In 2006, the European Union’s (EU) RoHS directive (Note 1) first took effect to restrict the use of some hazardous substances in the world marketplace. This legislative mandate forced worldwide consumer electronics manufacturers to implement a lead-free electronics assembly. At that time, medical electronics were exempted from this directive. But when the EU further published its RoHS directive revision, this had been amended and the exemption had also been eliminated, in which medical electronic devices (2014), in vitro diagnostic medical devices (2016), and industrial monitoring and control instruments (2017) are covered.
The EU enforced the RoHS directive on consumer electronics products in 2006, in which it restricted the use of substances including lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ether (Note 2). Since lead (Pb) is included, manufacturers have been forced to convert from the tin-lead solder (SnPb) that they have been using for 50 years to lead-free solder. The soldering temperature of the SnPb solder alloy (Pb Alloy 63/67) is 183℃ whereas the lead-free solder is in the range of 217℃ ~ 220℃.
Consequently, the lead-free electronics assembly process has had to raise itself to a higher temperature by 30~40℃ compared to the SnPb process. This places an extremely high burden on the tin-lead production machines. In addition, the elevated temperature will exert an adverse effect on the electronics components and printed circuit boards (PCBs), which in turn may affect the product life and reliability.
In 2014, the EU will cover medical electronics devices in the RoHS directive, which means that many medical electronics devices being developed now will have to conform to the lead-free requirements when they are commercialized in 2014. As such, the medical electronics manufacturers should immediately develop a reliability verification program for conversion to the lead-free assembly process and shall also start to implement it. Only in so doing can they successfully export their products to the European market as of 2014. The high reliability requirements for medical electronics devices in particular must be considered.
The laboratory statistics of reliability and failure analyses developed by us, Inc, have shown that under “good” lead-free assembly conditions, the lead-free solder joint outperforms the SnPb solder joint in the pull/shear and the temperature cycling tests. But the lead-free solder joint is substantially weaker in terms of mechanical stress as shown from the test results of mechanical shock, vibration, and the board bending test.
Moreover, many cases have also shown delamination between the IC and the PCB, electrochemical migration, creeping corrosion, as well as tin whisker and heat damage being more significant than in the SnPb process. These are very common problems occurring in the lead-free process transition for consumer electronics products. Therefore, the medical electronics industry should have a complete verification and analysis program to effectively mitigate the risk. In addition, special caution should be taken in the lead-free electronics assembly for any solder joint cracking due to overstress caused by improper application of force during assembly (e.g. cutting, assembly stress) or generated in the functional tests (e.g. plug-in/out, in circuit test – probe force).
Fortunately, the medical electronics industry can benefit from those problems concerning the lead-free conversion by the consumer electronics industry in the last few years. Nonetheless, the lead-free conversion has its apparent risks and difficulties. From the experience of lead-free conversion by the consumer electronics industry, to effectively control the product quality after conversion, the management of the company should give full support and also concentrate available resources on the reliability verification and analysis for lead-free products, which are of utmost importance. Some companies may also use this opportunity to carry out a comprehensive quality diagnosis for improvement, e.g. assembly control and work flow improvement, supplier re-evaluation, supply-chain management skills, and knowledge buildup.
Some companies may even use this opportunity to carry out a comprehensive update or look for more effective reliability verification and a better way for failure analysis, which certainly includes the phasing-out of some out-of-date testing procedures and the like. This article will describe some common methods of reliability testing for conversion to the lead-free assembly. Also, this will provide ideas on how to develop a lead-free reliability test program and our recommendations.
Classification of application ratings
Depending on the application requirements of the medical electronics products, we can develop a prudent reliability verification strategy for the lead-free conversion process with reference to the FDA rating scheme. For example, the equipment can basically be divided into 3 ratings depending on its application:
Equipment rating 1
• Equipment that uses less layers of PCB.
• Equipment that does not use components such as BGA or Flipchip.
• Equipment that has an expected service lifespan of 3-5 years.
• Equipment that has no critical correlation with the patient’s life in its application.
Equipment rating 2
• Equipment that uses thicker and multiple layers of PCB.
• Equipment that uses components such as BGA and Flipchip.
• Equipment that has an expected service lifespan of 5-7 years.
• Equipment that has no critical correlation with the patient’s life in its application.
Equipment rating 3
• Equipment that has an expected service lifespan of at least 5-10 years or longer
• Equipment that has a close correlation with the patient’s life maintenance
• For different level products, SIR and CAF related tests must be conducted
• Where the tin whisker risk shall be evaluated more cautiously
Three critical stages of reliability
To meet the high reliability requirements of medical electronics products, it is important that we shall clearly define the requirements of each stage in product development, which will normally be divided into 3 stages.
