Chip-on-Board LED Modules Standardized in Zhaga Book 12
Posted on Monday October 19 2015, by Tim Whitaker
This article was published in LED Professional Review, Issue 51, September/October 2015
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In order to address the complexity of dealing with a wide range of Chip-on-board (COB) LED modules, the Zhaga Consortium has standardized a family of form-factors in its new specification, Book 12. Martin Creusen and Charles Knibbeler from Philips Lighting, Manfred Scheubeck from OSRAM Opto Semiconductors, Ingo Arnrich from BJB and Nico van Stiphout from Molex discuss the numerous issues that have been addressed by Zhaga, leading to a mechanical interface specification for these components.
Chip-on-board (COB) LED arrays are already in widespread use throughout the LED lighting industry, serving a broad range of indoor and outdoor applications. To address this market, many alternative COB form-factors have been developed and are being supplied by multiple COB manufacturers. However, most of these form-factors offer only arbitrary variations in non-competitive properties such as outer dimensions and electrode positions. This increases the complexity of luminaire development and of the supply chain for lighting OEMs.
As a result, stakeholders such as suppliers of COB holders or optics have to accommodate numerous slightly different solutions to supplement this complex COB portfolio. In addition, for luminaire makers this also limits the options to use alternative products from different suppliers without the need to change their luminaire designs.
Many Zhaga members were concerned about this growing redundant diversity. Consequently, they initiated a standardization request and started a so-called Zhaga taskforce. The first task was to analyze and make an inventory of the existing COB market landscape by consulting many different stakeholders, such as manufacturers of COB modules, COB holders and LED luminaires.
Although focusing only on the mainstream COB market, with COB dimensions from 12 to 28 mm, this quickly resulted in a long list of more than 50 different COB form-factors. A short list of five categories was derived, applying a 4-mm interval between 12 and 28 mm, to attain five evenly-spread COB categories of rectangular and square COB modules.
To ensure the future robustness of the upcoming Zhaga COB specification, a rectangular shape was selected for the smaller form-factors (i.e. 12x15, 16x19 and 20x24 mm). For the larger form-factors (i.e. 24x24 and 28x28 mm) a square shape was selected. This was to safeguard sufficient area for future integration of additional functions and features, as well as to ensure ease of manual handling in the case of smaller COBs. The 19x19 mm form-factor was added alongside the 20x24 mm category as a compromise between cost effectiveness and long-term robustness of the specification.
Note that the first phase of Zhaga COB standardization focuses only on the basic COB functionality, and does not include additional functions such as multi-channel LED driving or auxiliary sensors. Also, COB dimensions larger than 28 mm or smaller than 12 mm have not been taken into account (yet).
For each COB size, the functional COB area can be calculated by subtracting the area of the maximum light-emitting surface (LES) from the overall area of the COB module. In Figure 2, the remaining functional COB area is plotted as a function of the smallest COB dimension. For the smaller COB form-factors, the rectangular shape ensures that at least 100 mm2 is available for further function and feature integration, whereas for larger square COB form-factors the remaining area is already sufficiently large.
Also, another advantage when using rectangular-shaped COBs is LES diameter maximization. This is because the creepage distance and clearance (C&C) requirements between contact pads and active dies are not a bottleneck in that case.
The resulting six COB form-factors defined by Zhaga allow COB-array makers to focus on areas where they can offer value-added differentiation to customers, such as thermal properties, quality of light or luminous efficacy. For supporting companies supplying complementary products such as COB holders and optical elements, the concise set of six form-factors will help to proliferate the accompanying ecosystem. Moreover, for lighting OEMs and end customers, this Zhaga specification simplifies the comparison and selection of products.
Mechanical Interface Specification
For COB product specifications, a wide range of different mechanical interface definitions are currently being used, and these vary significantly between the different COB manufacturers. Also, not all the key tolerances are always included in the accompanying product drawings. This unnecessarily complicates the design-in process for suppliers of components such as COB holders, heatsinks or optics. Moreover, most of the COB specifications do not take into account the risk of tolerance stack-up, which is critical to determine the mechanical fit between, for example, COB modules and COB holders.
Therefore, the Zhaga taskforce introduced a generic system for defining and communicating engineering tolerances, a method called geometric dimensioning and tolerancing (GD&T).
