Must List for Aviation PCBs
Three key aspects a contract manufacturer (CM) or EMS Provider should be well armed with to effectively produce PCBs targeted at aviation and aerospace PCB applications are (1) AS9100 certification, (2) RoHS expertise and/or hard to find leaded components, and (3) a bullet-proof process to fend off counterfeit components.
AS9100 is defined as the international management system standard for aircraft, space, and defense industries. It demands tightly and reliability controlled and repeatable processes. It’s targeted at having aerospace industry manufacturers produce world-class quality products at a reasonable cost. In particular, at the CM level, AS9100 certification demands certain quality and reliability measures be put in place for each and every aerospace and military project.
As for RoHS, many aviation systems makers remain with leaded components, although a few are making some inroads into lead-free, creating hybrid PCB assemblies. Here, the CM must be extra vigilant and careful in assuring assembly and manufacturing processes are fine-tuned to reliably produce these mixed boards. Leaded and lead-free-combination PCBs demand a disciplined list of checks and balances to meet customer specifications.
Lastly, CMs must take extra care in avoiding counterfeit components. Critical procedures and processes should be in place to detect the counterfeit components, if and when they creep into the system. Establishing sound relationships with reputable distributors and value-added resellers is also extremely necessary to make sure that all the components that are procured there are original and not counterfeit.
Sub-assemblies designed and manufactured for military and civil aviation systems must possess the highest levels of robustness and high reliability life-cycles. According to the experts, the average defined age for an aircraft is 50 years or more, thus supporting systems and sub-systems must have that extended longevity and associated reliability built-in.
Therefore, contract manufacturers and EMS Providers serving aviation OEMs must pay particular attention to establishing disciplined plans and procedures to assure total quality systems to include meeting special service, maintenance, safety precautions, and equipment requirements. Also, record-keeping, documenting detailed work performed during maintenance, the equipment that is used, and people who perform the work — all of it is logged and maintained for a specific period of time depending on the product being made. These and a host of other procedures are at the heart of AS9100 Certification.
To adhere to AS9100 requirements, a CM must operate a quality conscious shop. It must demonstrate the highest levels of quality by clearly proving it has instituted and formalized various critical verification steps at key design and manufacturing stages. In order to do that, all the process and manufacturing technicians must go through a rigorous training to be able to handle these types of projects. The flowchart shows the manufacturing operations of AS9100 based projects.
It is imperative for the CM to make sure that there are a lot of embedded QA and QC steps in the overall manufacturing processes so that it is a self correcting mechanism in itself and there are not a whole lot of surprises in the end.
As part of that process verification, assurances must be made that there are internal control checkpoint loops so that when inaccuracies start to surface, they are immediately flagged. Once identified, those mis-cues can be quickly adjusted, thanks to those checks and balances in place.
Traceability is among the major disciplines within AS9100 processes that must be stringently applied and diligently followed. Traceability is not only applied to an entire bill of material (BOM), but in some cases it is applied also to each single component in that particular BOM. Here, we’re talking about traceability as it pertains to the lot code, date, year, and manufacturing codes of each component. Therefore, component-level traceability of this high caliber demands extraordinary special attention by well-trained assembly personnel who not only can procure these components but also store them discreetly for future use when needed. Plus, traceability under AS9100 requires the CM to strictly maintain such projects in-house without consigning any or all of its parts to outside sub-contractors.
In case of consigned assemblies of extremely sensitive nature, when a component is damaged or isn’t soldered properly during the manufacturing process, CM assembly personnel must remove it, package it, correctly label it, and return it to the aviation OEM customer. The customer will then send a new component to the CM with complete traceability data. Even if there are multiple identical PCB units in the system, there is a separate BOM, a separate kit, and separate work order instructions for every single build.
While on some projects having AS9100 requirements, traceability on virtually every PCB is required. But on top of that, there is still another requirement some regulatory agencies demand such as FAA, NASA, JPL or such other agencies. Then there is detailed first article of inspection or FAI process, frequently performed at the source or at the CM floor conducted by the inspectors of the agency or its representative directly.
This is extremely time consuming and detailed process and is called Source Audit or Source Inspection. The purpose of this inspection is to demonstrate product conformance, process verification, and verification of engineering requirements. It also deals with approving any deviations encountered during the manufacturing process so that the rest of the build can go through smoothly.Other regulatory agencies may require such things as so-called potting and ionic cleaning to be performed after a PCB assembly is completed. As shown in Fig. 1, potting covers devices, which are sensitive, as well as to protect the intellectual property (IP). It is also applied to prevent humidity or moisture from seeping into the components since they’ll be operating in the upper atmosphere. Also, at times, the protection of intellectual property is at stake. In cases like this, conformal coating is used on such intellectual property to prevent unauthorized personnel from decoding it, as well as keeping “foreign objects” away from it.
