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Soldering LEDs

 The LED construction diagrams below illustrate that the lead frame is directly connected to the LED die and the Anode bond wire.  Heat used during the soldering process will be transmitted to the chip and bond wire structure. Although LED devices are designed to be soldered using all the modern soldering processes, adherence to the recommended soldering profiles is critical to the reliable performance of an LED device.

 

Fig 1    LED external                                                  Fig 2.   Internal LED construction

 

General Rules for lead bending; -

1.    Any lead bending should be done prior to soldering, never after or during soldering.
2.    Avoid bending the leads at the same point more than once
3.    Never bend the leads at the junction with epoxy, there should be a clearance (min 2mm) between the base of the LED and bend point
4.    During the lead forming (bending) process the leads must be held securely to prevent any bending stress being transmitted to the  internal structure of the LED
5.    Avoid placing any stress on the LED lens in order to prevent fracture of the epoxy layer and damage to the internal structure of the LED.
6.    Avoid any stress to LED once it has been mounted, i.e. all lead forming should be undertaken prior to soldering
7.    When using an unformed LED, the mounting holes in the PCB must match the lead pitch of the LEDs.  Where the pitch of the PCB mounting holes and the LED lead pitch differs, then the LED leads must be preformed to exactly match the mounting hole pitch.  This should be performed using the above guidelines.

The diagrams below illustrate of acceptable and unacceptable lead forming practices: -

Acceptable

 

Not Acceptable

 

Diagrams courtesy of SunLED Company

 

Manual soldering

  LED can be hand soldered, but the following Guidelines should be observed: -

1. Should only be considered only for development repair and rework processes.  Manual soldering should be avoided for production purposes as it will be difficult to maintain the repeatability of the process and this will impact on overall quality levels.
2. A modern temperature controlled soldering iron should be used. The maximum tip temperature is dependant on the soldering process used and the technology of the device being soldered: -
   1. Lead free - 350??C compulsory for RoHS compliency
   2.  Pb-Sn (Leaded) solder - 300??C
   3. All InGaN (Blue / Green) - 280??C
3. Maximum soldering time (tip in contact leads) 3 seconds
4. The soldering iron should never touch the epoxy lens
5. The LED should not be stressed during the soldering process.

Note: These are general guidelines and the manufacturer’s data sheet for the specific part being soldered should be consulted prior to the soldering process.

In order for an electrical device or component to stay RoHS complient the Lead free solder process should be adopted along with lead free solder.

Wave Soldering

This process will generally be used for soldering through-hole (leaded) components, i.e. 5mm 3mm LED lamps, Top View LEDs, Seven Segment Displays and bargraph Displays.  In this process a PCB board complete with all the components is passed over a wave of solder, such that only the underside of the PCB is in contact with solder wave.

The resultant process produces mechanically strong and reliable connections. Any thermal is reduced because the component leads are in contact with the solder wave for a relatively short period of time.  The process is highly repeatable and controllable meaning that it is very suitable in a production environment.  However, like all processes it needs to be carefully set up, monitored and calibrated.

 The process has several distinct phases that need to be controlled: -

 Pre-Heating: - Temperature and time

Soldering: - Temperature and time

Cooling: - Time and slope of the cooling curve

 The graphs below illustrate typical wave soldering profiles for Lead Free and  Pb-Sn (Lead based Solder) soldering processes: -  

 

a) Lead Free process (Compulsory for RoHS compliency)

 

 

b) Lead (Pb-Sn) process

 

 

Courtesy of SunLED Company

Note: - The above are illustrative and it is important that reference is made to the manufacturer’s data sheet for the specific LED component(s) being soldered.

It is critical to the long-term reliability of the components that soldering process keeps with the maximum temperature and times.  Soldering equipment needs to be maintained and calibrated in line with the equipment manufacturer’s recommendations.

Reflow Soldering

This process is used for soldering Surface Mount (SMT / SMD) components, e.g. SMD Lamps, Chip LEDs, PLCC Power LEDs.  Also it is possible to form some leaded components to be suitable for use in the Reflow Soldering oven. 

Similar to the Wave Soldering process, Reflow soldering has three distinct phases that need to be controlled: -

 Pre-Heating: - Temperature and time, used to help minimise thermal stress

Soldering: - Temperature and time

Cooling: - Time and slope of the cooling curve

 The graphs below illustrate typical wave soldering profiles for Lead Free and Pb-Sn (Lead based Solder)  soldering processes: - 

 

a) Lead Free process (Compulsory for RoHS compliency) 

 

 

b) Lead (Pb-Sn) process

Courtesy of SunLED Company

 

Note: - The above are illustrative and it is important that reference is made to the manufacturer’s data sheet for the specific LED component(s) being soldered.

It is critical to the long-term reliability of the components that soldering process keeps with the maximum temperature and times.  Soldering equipment needs to be maintained and calibrated in line with the equipment manufacturer’s recommendations.