This GD&T system defines the basic dimensions as nominal values and outlines the allowed variation in form and size. Figure 3 shows an example of the center position and minimum size of the electrical contact pads and mounting holes as specified according the GD&T method.
To avoid the need for manual soldering, COB modules are often used in combination with COB holders. For this application, the position and minimum size of the COB electrical contact pads are key dimensions and therefore are also part of the Zhaga specification. The required minimum size of the contact pads is actually a result of the overall mechanical tolerance chain of both COB module and holder.
In order to avoid needlessly-large minimum contact areas, Zhaga requested data for the actual tolerances in COB and holder manufacturing instead of the usually-communicated commercial tolerances.
A square-shaped minimum area with rounded corners was agreed upon to allow sufficient design freedom for the COB holder manufacturers. A rectangular- or triangular-shaped contact pad would also have been possible, but would have constrained the position and direction of the contacting beams in the COB holder.
Next, the center positions of the electrical contact pads were designated, taking into account typical creepage distance and clearance (C&C) requirements. Table 1 shows the resulting contact-pad positions (Dx, Dy) and minimum contact-pad dimensions (Lp) for the different COB categories.
The keep-in zone defines the region of the COB where it is permitted to place LED components as well as other features such as the dam surrounding the phosphor area. The center of this keep-in zone coincides with the mechanical reference point. Outside this keep-in zone, the LED module is not permitted to have any features that protrude above the PCB top surface, with the exception of PCB tracks and contact pads.
The Zhaga specification defines the maximum keep-in zone diameter. For a specific COB module, this depends not only on its COB category (i.e. the overall module dimensions) but also on its LES category. The COB category limits the keep-in zone diameter due either to the overall COB outline, or to the position of the contact pads as listed in Table 1. In addition, the keep-in zone is restricted by the LES category. Each LES category has a maximum diameter for the optics contact area (OCA) of the LED module, which corresponds to a minimum reflector opening.
The LES and OCA categories in the COB specification are the same as those used in the different Zhaga specifications covering LED modules for spotlighting-type applications. Currently these specifications are Books 3, 10 and 11 (Books 3 and 11 are published, Book 10 is in development), but in future they will be combined into a single Book.
The combination of a COB module and a suitably-sized COB holder can create a product that effectively replicates a spot LED module as described in either Book 3, 10 or 11.
Table 2 shows the resulting maximum keep-in diameter both as a function of the LED module category and the LES category. The keep-in zone typically includes the dam surrounding the LES area.
Mounting holes and mousebites
For holderless mounting, the COB modules can have optional mounting holes or indents with a designated position and diameter (Figure 3). The larger COB categories (i.e. C28x28 and C24x24) can have mounting holes suited to accommodate M3 screws. The smaller COB categories can have indents at the corners, also known as mousebites, with a radius intended also for M3 screws.
For best performance and a secure fit of the COB holders it was necessary to limit the thickness variation of the COB modules to 1.0±0.15 mm. Therefore, the thickness does not include a thermal interface material (TIM). Usage of a suitable TIM is always recommended but it should be either thin enough to keep the COB within the thickness tolerance or the TIM area should be extended and overlap with the holder.
In a very short period of time, Zhaga members have worked together to produce the first edition 1.0 of Zhaga Book 12. This defines a family of six form-factors, and includes critical dimensions such as the position and minimum size of the electrical contact pads, and the size of the keep-in zone.
An updated version of the specification, which will also include the definition of the COB module-holder interfaces, can be expected in due course. Moreover, Zhaga has already decided that the COB-holder-to-luminaire interface definition will be included in future editions of the Zhaga Books covering LED modules for spotlighting applications.
Testing for COB product compliance with Zhaga Book 12 will be carried out by independent testing facilities operated by one of several Authorized Testing Centers appointed by Zhaga (see www.zhagastandard.org/books/certification). Compliant and certified products are listed on the Zhaga website and are entitled to carry the Zhaga logo.
After finalization of phase 1 of COB standardization, which is limited to basic COB functionality, work will be started on a second standardization phase. This will aim to include additional features and higher integration levels, for example multi-channel, optical feedback and sensors.
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