Stage 1: Selection of materials and parts
1-1 Selection of materials
• Reflow solder paste, alloy and flux (rosin flux, non-clean process, etc.)
• Wave soldering alloy and flux
• Number of layers of PCB
• PCB surface treatment (e.g. Im.Ag, ENIG, OSP, SAC HASL, etc.)
• PCB copper layer thickness, blind / buried via sizes
• Protective coating material
• Rework material
The goal of this stage is to verify and select the materials to be used by the end product using an experimental method. The quality of the assembled product will best be assured only if these materials conform to the reliability requirements. After being tested by the most common verification methods for PCB, such as reflow simulation, solderability, thermal shock, conductive anode filament, surface insulation resistance, copper thickness value, Tg/Td measurement, and CTE (coefficient of thermal expansion) analysis, the materials will further be inspected by a failure analysis method (e.g. X-ray, SAT, Cross-section, SEM, TEM, etc.) to ensure the quality in actual assembly practices.
1-2 Selection of parts
• Confirming the moisture sensibility level of IC components
• Confirming the electromagnetic radiation interference strength level of IC components
• Confirming the electrostatic discharge capability of IC components
• Confirming the service life of IC components
• Confirming the risk rating of tin whisker; confirming thermal resistance capability of parts
• Confirming soldering ability of parts
Stage 2: PCBA reliability test
• This is a very important stage for the end products. At this stage, the reliability verification is carried out in real PCB assembly practices. Its purpose is to confirm that after lead-free electronic assemblies at an elevated temperature, the active/passive parts and PCBA are not affected in their reliability. In the meantime, the solder joint quality and the assembly capability of the production line are also verified.
• The test items normally include thermal cycling, mechanical shock, vibration and HALT (high acceleration life test). In the process of each test, analyses such as stress/strain, resonance response, shock frequency spectrum response, voltage/current/temperature distribution can also be used to assist in identifying the causes of failure so that the defects can be corrected.
• Selection of test sample quantity has a correlation with the confidence of the result of each test item and the overall testing costs. Therefore, both parties of a trade shall negotiate based on product market price, repairable failure cost, warranty, or seek recommendations from a professional laboratory.
• Before and after the reliability test, aside from the appearance check and functional tests of PCBA, the quality shall further be verified with an analytical means (such as SAT, Cross section, SEM, Dye/Pry, etc). Only in so doing can the correctness and trustworthiness of each testing data result be ensured.
Stage 3: Finished product qualification verification
• At this stage, the final product is to be verified after completion of assembly in the production line. Normally, it focuses on how to handle the environmental stress that may appear in the product life cycle and the test is often conducted in a way that the product meets a certain test specification. Test items include thermal cycling, temperature, humidity, thermal shock, vibration, mechanical shock, drop, altitude, corrosion, etc. For products that are portable or suitable for outdoor use, items like salt, solar, rain, dust, water immersion,, etc. are also taken into consideration.
• It is important that after tests, while detailed appearance and electrical characteristics are checked, any physical failure of the product shall also be confirmed. For example, it is likely to misjudge a broken solder joint even if it still has sufficient strength in contact. Therefore, after testing, it is mandatory to check for any cracking of the solder joint, delamination of PCB, etc. by using cross-section or dye and pry, particularly after the thermal cycling, vibration and mechanical shock tests.
Developing a reliability test program
• Once the strategy and framework of overall conversion to a lead-free assembly has been finalized, the reliability test program can start to be developed. The test program shall include product ratings, test items, parameters, measuring methods, number of samples, testing procedures, and definitions of “Pass” and “Fail” for each test, etc.
• In addition to the traditional qualification verification, the test shall also include additional test items and inspection methods that can evaluate the reliability of lead-free products while in transition to the lead-free solder.
• Investigate resources to make sure that the capability of each verification item and analysis or its source is made available. It is also advisable to choose and cooperate with a laboratory that has practical experience in complete supply chain verification.
Malaise of product in the corrosive gas environment
• Global air quality has continuously been worsening in the last two years. The failure cases of electronic products due to corrosion problems have increased year by year. The survey report of 4 different industries (Note 3) by the International Electronics Manufacturing Initiatives (iNEMI) in 2009 has shown that 67% of 45 interviewees from different industries have seen product failure due to corrosion. Further investigation also showed that 90% of MTBF (mean time Before failure) fell at 1-3 years. Most cases of corrosion appear at non-soldering places, e.g. copper exposure on the PCB or non-wetting part, out of which the majority are silver plated boards. Some failures also appear on silver-plated terminals. Most failures caused by creeping corrosion lead to a short-circuit.
