Thursday, April 26, 2018
Publication Date: 02/1/2008
Archive >  February 2008 Issue >  Electronic Mfg. Services > 

Custom LED PCB Assembly, From Start to Finish
Chip mounting line.

Whether you're evaluating your current CM, or sourcing a specialist PCBA manufacturer, especially one you have not worked with previously, the first thing to check out is certifications. Manufacturers with a commitment to quality should have an ISO 9001:2000 certification and adherence to an IPC-6000 series level of inspection. Having this certification shows a general commitment to producing quality products.

If your industry requires more specific certifications, such as MIL-Spec, RoHS compliancy, the TS 16949 for automotive or the ISO 13485 for medical, you will be pressed to look beyond the norm for PCBA providers. Even if your industry does not require these more strenuous certifications, choosing a manufacturer that meets them will also ensure a higher quality product. If you don't find Certification information on the prospective manufacturer's web site, call and inquire directly. If they are not certified, and you are not familiar with them, outsource elsewhere!

Alternatively, search out as your CM a company that is an LED manufacturer/packager, or has extensive experience in LED PCBA. Using a CM who is a manufacturer or packager of LEDs provides assurance that their experience manufacturing LEDs will translate into improving the overall assembly of your product.

Unfortunately, the number of LED manufacturers/packagers providing CM services is relatively small, which may send you back to looking for a CM with experience building LED PCBAs. Here are some important things to look for:

  • Are they able to do the kind of custom manufacturing you require?
  • Do they know the properties of LEDs, such as bins, electrical and optical characteristics, and conditions that may cause an LED to fail?
  • Can they accommodate a difficult binning and matching process, which can complicate production? (If they know how to do match testing before production, it will prevent a lot of headaches.)
  • What are their LED testing capabilities?
  • Do they have expertise in voltage driving conditions for LEDs? It is fairly common to spec in an LED that cannot withstand the driving conditions of the assembly. An experienced LED PCBA provider should be able to adjust resistor values to accommodate different Vf and intensity bins.

Binning Capabilities
Generally LEDs come in different bins that were pre-binned according to the industry standard. What happens if you have to do a narrower or broader re-binning of your LEDs? The narrower re-binning process may come from a stricter consistency requirement for an individual module, but not necessarily between modules. The broader re-binning process may come from a less strict consistency requirement for an individual module, but a stricter consistency requirement between modules. The first situation may apply to a large outdoor video display and the second situation may apply to multiple light boxes along one street that has LED backlighting panels inside. If your potential CM has the capability to re-bin the LEDs, that will help address any current and future problems, and can reduce lead time.

Testing Capabilities
LED testing is what really separates the men from the boys. You'll find CMs boasting about their experience dealing with LEDs; and now you have to differentiate between the one who really knows and the one who just knows a little bit. Their capabilities to do LED testing may guide your selection, and the LED testing lies in two levels:

Individual LED testing.: As an ISO certified manufacturer, every CM must perform incoming material inspection, especially for the most costly component — LEDs. It's too late and too costly to discover non-conforming materials after every component is populated to the board or assembled into a display. LED incoming inspection can be as extensive as requiring 6 tests: intensity, view angle, forward voltage, reverse current, thermal shock, high temperature/high humidity.

To perform these tests, the contractor will need very delicate and expensive optical, electrical and environmental testing equipment, along with the knowledge of how to do the tests. Finding out if the prospective CM has the required testing equipment can help you filter out inappropriate candidates.

Functional test, burn in and consistency test of the LEDs at the PCBA level. Generally an electronics engineer can provide testing protocol and testing fixture to the CM on how to do the functional tests. However, to do the burn-in and consistency test may require specialized test equipment such as a precise spot meter or other CCD LED measurement system. You can feel much more comfortable if you know that your future CM has this type of equipment in house.

Once you have narrowed down your roster of prospective CMs, make sure to have a signed non-disclosure agreement (NDA) prior to turning over any technical files. This is probably already a prerequisite in your company, but one reason for a checklist is to remind us to never assume! It is better to be safe than sorry, especially when it comes to your intellectual property, which you and your company have worked so hard to develop.

What technical files does your vendor need from you?

  • Gerber Files (PCB fabrication files).
  • BOM.
  • Labeling specifications.
  • Burn In or Test Specifications.
  • Any assembly instructions critical to your finished product not contained in the above documents, such as conformal coating and/or potting.

A good CM will review the files and give recommendations if they see a potential problem. Recommendations can range from parts to packaging.

If the LED PCBA being produced by the CM is a major piece of your end product, have your engineer visit the CM on a regular basis. Although it's unlikely your CM will have a problem with this, it is a good idea to discuss regular visits prior to placing a purchase order.

Here is a quick rundown of what the production process should look like:

LED binning/Planning. After all the LEDs are binned as required, a plan should be set up for which bin goes to which board so you can utilize all the LEDs and meet the consistency and matching requirements.

Assembly. The LED PCBA assembly process is similar to other PCBA processes. However, two factors are critical for LEDs. First, make sure that all the components can withstand the soldering conditions or temperature profiles — especially the LEDs. Second, all tools and people need to be ESD-free.

Testing/Burn-In. The testing capabilities should have been determined when choosing the CM. The testing and burn-in protocol and fixtures should have been provided before production. This step will strictly follow those processes.

Results Documentation. Good documentation can save a lot of headaches if issues arise in the future. Besides the general labeling requirements, such as manufacturer name and lot number or date code, generally you will need to label the PC boards with specific LED bin codes so that it is easier to match and trace. Good documentation also means documenting the detailed testing and burning data, and possibly manufacturing process data.

Packaging. Besides the ESD-free packaging, the LED PCBA may require protective bags or layers, because almost all LED lenses are made of epoxy resin, and if scratched, the light output efficiency will be lower.

For more information, contact: CAO Group, 4628 West Skyhawk Drive, West Jordan, UT 84084 877-877-9778 or 801-256-9282 Web:

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