Getting ready for lead-free medical electronics.
Eutectic (or leaded) and lead-free designs are widely different. Lead-free sub-assemblies require, among other things, a higher-temperature assembly reflow. This means designing in lead-free components, laminates, surface finishes, solder pastes, and designating correct thermal profiles to comply with those higher temperatures. Plus, extra care and attention are vital for hybrid (eutectic and lead-free) designs.
The system designer must carefully designate these specific requirements in both fabrication and assembly notes. Those documents and notes are then passed on to printed circuit board (PCB) fabrication and assembly houses to finalize a lead-free product.
Overlooking any of these areas during the design layout phase can have devastating effects at assembly. Hence, it is imperative that system designers get a good handle on lead-free issues to maintain the high-reliability their previous eutectic-based product lines have maintained.
The reflow profile graph (Fig. 1) compares eutectic and lead-free reflow profiles with melting points of 205[degrees] to 230[degrees]C for eutectic solder and 235[degrees] to 255[degrees]C for lead-free solder. Eutectic assemblies have a 25[degrees]C tolerance or thermal profile window, which allows sufficient flexibility to create an effective solder joint. However, due to the higher temperature cycles associated with lead-free solder pastes, the tolerance window or temperature profile drops down to 10 to 12[degrees]C, thereby requiring a more precise thermal profile and process controls.
[FIGURE 1 OMITTED]
Consequently, the width of the temperature cycling window is significantly reduced, placing more attention on highly accurate temperature zones. With this in mind, system designers should pay special attention to lead-free PCB laminates, flux, solder pastes and thermal profiling.
At PCB design layout, calculations for impedance control need to be made to assure that the differential impedances the PCB fabrication vendor has specified are correct. These impedances could be single ended, dual strip-line or micro strip line. The purpose of these impedances is to make sure all signals coming out of the different components get to their destination as clean as possible with minimal noise or cross-talk in those signal transmissions.
Differential impedance is calculated based on the particular material, its characteristics, thickness, and level of internal layers in relationship to the ground plane. Different PCB laminates have different core thicknesses and characteristics. Impedance is calculated based on a particular laminate's thermal and electrical characteristics. If a different PCB laminate is used to accommodate lead-free manufacturing, then this calculation changes; meaning the system designer or fabrication vendor must use different cores and thicknesses with perhaps different dielectrics characteristics and pre-preg material to fabricate the board.
It's important to note that higher temperature laminates are normally used for lead free fabrication requirements such as FR406, IS410 or HR 370 that can withstand upwards of 270[degrees]C or higher.
As for surface finishes, hot-air solder leveling or HASL is used for eutectic PCBs, but is not appropriate for lead free assembly. For lead-free assembly, there are such PCB surface finishes as electro-less nickel immersion gold or ENIG, immersion silver, organic solderability preservatives (OSP), and a special Pb-free brand of HASL mostly used in ASIA to do Pb free assembly.
Surface finishes are important to lead-free PCB fabrication because conductivity of immersion silver and immersion gold is considerably higher compared to tin lead used for eutectic soldering. These finishes withstand higher temperatures and there is less likelihood of the pads being peeled off from the board surface when it is exposed multiple times to higher temperature cycles.
Design tradeoffs involved with there surface finishes are shelf life, cost, reflow cycles, and solder joint flatness. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost five to 10 percent extra, depending on the amount of exposed surfaces. Therefore, it's best from a cost/performance point of view to select a finish that can be cost justified in a particular end product
As for shelf life, HASL has about 18 months, but OSP has only six months. Immersion silver has a shelf life ranging from 12 to 16 months, while immersion gold is the most durable at 24 months.
OSP cannot undergo more than two to three reflow cycles. If the need for rework continues, then the surface mount (SM) pads on the OSP finish begin peeling off. Thus, OSP is not the best finish for rework. On the other hand, immersion silver or gold can undergo six to eight reflow cycles.
Using immersion gold or immersion silver comes with an added advantage, which result in a flatter PCB surface finish. Consequently, the flatter PCB surface finish is considerably more conducive to a perfect PCB assembly compared to HASL finish.
Some medical electronics systems may require the finish to have low contact resistance throughout a product's life time. Contact resistance is in two forms: (1) touch pads. such as keyboards and (2) plated edge rails, which make electrical contact with the system's chassis.
As for Pb-free solder paste, selecting the most efficient one for the surface mount process and specific application is a critical variable. Selection criteria include print speed, tack time, re-flow window, voiding potential, and several others factors, If paste is chosen with care and the surface mount process optimized, lead-free transition is achieved without jeopardizing reliablity and product yields.
Otherwise, defects like voids are associated with lead free assemblies. A void is a lack of solder paste on the SMT pad creating a solder deficiency (Fig 2). Other common defects associated with lead-free assemblies include open joints, non-wetting, mid-chip solder balling, and others. These defects can be avoided in a properly optimized process with a lead-free solder paste designed to provide adequate wetting, low slumping, and low voiding with Pb-free alloys.
[FIGURE 2 OMITTED]
Lastly, applying a wrong thermal profile without care-fully considering PCB materials, surface finishes, and/or the right-free solder can have catasterophic effects during assembly. This is particularly true for hybrid sub-assemblies fitted with both eutectic and lead-free components.
When outsourcing lead-free designs, a most important OEM precaution is to avoid selecting separate design layout, fabrication, and assembly contractors for a single lead-free PCB project. For example, using an inexperienced lead-free PCB design house may inadvertently spec out fabrication material using a leaded HASL surface finish. Or, it may unknowingly use a leaded surface mount component on the PCB design stage that only withstands 230[degrees]C. In both cases, severe damage results, destroying expensive assemblies.
By Zulki Khan
NexLogic Technologies, Inc.
Zukki Khan (Zk@nexlogic.com) is the Founder and President of NexLogic Technologies, Inc., San Jose, CA, an ISO certified since 1995 and is a RoHS compliant EMS provider. He conducts lead-free PCB workshops and is a frequent author of contributed articles to EMS industry publications.
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|Title Annotation:||Medical Electronics|
|Publication:||ECN-Electronic Component News|
|Date:||Nov 15, 2008|
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