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2022 Ultimate Guide to LED PCB Assembly and Defect Analysis


Thur, Sept 08, 2022

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This guide gives an introduction to LED PCB assembly and an analysis of possible issues occurring to your LED circuit boards. If your LED lighting products have any issues during PCB assembly or in use, the cause probably hides in the improper LED PCB design or assembly process.

flexible LED PCB assembly

Part 1: What is LED

LED is a light-emitting diode. It is a moisture and thermal sensitive component sealed in encapsulation glue (silicon resin or epoxy resin). Inside the LED, there's a die, phosphor powder, and die-bonding glue(except wire bonding COB LEDs). An LED is a constant-current diode, and its volt-ampere (V-I) graph is non-linear.

Understanding the characteristics of LEDs is important for proper LED PCB design and assembly.

SMD LED PCB assembly

Now, most LEDs are SMD LEDs and chip-on-board (COB) LEDs. SMD LEDs are surface mounted on the printed circuit board with other SMDs. For COB LEDs, the dies (ICs) arranged in a circle or regular polygon are directly bonded to the PCB and then sealed together by glue. The COB LED bonding methods are wire-bonding and flip-chip bonding.

Part 2: LED PCB Assembly Process

LED PCB assembly means mounting all the required electronic components including LEDs on the printed circuit board.

If the LEDs are the SMD type, the LED PCB assembly is surface mounting (SMT) components on PCBs, and the LED PCB assembly process is: silkscreen printing solder paste on PCB pads > solder paste inspection > placing SMD components on the PCB pads > reflow soldering to bond the component pins to the PCB > automatic optical inspection (AOI) to make sure no significant issues happen.

If the LEDs are COB LEDs, the LEDs are directly bonded on the PCB without surface mount and reflow soldering. The two methods are wire bonding and flip-chip bonding.

For wire bonding COB LED PCBs, the chips are connected to the positive and negative electrode pads using gold wires.

COB LED PCB assembly

For flip-chip COB LED PCBs, the chips with solder balls at the bottom are picked and placed on the PCBs. And then at high temperatures under a force, the solder balls melt and cure the chips and the PCB.

After bonding, all COB LEDs are sealed together by encapsulation glue.

You can check out wire bonding and flip-chip bonding in the article "Wire Bonding VS Flip Chip Bonding: How to Customize COB LEDs".

After LED PCB assembly, the single boards are separated from the panelized board. And then the LED PCBA one-by-one undergoes functional testing and thermal aging testing.

Part 3: LED PCB Assembly Defects and Analysis

After LED PCB assembly, it is possible that some LEDs don't illuminate or the light emitted by the LEDs has severe color differences. The LED lamps might also have too early light decay in use.

You can find the cause in the LED PCB design and assembly process. And here we list some cases.

Issue 1 -- LEDs not illuminate due to improper wiring

The LED PCBA panelization board passes the electrical and AOI tests, but after depanelization, some LEDs don't illuminate. In this case, you should check the direction of the LED pads.

improper LED PCB assembly

In LED PCB design, you need to keep in mind that the LED pads' direction had better be perpendicular to the stretch direction of the PCB. Especially for aluminum LED PCBs that are depanelized by the V-cuts, if the LED pads' direction and the PCB stretch direction are the same, the bending force during panelizing of the boards may pull off the gold wires and causes an open circuit of the LED.

Solution - in LED PCB design, try to make LED pads in the perpendicular direction with the PCB stretch direction.

Issue 2 -- LEDs have chromatic aberration without baking before assembly

If the LEDs had been unpacked for a period, you should keep in mind that it is necessary to bake LEDs before LED PCB assembly.

LED PCB assembly color different

If there's moisture inside the LED and you don't bake it, in reflow soldering, the water vapor will rapidly expand inside and cause the glue to crack. Thus, the path of the light changes and the light color looks more yellow than the qualified LEDs.

Solution - store LEDs in sealed moisture-proof preservation conditions with a moisture indication card inside. If the LEDs have been unpacked or wet, bake them at 70℃ for 24 hours before LED PCB assembly.

Issue 3 -- flexible LED strip doesn't illuminate due to low-temperature solder paste

LED PCB assembly open circuit

If you use low-temperature solder paste to solder flexible LED PCBs, after reflow soldering, the solder paste may drop off and cause an open circuit. Why? Because low-temperature solder paste will be fragile after reflow soldering (the peak temperature of lead-free soldering is from 245℃ to 265℃.). However, flexible LED strips are bent frequently in use, so the solder joints will drop off, and low-temperature solder paste should not be used for soldering flexible LED PCBs.

Solution: when choosing the solder paste type, make sure the solder paste viscosity, wettability, and melting point match the LED PCB and product use.

Issue 4 -- a too early light decay due to poor thermal dissipation

LED light decay means the light brightness gets lower after working for a period, and this is most likely due to poor thermal dissipation.

The thermal can't be dissipated effectively from the LEDs, and the temperature of the LEDs gets higher and higher when working. Under high temperatures, inside the LEDs, both the phosphor powder and die-bonding glue deteriorate, and the chip's light efficiency lowers. Also, the encapsulation glue becomes yellow.

Solution: pay attention to LED PCB thermal management carefully, in regards to the PCB substrate material, copper thickness, effective heat transfer areas, etc. You can get help from PCBONLINE, a power PCB manufacturer with a thermal management expert team.

LED PCB assembly thermal management

Part 4: One-Stop LED PCB Assembly Manufacturer

Now you understood the LED PCB assembly process of SMD LEDs and COB LEDs, and that improper LED PCB design and assembly process can lead to failures. How to make sure you can get successful LED products out of LED PCB assembly? You can work with the one-stop LED PCB assembly manufacturer PCBONLINE, which provides free LED PCB design for manufacturing (DFM), design for assembly (DFA), and design for excellence (DFX) of the box builds.

PCBONLINE, founded in 1999, has two large advanced PCB manufacturing bases and one PCB assembly factory. You can custom order LED PCBs of various types, including aluminum LED PCBs, AlN LED PCBs, alumina LED PCBs, flexible LED PCBs, FR4 LED PCBs, and hybrid LED PCBs.

LED PCB assembly manufacturer

Below are the benefits of LED PCB assembly from PCBONLINE you can enjoy:

    • One-stop LED PCB design, fabrication, assembly, and box build assembly from prototype to bulk production without a quantity limit.
    • Provides SMD LED PCB assembly and COB LED PCB assembly (wiring bonding and flip-chip bonding).
    • We can adjust the LED light wavelength and custom encapsulation glue colors according to your products' specific requirements.
    • We can custom design and manufacture RGB and RGBW LEDs that are flip-chip.
    • Complete inspections and value-added services to LED PCB assembly, including functional jig testing, thermal aging, conformal coating, IC programming, and box-build assembly.
    • LED PCB assembly following IPC-A-610 2/3 standards verified with ISO 9001:2015, IATF 16949, REACH, RoHS, and UL.
    • Free complete LED PCBA sample and functional jig testing for bulk production.

If you have any needs for LED PCB manufacturing and assembly, please feel to contact PCBONLINE by email at info@pcbonline.com to get a quote.


SMD LED PCB assembly is SMT assembly, and COB LED PCB assembly is classified into wire bonding and flip-chip bonding. Proper LED PCB design and assembly process are critical to the success of an LED light project. If you need reliable LED PCB assembly and successful LED products, the 23-year-old LED PCB assembly manufacturer won't let you down.

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