In other cases, AS9100 calls for special processes to ensure product longevity and reliability. This means when a CM builds a PCB per the AS9100 Standard, the aviation system OEM will require a separate line and associated workstations to comply with certain requirements. For example, an OEM may have special ESD requirements. ESD0 is an especially tight ESD requirement in civil and military aviation systems, and the OEM doesn’t want any other components, boards, or fluxes, other than those originally specified.
ESD Control Class 0 covers electronic parts and assemblies that are susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM). When dealing with these highly susceptible devices, a more stringent ESD control program is required. It involves improved grounding of wrist strap and footwear for assembly personnel, work surfaces, bonded grounds and minimizing conductors. This extra precaution ensures the electronic assemblies that are following these conditions are 100% ESD compliant to the highest degree.
RoHS Reluctance Persists
Some highly visible aviation programs still deal with leaded or eutectic components and are reluctant to adopt hybrid ones. Therefore, the vast majority of aviation PCBs remain leaded. If Aviation OEMs find components with gold leads, they will require pre-tinning them so that component can be run through the regular eutectic process.
A general rule of thumb to follow when producing hybrid leaded and lead-free aviation projects is to reflow the board after loading the lead free components because their thermal profile has higher temperature ranges to deal with. This is followed by loading leaded components and assembling them, utilizing manual placement techniques.
With other markets such as industrial, networking, computer, and consumer moving rapidly to RoHS, the majority of suppliers are producing lead-free components with great numbers of leaded ones becoming obsolete. That’s why a particularly experienced CM and its purchasing staff are especially valuable to the Aviation OEM because they maintain a good supplier network and can locate hard to find leaded or eutectic components. CMs like that must manage their supplier base throughout the aviation/aerospace supply chain, which remains a major challenge. It is a challenge because the supply chain is extremely long and hard to manage.
Within that supplier chain base, suppliers serve other industries, and those industries may not be as quality or reliability conscious to effectively serve aviation/aerospace OEMs and their CMs. The savvy CM plays a special and pivotal role in this case to make certain leaded components are not only available, but are reliable and not counterfeit. This involves close supplier control that include spot checks and audits of both primary and sub-contractor suppliers to maintain the integrity of the supply chain.Considering special projects of extreme sensitivity that aviation and military demands, all operations and sub operations require step-by-step inspection and verification. Take printing paste for example. There are certain internal procedures that are to be followed. When printing on a surface mount (SM) pad, there are specific requirements for assuring that a certain specific amount of paste is dispensed on a specific SM pad, and a paste height inspection machine inspects and insures that has indeed occurred, Fig. 2. All along the process, multiple automated optical inspections may be required. The purpose is to make sure there are no irregularities creeping into the sub-system build, and thus these processes are 100% compliant to the MPI documents.
Every manufacturing step is listed in extreme detail in the new product introduction (NPI) with specific figures, numbers, charts, and directions. This means the savvy CM must be detailed oriented, thus avoiding ambiguities or faulty assumptions and subsequent errors. The NPI anticipates any and all questions and leaves no room for misunderstanding. This frees assembly personnel from having to make any spot decisions since their directions and instructions are clear, direct, and detailed step-by-step.
The main rule of thumb as far as counterfeit components is to stick to reputable and well-known value-added resellers, when purchasing components. But even better is for the CM to have direct links to top of the line chipmakers, and that connection has two benefits to the aviation OEM. First, it eliminates a distributor or middleman who is carrying the product and therefore provides some cost savings. However, more important is the fact the OEM and CM are getting 100 percent guaranteed ICs directly from the manufacturer, thus virtually eliminating questionable, bad or counterfeit devices.
But let’s say a counterfeit component gets into CM floor. The experienced CM knows to deploy certain assembly tests at the first signs of component failure. Among the top on the list is to have the best incoming inspection components audit team that is capable of identifying these counterfeit components and rejecting them at the entry level. If there are some doubts, they can use these components on an existing board to do a quick functional test. Following disciplined processes and procedures, the CM prepares the device for shipment back to the OEM customer complete with traceability data and then have the OEM provide a genuine version of that component along with traceability data.Advanced x-ray and automated optical inspection (AOI) play vital roles to ascertain wire-bonding integrity, as well as to ferret out counterfeit components. Fig. 3 shows an x-ray image Indicating wire bond integrity. Also, functional test is applied to ensure a PCB is not populated with counterfeit devices erroneously coming onto the assembly floor though the supplier chain. But as stated earlier, smart OEMs and CMs like to go to the IC manufacturer, although they have to pay a little extra for such purchases. However, to meet the AS9100 Standard, pricing is not as important as the reliability and consistency of a quality PCB being manufactured.