Lead-free DFM: demands new tricks: lead-free DFM is a prime target for Murphy's Law. So make sure every detail is handled accurately.

Leaded design for manufacturing (DfM) involves proven, well-established practices in PCB design layout, fabrication, assembly and component management. However, lead-free DfM significantly changes the landscape of each of those four areas. OEMs outsourcing lead-free PCBs, but not completely savvy about lead-free DfM issues, can fall victim to costly re-work and lost time-to-revenue if they are not in tune with their EMS providers.

At PCB design layout, key concerns should focus on component footprints, placement, routing and clean up during post-processing after the layout is finished. Also, the fabrication and assembly drawings are generated as output of the layout process as seen in FIGURE 1. These fabrication and assembly drawings are of utmost importance going forward in the DfM process because they call out for special materials and PCB surface finishes that can withstand the higher temperature required for the lead-free process. There are other call outs that are required in the assembly process, like thermal profiles, special flux requirements and some additional process changes.

[FIGURE 1 OMITTED]

It's critical for the OEM to understand that packaging and footprints of leaded and lead-free components can be different, and that the differences could be the basis of a costly manufacturing error. Lead-free components are packaged to withstand higher reflow temperatures and they may or may not have different footprints. Therefore, it is important that component footprints, package types and associated component spec sheets be carefully studied during the PCB layout process.

This particular DfM step can be easily overlooked, and as a result, cause the OEM to re-spin a first article PCB at a cost ranging from few hundred dollars to an excess of $20,000. A case in point is when an OEM gives the EMS provider a lead-free PCB order with the wrong spec sheet for the most important component, which was designed at the lead-free assembly level. The footprints will not match, resulting in re-spinning of the bare board, which is expensive. Instances like this are rare, probably arising one or two times out of hundreds. Still, they occur and usually stem from OEM engineering assumptions and lack of careful spec sheet review.

Component placement can also introduce new and costly mistakes, especially if the PCB is a mixed technology board with leaded- and lead-free components. Keep in mind that these components require different thermal profiles. This issue becomes even more significant in case of rework. For example, removing a lead-free BGA from the PCB requires a different nozzle with more heat on the periphery to pull it since a higher temperature is required to melt the solder properly (FIGURE 2).

[FIGURE 2 OMITTED]

Leaded and lead-free components should be properly and clearly specified on the assembly drawing, leaving no doubts, ambiguities, misunderstandings or room for faulty assumptions. Detailed instructions are especially important for mixed technology PCBs to include how engineering change orders are to be addressed and how clearly they are listed in the assembly drawing. A poorly documented assembly drawing can result in issues such as melted solder balls of a leaded BGA or even completely melted ICs.

An example of poor OEM documentation would include failure to specify which components are lead-free, especially problematic for a mixed technology board. In such a situation, the lead-free components would fall off the board after the board underwent reflow. The reason? The assembly did not undergo the correct high temperature thermal profile; hence those components were not properly soldered on the board. This particular type of error is hard to catch even with a good quality control process in place, because in some cases the part number of the leaded component and the lead-free component is the same.

Fabrication

PCB material types play a major role in lead-free DfM. Considering that lead-free projects require a higher temperature profile, it is important to use base material that is compatible with lead-flee fabrication and assembly of the PCB. FR-4 has been the traditional material for leaded-based PCBs. Laminate materials with higher thermal profiles such as FR408 are more suited to lead-free assembly. TABLE 1 shows a thermal profile comparing traditional FR-4 and the higher Tg material, FR408.

During the material qualification process spec sheets, the manufacturer's reference data and other documentation require careful review to fully understand what the best PCB laminate material based on what characteristics it needs to have for processability. If clarification is needed, it is best to directly contact the manufacturer and ask specific questions. The EMS provider must also continuously calibrate and fine-tune its assembly and manufacturing equipment. It must be in top-notch shape so the equipment can effectively and accurately handle the tighter lead-free assembly process windows compared to leaded assembly.

Since these quick thermal cycle windows are so closely linked to lead-free assembly, there is little room for margin of error. With leaded thermal cycles, there is 20[degrees] to 30[degrees] latitude, and components can stay in reflow another 30 to 60 seconds without adverse effects. That's not possible with lead-free because the operating window is considerably shorter.

In many cases the lead-free PCB surface finishes are different from those used for conventional leaded PCBs. Hot-air solder leveling (HASL) based on leaded solder is common for leaded boards. A lead-containing material is not compatible with lead-free assembly. For lead-free assembly, there are PCB surface finishes such as electroless nickel/immersion gold (ENIG), immersion silver, high temperature organic solderability preservatives (OSP) and a special lead-free HASL. These surface finishes are important to lead-free PCB fabrication. They need to withstand the higher temperatures with out affecting their wettability and be compatible with the lead-free assembly materials.

