Medical electronics products have been exempt from the European Union's (EU) Restriction of Hazardous Substances (RoHS) directive since it went into effect July 1, 2006. However, compliance is right around the corner because medical electronics are expected to be included in the directive's next revision scheduled for about 2010.

Today, RoHS and ‘lead free’ are synonymous, but the directive also covers cadmium, mercury, hexavalent chromium, and flame retardants used in new electronic and electrical equipment sold in EU countries.

The savvy medical-electronics OEM is well on its way to RoHS and lead-free compliance by taking early steps to learn about lead-free procedures and processes, while organizing and implementing its resources. But for procrastinating or complacent OEMs, costly and time consuming issues are looming because the OEM landscape is rapidly changing.

Supplier transition from leaded to lead-free components and subassemblies is among the most prevalent changes. It's been so quick that many suppliers are placing leaded components on a non-cancelable list and a non-returnable availability. In some instances, suppliers have stopped producing certain leaded components because there is little demand.

This change translates into costly and time-consuming redesigns with alternative lead-free components and circuit boards. A completely new PCB design layout incurs a major engineering cycle to include new board fabrication and assembly. It also involves OEM management approval and quite likely, FDA reapproval. All these approval cycles devour considerable time, excise large dollar slices from time-to-revenue, and perhaps create loss of market share.

But issues don't stop with this change, because there is a whole gamut of things that can go wrong when OEMs wait too long to engage lead-free and then hasten the transition. Major areas of concern include PCB materials and surface finishes, solder joint reliability, incorrect thermal profiles during assembly, hybrid lead and lead-free assemblies, and using separate PCB design layout, fabrication, and assembly contractors.

Using leaded materials such as FR4 in lead-free boards can severely damage the PCBs because a lead-free reflow temperature profile ranges from 255 to 260C compared to the lower leaded range of 230 to 235C. Also, board surface finishes must withstand these higher reflow temperatures. Lead-free finishes include electroless nickel immersion gold, immersion silver, tin, organic solderability protectants, and a special lead-free brand of hot-air solder leveling (HASL) used mostly in Asia.

Solder joint and assembly reliability come when trained specialists carefully implement process requirements. Variables to consider include the alloy's melting temperature, wetting characteristics, surface-tension properties, solder balling and bridging, and cosmetic effects of flux at higher reflow temperature.

Applying a wrong thermal profile without considering PCB materials, surface finishes, or the right lead-free solder can have catastrophic effects during assembly. The same holds true when poor PCB design and leaded and lead-free components are incorrectly used on hybrid assemblies. Lastly, the most important OEM precaution is to avoid selecting separate design layout and fabrication contractors for a single lead-free PCB project. For example, using an inexperienced lead-free PCB design house may inadvertently spec out fabrication using a leaded HASL surface finish. Or, it may unknowingly use a leaded surface mount component on the PCB's bottom side that only withstands 230C.

Hence, design layout, fabrication, and assembly done by one EMS provider or contract manufacturer (CM) are linchpins for successful lead-free PCBs. Close coordination among these entities is vital to avoid miscues.

Tardy OEMs should now move quickly toward lead-free. A prudent route takes direction from a lead-free EMS provider or manufacturing veteran capable of teaching the basics of lead-free. Savvy resources such as these can help OEMs effectively transition a product from leaded to lead-free by avoiding pitfalls and costly mistakes.

A first step is to experiment with a small prototype run of 10 to 20 boards to determine whether or not all critical lead-free elements are in place to produce an accurate product. If an OEM has 10 different product lines each with three or four boards, there may be 40 to 60 PCBs that must transition to lead-free. In cases like this, the OEM may want to consider testing a sample of five boards to gain confidence with this transition process.

Time is of the essence because some leaded components are fast becoming obsolete. If certain designs call for maintaining these particular components, it is wise to buy sufficient inventory. But these are things OEMs cannot know until they do a pilot run and get a good handle on lead-free.

How different board finishes compare

The chart shows trade-offs associated with different finishes. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost 5 to 10% extra, depending on the amount of exposed surfaces. Therefore, it's best from a cost-performance standpoint to select a finish that's cost justified in a particular medical electronics product. It is important to note that OSP cannot undergo more than two to three reflow cycles. If extra rework is required, surface mount 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.