-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Industry View: Medical Electronics OEMs: Prepare for Lead-free
December 31, 1969 |Estimated reading time: 4 minutes
The old saying, “It’s never too late,” applies to some Johnny-come-lately medical electronics OEMs that aren’t ready to comply with the EU’s upcoming new RoHS directives. The initial 2006 directive didn’t include medical electronics, but now RoHS will. This means medical electronics OEMs must discard eutectic SMT-based designs and think of SMT lead-free products.
In the past year, I’ve worked with leading medical electronics OEMs to prepare them to comply with this new directive. Companies like these have organized plans to enter into lead-free product designs successfully, and thus have a competitive edge. Others who have remained complacent and have not given lead-free much thought may be in for some costly surprises.
Key questions asked during our lead-free workshops involved the exact time for medical products’ RoHS exemption to expire, the durability and reliability of medical applications using lead-free materials, and details associated with tin whisker formation that can cause latent field defects, among a host of other pertinent queries.
Aside from those issues, medical electronics OEMs must ensure lead-free components are used on assemblies, and not eutectic ones. This may seem like an elementary step that can appear as a no-brainer. But lax practices in this regard can lead to costly mistakes. For example, SMT component selection during PCB layout can have adverse effects at assembly if one or more components don’t meet the higher lead-free temperature requirements.
It is vital to identify the component and its package clearly to determine whether or not it can withstand certain reflow temperatures. Can it be reflowed at temperatures in excess of 250° or 260°C? If so, it suits use in the lead-free temperature profile, ranging from 255° to 260°C. But if the selected component can only withstand 230° to 235°C, then it is important to replace the component with one that is properly packaged to reflow at higher temperatures.
Other areas that demand close attention include thermal profiles, PCB laminates, board surface finishes, and solder pastes.
Thermal profiles for lead-free assemblies require considerably higher reflow temperatures than those for eutectic ones. OEMs and their EMS providers must check that eutectic thermal profiling isn’t called for inadvertently in assembly notes. Lead-free solder has a higher melting point than solders used for eutectic assemblies. Eutectic solder melting points range from 205° to 230°C; lead-free solder’s melting points hit 235°–255°C.
Eutectic assemblies have a 25°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 for a temperature profile drops to 10°–12°C. Consequently, the width of the temperature cycling window is reduced significantly, placing more attention on highly accurate temperature zones.
FR-4 has been the standard for eutectic PCB laminate. Now, different laminates with higher temperature cycle ranges are required for lead-free assembly, such as FR-406 and FR-408, and IS-410. As for surface finishes, hot-air solder leveling (HASL) is used for eutectic boards, but is not conducive to lead-free assembly. For lead-free assembly, there are such PCB surface finishes as electroless nickel immersion gold (ENIG), immersion silver (ImmAg), and organic solderability preservatives (OSP).
These 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 SMT pads being peeled off from the board surface when it is exposed multiple times to higher temperature cycles.
There are several tradeoffs involved with these surface finishes. Shelf life, cost, reflow cycles, and solder joint flatness all require attention. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost 5–10% percent extra, depending on the amount of exposed surfaces. Therefore, it is best from a cost/performance point of view to select a finish that can be cost-justified in a particular end product.
HASL has about 18 months shelf life, but OSP has only six. 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 reflowing continues, as for rework, then the SMT pads on the OSP finish begin peeling off. Thus, OSP isn’t the best finish for PCB applications that may require considerable 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, a flatter PCB surface finish. The flatter PCB surface finish is considerably more conducive to a perfect PCB assembly, when compared to HASL finish.
As for lead-free solder paste, the rule of thumb is to select the most efficient one for the surface mount process, and the specific medical device application is a critical variable. Selection criteria include print speed, tack time, stencil life, reflow window, voiding potential, and several others. If the paste is chosen with care and the SMT process optimized, lead-free transition is achieved without jeopardizing reliability and product yields. SMT
Zulki Khan, founder and president of NexLogic Technologies Inc., 2075 Zanker Road, San Jose, Calif., 95112, conducts lead-free PCB workshops for medical device companies. Contact him at (408) 436-8150 ext. 102; zk@nexlogic.com; www.nexlogic.com.