As seen in Table 1, several tradeoffs characterize these surface finishes. Shelf life, cost, reflow cycles and solder joint flatness are all unique. Electroless nickel/immersion gold and immersion silver processes use precious metals. These finishes could cost extra, depending on the amount of exposed surfaces. Therefore, its best from a cost/performance point of view to select a surface finish that can be cost justified in a particular end product application.

Assembly: More Defects

Since lead-flee components require higher temperatures at assembly, the routine that is critical to follow involves careful study of component spec sheets, thermal profiles, flux and solder pastes and their different temperature characteristics that help to create a first article. A first article in DfM is important for any type assembly, but more so for lead-free since different thermal profiles are used. Also, for lead-flee PCB assembly the search for possible defects becomes more intense than leaded assembly. Quality control measures embedded in assembly processes must get more detailed attention than that given to leaded assembly. The possibility exists for lead-free related defects such as BGA voids, tomb-stoning effects and tin whisker formation.

The importance of using advanced equipment and trained technicians to target these defects cannot be understated. High-caliber AOI and high-resolution x-ray systems are top priority to track down problems. These systems and their supporting software not only find defects, if any exist, but they'll also verify whether they are true defects (FIGURE 3).

[FIGURE 3 OMITTED]

Labeling equipment for lead-free use is another aspect of effective DfM. This is important to avoid the possibility of mixing regular leaded processes with lead-free and incurring extra cost and time delays. Again, technicians and other support personnel should be properly trained to manage and operate these lead-free systems. A trained and educated eye can detect whether a component or assembly is faulty.

Component Management

The buyer is the linchpin for effective lead-free component procurement. They must be well trained, educated and qualified to do this job efficiently. This position within an EMS provider is critical as some component vendors fail to change component nomenclature to clearly identify lead-free components. The burden and responsibility therefore fall on procurement to cross-referencing leaded parts being replaced with lead-free devices, plus the buyer must be able to suggest the correct drop-in replacement equivalents. Procurement management and a staff of trained personnel should also be familiar with incoming component inspection to accurately separate and detect leaded and lead-free components. The group should include strong inspection criteria to closely review and verify component value, footprint, package type and lead-free status.

A lead content detector device is an important tool for incoming inspection management. It not only checks for lead content levels, but also searches for all the other RohS substances to ensure the component is compliant. Incoming inspection prevents the use of wrong leaded components, and serves to verify questionable components coming from brokers. The best practice is to buy from reputable manufacturers or resellers. While brokers also sell components, in some cases they are pulled from discarded PCBs or recycled systems. Using pulled components is not a good practice as their reliability may be questionable. As for component changes or replacements during DfM, they must come via a properly documented engineering change order and they, in turn, should be directly released from the document control department. This procedure is key and should not deviate because it ensures that a repeated project will be accurately supported by its associated documents. This is especially valuable for lead-free DfM because those PCBs require considerably more attention to detail.

Lastly, "return to stock" is an equally important aspect of lead-free DfM. Here, crosschecks are vital to make sure lead-free and leaded components are separated when they're returned to inventory/stock. These different components are relatively easy to keep separate at incoming. But the return to stock process can be difficult and error-prone when components are in a bin with thousands of others that may or may not be lead-free.

ZULKI KHAN is president and founder of Nexlogic Technologies in San Jose, CA. He can be reached at [e-mail].

TABLE 1. Various Lead-Free Materials *

SUPPLIER    MATERIAL         RESIN             TG           TD
              TYPE           SYSTEM        ([degrees]C  ([degrees]C
                                              DSC)         TGA)

Polyclad     FR 226           FR-4             140          320
             FR 370           FR-4             170          310
            FR370HR    FR-4 with Phenolic      180          350
             Getek            PPO              180          340
             LD 621           APPE             190          400

Isola        FR 406           FR-4             170          290
             FR 408      Modified FR-4         180          360
             IS 410    FR-4 with Phenolic      180          350
             IS 415      Modified FR-4         220          360
             IS 620      Modified FR-4         205          330

Nelco      N4006-6FC          FR-4             175          300
            N4000-11   FR-4 with Phenolic      175          360
            N4000-13     Modified FR-4         210          350
            N4000-12     Modified FR-4         190          370
           N4000-13SI    Modified FR-4         200          350

SUPPLIER    MATERIAL       LEAD-FREE         HALOGEN
              TYPE         COMPATIBLE         FREE

Polyclad     FR 226            No              No
             FR 370            No              No
            FR370HR           Yes              No
             Getek            Yes              No
             LD 621           Yes              No

Isola        FR 406            No              No
             FR 408           Yes              No
             IS 410           Yes              No
             IS 415           Yes              No
             IS 620           Yes              No

Nelco      N4006-6FC           No              No
            N4000-11          Yes              No
            N4000-13           No              No
            N4000-12          Yes              No
           N4000-13SI          No              No

* This is not a complete list of vendors or materials.