°C or 40°C, R. H. = 90°||-|
|Enviromental||Immersed for 10 sec. at 260° up to 2mm from the body||2031.1|
|Enviromental||Tstg. Min. ~25°C~ Tstg. Max. ~25°C (30 min.) (5 min.) (30 min.) (5 min.)
|Enviromental||100°C or Tstg. Max. ~0°C
|Enviromental||100~2000~100 Hz 4 cycles each X, Y, Z at 20G
|Enviromental||3 blows, 1500G, 0.5 (lamps)|
3 blows, 500G, 1 ms (displays)
|Enviromental||1 minute each X, Y, Z at 20,000 (lamps)|
1 minute each X, Y, Z at 5000G (displays)
|Enviromental||Weight 250 g, 0° ~ 90° ~0° bend, 3 times||2036.3|
|Enviromental||Immersed for 5 sec. at 230°C flux: 75% isopropyl alcohol, 25% WW resin||2026.2|
Reliability test methods - life tests
|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|
Operating Life Test: To confirm the stability during usual operation, the maximum rating forward current at room temperature is applied to the LEDs.
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.
*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