The sensitive nature of LEDs underscores the need for minimizing “waste.”
Manufacturers of printed circuit board assemblies, in general, have been experiencing relatively high first-pass yield infant mortality rates due to the nature of smaller LED component design and manufacturing itself. These are very delicate, wire-bonded parts. Common component failures include silver-filled epoxy die attach de-bonding issues, wire-bond lifting due to encapsulation around the LED die or the wire bonding process itself, and broken wires due to PCB flexing or solder wicking stress on the wire bonds.
Not surprisingly, addressing challenges represented by smaller LED components aligns closely with the focus on eliminating several of the seven wastes identified by Taiichi Ohno, the key founder of the Toyota Production System (TPS). These inefficiencies or wastes are:
While focus on all seven of these wastes is a hallmark of a well-run production operation utilizing Lean manufacturing principles, three wastes in particular relate strongly to LED manufacturing: defects, inappropriate processing, and transportation. This column looks at the steps being taken in each of these areas.
Eliminating the waste of defects. The challenge in LED assembly is the sheer number of defect opportunities that exists due to the delicacy of small parts. The discipline of eliminating the potential for defects to occur is critical and requires a combination of close collaboration with the LED component supply chain; strong focus on design for manufacturability (DfM) and design for assembly (DfA); and tight process controls and specialized handling during manufacturing.
Selecting LED component suppliers capable of providing consistent quality parts is a key first step. Additionally, hue selection is very important. LEDs are bought by the bin. While providers are good at keeping hue consistency within a specific bin, the correct bin for the desired hue must be specified.
The delicacy and nature of LED manufacturing drives more stringent process control in PCBA manufacturing. Adhering to each LED manufacturer’s component data sheet guidelines is critical. As an example, some 0603 LEDs start out as a thin specialized printed circuit board panel containing thousands of LEDs. This large PCB panel is drilled at two locations at each individual LED site. These holes are plated through and will facilitate the electrical connections between the bottom mounting side and the topside, where the wire bonds or silver die attach epoxy connections are placed. When the LED panel fabrication is completed, the panel is sawed along the hole axis. This leaves a portion of each hole present at each end of each LED or a castellated connection. Soldering must be tightly controlled; otherwise hot liquid solder will flow upward into the partial hole or castellated termination and come in contact with the LED lens and attempt to dislodge it.
From a DfM standpoint, stencil design is very important for ensuring the correct amount of solder is deposited. In SigmaTron’s process, the apertures in the solder stencil use a trapezoid shape because controlling the amount of solder paste, aperture and shape of the print is critical for keeping the solder from wicking up and coming in contact with the clear lens. Otherwise, the excess solder can peel off the lens, or put too much strain on the wire bonds holding the lens.
This contractor’s Acuna, Mexico, facility still occasionally runs through-hole LEDs in low volume for legacy products. DfM and adherence to IPC design guidelines is important with these PCBAs, as the hole sizes for leads and spacing between holes is critical. If spacing between holes is too far or too close, it will place stress on the leads and weaken the part. Through-hole LEDs should also be inserted with a standoff.
There are also DfA considerations in field failure mitigation where 0603 LEDs are involved. If a plastic light guide or light-focusing device is placed too near the 0603 lens, shock and vibration in normal use can cause the device to touch the lens and break.
ESD damage is also a concern, and defect mitigation requires a higher level of ESD protection.
Ultraviolet (UV) or blue LEDs are considered the most ESD-sensitive, but in reality white LEDs are also blue LEDs. A phosphor coating is used to color the lens yellow so that human eyes interpret the light as white. So basically, all LEDs are extremely ESD-sensitive.
In this contractor’s process, ionizers are used in all areas where LED PCBAs are handled, in addition to the factory’s standard ESD protective flooring and employee protective clothing.
Eliminating the waste of inappropriate processing. The delicacy of wire-bonded 0603 LEDs also creates challenges in reflow, particularly on Pb-free products. The process window is very narrow. When possible, SnPb solder is preferred because peak soldering temperatures run 217˚-220˚C. Conversely, the peak soldering temperature is 242˚C with Pb-free solder. The temperature profile needs to be as gentle as possible to minimize the impact of peak temperatures on the wire-bonded die. If the profile is not hot enough and long enough in duration, the flux does not properly activate and then deactivate. If peak temperatures are held too long and higher temperature profiles are used for Pb-free solder, yields go down due to silver die attach failures. The die attach failure is normally caused by some type of force acting on the lens or due to an LED vendor wire bonding issue.
Mitigating the waste of inappropriate processing also plays a factor in test strategy. In this case, the issue is most related to cost of test. Simpler equipment achieves the desired result at lower cost.
LED PCBAs are typically tested using in-circuit test (ICT). However, standard ICT will not test for brightness. At this contractor, testing is done using cameras and software that analyzes pixel values. The alternative test option measures LED hue using a spectrophotometer with an integrating sphere, which requires precise distance between the LED and test equipment. From a cost and test time perspective, the latter method was deemed too costly when the two methods were analyzed in terms of cost and accuracy. In the camera method for white LEDs, the ratio of blue pixel intensity to red pixel intensity is measured to verify the correct hue.
Eliminating the waste of excess transportation. Handling following reflow is also critical. If the PCBAs are moved when hot, the LED will separate from the substrate. This contractor addresses this with a chiller in the reflow oven followed by a fan tray as the PCBAs exit the oven so boards are room temperature when handled. Even at room temperature, handling can be an issue. A shock or bump will break the lens of a 0603 LED. Consequently, the production process is designed to minimize the need for production operators to handle PCBAs.
With careful planning LED manufacturing can generate high yields. However, the margin for error in process or handling is much smaller for packages such as the 0603 SMT device. As noted, many of the foundation disciplines utilized in Lean manufacturing can pay big dividends in improving LED assembly production processes.