Military and Aerospace XRF Verification: Keep the Lead In?


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By Scott Mazur, Benchmark Electronics Inc.

Various industries such as military and aerospace sectors continue to be exempt or not covered by RoHS. These industries require leaded assembly, given the risk of growth of tin whiskers and lack of field reliability data for lead-free components and subassemblies. Military and aerospace companies are searching the supply chain for COTS leaded components. Tin whiskers can grow unpredictably, and are mitigated by leaded materials.

For the last three years, EMS companies have converted processes and equipment to become Restriction of Hazardous Substances (RoHS) compliant. RoHS verification tools have been deployed to verify that components and products are lead-free. Various industries, however, such as military and aerospace sectors, continue to be exempt or not covered by the RoHS directive. These industries require lead-terminated components and lead solder processes, given the risk of growth of tin whiskers and lack of field reliability data for lead-free components and subassemblies. Tin whiskers can grow unpredictably (Figure 1), and potentially short to adjacent terminations. They are subject to many variables, such as plating finish and environment. Military and aerospace products require high reliability, and must withstand long product life and harsh environments. Several methods can be used to mitigate the tin whisker growth, the most effective being the addition of lead (Pb); as supported by years of field reliability data. Given the changeover to lead free, military and aerospace companies are searching the supply chain for commercial off the shelf (COTS) leaded components, due to the concerns stated above.1 COTS material has been used for decades instead of mil-spec components with benefits being that they are cost effective and have years of field reliability data.

The identification of leaded or lead-free COTS material can be convoluted given that many manufacturers instituted RoHS date codes without changing the manufacturer's part number. Because of the ambiguous control of this changeover and the critical need for leaded components, a process must be implemented to verify that lead is present in a component termination. Various testing options are available, either destructive or nondestructive to the component. One option, which is nondestructive and can be used effectively, is X-ray fluorescence (XRF). The process has been used by many facilities as a RoHS verification tool, to demonstrate the absence of lead. This application is the reverse, to validate the presence of lead.

XRF TechnologyXRF technology is a method in which a sample is exposed to X-rays of energy to determine its atomic material composition. Several XRF equipment options are available, such as fixed or portable. One advantage of the portable unit is that it can be used throughout an electronics manufacturing operation, including at receiving, surface mount technology lines, and an entire printed circuit assembly manufacturing process. The advantage of most fixed units is smaller spot size, important for small SMT components such as chip resistors, capacitors, and integrated circuits (ICs).

Lead Verification ProgramThe lead verification program must start during the receipt of incoming material to prove lead is present in the terminations. The testing can provide a "trust but verify" approach versus material acceptance using only certificate of compliance (CoC) or manufacturer part number validation. During the receipt verification, several factors need to be considered, since lead is used in many component applications. Lead is present in glass in resistors, ceramic in capacitors, and internal die attach materials of ICs. A smaller spot size may be required to focus on the termination with miniaturized package sizes challenging the inspector. If a larger spot size is used, the result may be false due to the reading registering lead from the inner package and not from the termination. The limitation is that although XRF reliably detects the presence of lead care must be taken to isolate the lead's location.

In addition, the XRF process can be used to verify tin whisker mitigation processes (nickel under barrier) and non-lead termination finishes (i.e. gold, nickel, palladium, etc). Similar processes can be implemented for components that have been qualified for re-tinning or lead solder dipping to mitigate tin whiskers. Validation would ensure that the correct lead alloy was used for dipping and no components were missed by the operation.

Additional detection points can be included in the manufacturing process to ensure that lead-free materials are not introduced into the process. This is especially important with manufacturing facilities producing leaded and lead-free product. If lead-free solder is introduced into a leaded assembly, the risk is not primarily tin whiskers, but more in the area of long-term joint reliability. Lead-free solders have not been approved by military, aerospace, or some medical and telecommunications industries. The equipment of choice for lead process validation is typically portable XRF, given its handheld and lightweight design and the nondestructive character of its analysis and ease of use by the operator. Figure 2 details the verification of a printed circuit assembly.2 Various process points that can be tested include SMT, wave solder, and hand soldering operations. Additional process inputs to be considered for cross contamination include manufacturing materials such as solder paste, bars, and solder pots for wave soldering equipment. Lead solder pot contamination with lead-free solder changes the alloy composition, creating unknown reliability risks. The measurement is specific to process location or solder point with the critical focus being the risk areas when validating.

ConclusionInstituting a lead XRF program enables a manufacturing facility to verify the presence of lead in the terminations, manufacturing processes, and materials. Without the XRF verification process, electronic manufacturers will be completely dependent on the supply chain and acceptance of material using documentation such as declarations and manufacturer part number validation.

References:1. "Leaded parts still in high demand," Electronic Business, 1/20/09.2. "Green Electronics Design and Manufacturing Book," McGraw Hill Publishing, authored by Sammy Shina; contributing authors Bob Farrell and Scott Mazur, Chapter 6.

Scott Mazur, manufacturing staff engineer, may be contacted at Benchmark Electronics Inc. Hudson, NH Division, 65 River Road, Hudson, NH 03051; (603) 879-7000 Ext 8004; scott.mazur@bench.com.

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