• Most current international standards use 70% of humidity mixed with 20-200 ppb sulfide gas and chlorine as the test standard. But some of the recent international survey results have shown that the concentration of the gas mixture specified in current standards is not able to reflect the real conditions that cause product failure. Some international organizations (e.g. IPC, iNEMI) have organized task forces to seek a more representative test method.
• Recent research has found that high content of sulfide gas in the environment is the major cause of PCB failure by corrosion. But the corrosion severity is difficult to quantify with figures. Accordingly, increasing the concentration of sulfide gas may be a plausible way of evaluating resistance of lead-free products in a corrosive environment. This method can also be used to evaluate the resistance of gold-plated surfaces of alloys, flexible flat cables (FFC/FPC) and golden fingers in a corrosive environment.
Along with the evolution of technology, the density of modern electronic circuits has become increasingly greater, lessening the amount of free space as compared to before. It can be expected that the creeping corrosion problem will receive higher concern. The iNEMI is expediting its study of the effects of corrosive environments to PCB, flux, etc. that have different surface treatments.
As the year 2014 is approaching, the medical electronics industry is facing more and more pressure on lead-free conversion. Fortunately, the consumer electronics industry introduced the lead-free assembly process in 2006. Some components and PCBs have met the lead-free requirements. Some lessons have also been learned from the reliability and failure modes of the tin-lead solder (SAC). Also, some international organizations have published relevant specifications of reliability verification of lead-free products for reference.
Therefore, in the face of the lead-free transition period, it is important to seek an experienced laboratory and, according to different product ratings, develop a testing program that can identify product defects and verify product reliability. This program shall include the reliability verification items, analysis method, and determination criteria of IC components, PCB, PCBA, finished products, etc.
A good quality product relies on every one in the supply chain. The product reliability verification at different stage shall be supported with failure analysis technique so that defects can be identified and then be corrected. With assisting the major local and international manufacturers, we obtained years of experience on lead-free product reliability verification / analysis. Our laboratory is also the only one across both sides of the Strait that has the reliability verification and failure analysis capacities in the entire supply chain. We welcome customers to visit us and engage our technical experiences.
1. RoHS: Restriction of Hazardous Substances directive for the electronic and electrical equipment.
2. RoHS directive control scope: Lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB) and polybrominated diphenyl ether (PBDE)
1) Lead: often used in lead tube, oil additive, packaging material, plastics/rubbers, dye, pigment, paints, electronic components, etc.
2) Mercury: often used in battery, packaging material, thermometer, electronic components, etc.
3) Cadmium: often used in packaging material, plastic/rubber material, stabilizer, dye, pigment, paint, electronic components, surface treatment, etc.
4) Hexavalent chromium: often used in packaging material, plastics/rubbers, dye, pigment, paint, electroplating treatment, surface treatment, etc.
5) Polybrominated biphenyls (PBB): often used in PCB, electronic components, wire flame-retardant, etc.
6) Polybrominated diphenyl ether (PBDE): often used in PCB, electronic components, wire
3. Industrial and automotive industry, telecommunication and server industry, computer and its peripheral industries, consumer electronics industry.
1. A Validation Method to Approach Creep Corrosion Occurrence on PCB/SMTA China East Technical Conference, 2011 (CE11-TC1.5)
2. Creep Corrosion on Lead-free Printed Circuit Boards in High Sulfur Environments/SMTA Intel Proceedings, Orlando, FL, Oct. 2007
3. Cherie Chen, “Investigation of Factors That Influence Creep Corrosion” / MPACT, 2011
4. Randy Schueller, “The revised RoHS directive will require medical electronics manufacturers to adopt new reliability testing strategies” available from Internet: http: // www.medicalelectronicssdesign.com/article/ get-lead-out
5. J Bath et al., "Comparison of Thermal Fatigue Performance of SAC105 (Sn-1.0Ag-0.5Cu), Sn- 3.5Ag, and SAC305 (Sn-3.0Ag-0.5Cu) BGA Components with SAC305 Solder Paste," (Haverhill, MA: Circuitnet); available from Internet: http://www.circuitmart.com/pdf/comparison_thermal_fatigue.pdf.
6. N Pan et al., “An Acceleration Model For Sn-Ag-Cu Solder Joint Reliability Under Various Thermal Cycle Conditions,” inSMTA International Conference Proceedings, (Chicago: Small Mount Technology Association, 2005); available from Internet: http://www.smta.org/knowledge/proceedings_abstract.cfm?PROC_ID=1815.