The impact of lead-free on design and fabrication.

The advent of lead-free manufacturing is impacting PCB design and fabrication, but its effect on design is minimal. Fortunately for designers, in general, the design rules for lead-free soldering are similar to those for tin-lead soldering. Many steps in the design process--bringing up the netlist, making physical aspects of components or footprints, assuring component connections are correct, and making board thickness calculations for impedance control or dielectric constant calculations--do not typically change with lead-free.

When designing a board that's bound for lead-free assembly, the main concern is the physical footprint on the board. If component packaging is manufactured to withstand the higher temperature for lead-free assembly, then there should be no change in the footprint and, consequently, no change in the layout.

However, there can be exceptions and in these instances, it is prudent for the designer to consult with a knowledgeable EMS provider to minimize design and fabrication concerns regarding the lead-free impact on design. For example, SMT component selection can be classified as a hardware issue, but it can have an impact on design layout if a particular component doesn't meet higher lead-free temperature requirements.

Therefore, it is important to clearly identify the component and its package to determine whether or not it can resist the higher reflow temperatures necessitated by lead-free. Because the lead-free temperature profile ranges from 255[degrees] to 260[degrees]C, components that can only withstand temps from, say, 230[degrees] to 235[degrees]C must be replaced.

Another concern when designing for lead-free assembly is a PCB with SMT components populated on both sides. Boards ready for lead-free assembly may have bottom-side glued and resoldered components. These must be able to withstand solder temperature of over 250[degrees]C; if not, they must be shielded using a wave soldering fixture (see FIGURE 1).

[FIGURE 1 OMITTED]

Also, while in design layout, calculations for impedance control need to be made to assure that the differential impedance specified by the fabricator is correct. These impedances could be single-ended, dual stripline or microstrip. The purpose of these impedances is to make sure all signals coming out of these components get to their destination as clean as possible with minimal noise or crosstalk in those signal transmissions.

Differential impedance is calculated based on the particular material's various characteristics, such as its thickness, and level of internal layers in relationship to the ground plane. Laminates have different core thicknesses and characteristics. Impedance is calculated based on that material's thermal and electrical characteristics. If a laminate is selected to accommodate lead-free manufacturing, then this calculation changes, meaning the fabricator must use different cores and thicknesses and perhaps different dielectrics and prepreg material to fabricate the board.

It's important to note that different PCB laminates used specifically to comply with higher temperature lead-free fabrication requirements can affect impedance control calculations. Normally, PCB materials like FR-4, FR406 or FR408 can withstand upwards of 270[degrees]C or higher. But on rare occasions, a different laminate may be required. In those cases, impedance control calculation changes would impact design layout.

Fabrication

The impact of lead-free on fabrication is a different story. Different surface materials with a higher temperature cycle range are required for lead-free assembly. In this case, higher temperature FR-4 materials such as FR406 or FR408 are required. Also, PCB surface finishes will be different; depending on cost and board application, immersion silver or gold or OSP can be used to withstand the higher temperatures of the reflow oven.

There are two considerations involved in preparing a bare board for lead-flee assembly: board thickness and surface finish. If the PCB is thicker than the regular .062", then a specific amount of solder and flux is needed. If it is even thicker than that, say, .093", then the board will require more flux activity to soak the pad. Plus, it will require even more flux. Contact time, bump speed and peak temperature must be increased. Lastly, a nitrogen reflow instead of regular hot air reflow may be considered to make boards more cosmetically pleasing.

As for surface finishes, hot air solder leveling (HASL) is used for eutectic boards, but is not conducive to lead-free assembly. Lead-free assembly requires surface finishes such as electroless nickel immersion gold (ENIG), immersion silver, organic solderability protectants (OSP), and a special lead-free brand of HASL used mainly in Asia to do lead-free assembly.

The reason these surface finishes are so important to lead-free board fabrication is because conductivity of immersion silver and immersion gold is considerably higher compared to the tin-lead variety used for eutectic soldering. These finishes withstand higher temperatures, and there is much less likelihood of the pads peeling away from the board surface when it is exposed multiple times to higher temperature cycles.

As shown in TABLE 1, there are several tradeoffs involved with these surface finishes: shelf life, cost, reflow cycles and solder joint flatness. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost 510% 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.

HASL has a shelf life of approximately 18 months, versus only six months for OSP. Immersion silver has a shelf life ranging from 12 to 16 months, while immersion gold is the most durable, clocking in at 24 months.

OSP cannot undergo more than two or three reflow cycles. If more rework is needed, then the SMT pads on the OSP finish begins 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.

There is another benefit to spending more on gold and silver. Immersion gold and immersion silver bring the added advantage of a flatter surface finish, which is considerably more conducive to a perfect assembly than a HASL finish.

Procurement

Due to the RollS Directive and the need for lead-free compliance, component supplier/EMS provider relations are now more important than ever before. Good lines of communications are essential to ensure that molding compounds used for component packaging are properly incorporated in the EMS providers' temperature thermal profiles to maintain thermal compatibility. EMS providers must make sure that they thoroughly understand the specifications for lead-free components and that there is thermal compatibility with eutectic components in case of hybrid assembly utilizing both eutectic and lead-free components. TABLE 2 shows the differences between temperature cycles on the different zones in the reflow oven for tin-lead versus lead-free assembly.

Component logistics and shelf life can also pose significant issues unless component vendors properly address them. Many major component suppliers are doing their part by effectively labeling lead-free devices. But others aren't, and these are the ones creating the problems. For example, some component vendors are reluctant to devote the necessary resources to separating and labeling eutectic and lead-free components, or even addressing this issue, period. So at times, both types of components are being shipped without proper labeling. Component vendors are concerned about incurring the high cost of making a separate lead-free part number and thereby doubling the inventory. Instead, the problem is passed on to the next level.

Consequently, in these instances, this lack of supplier attention poses a problem for the EMS provider, who must devise an exceptionally well-organized inventory separation system (see FIGURE 2). Separating eutectic and lead-free components is difficult at best, but is a nightmare when part numbers are the same for both leaded and lead-free components. And it's already happening.

[FIGURE 2 OMITTED]

The moisture sensitivity levels of SMT devices represent a shelf life issue. The advent of lead-free has created a major impact on moisture sensitive parts because higher reflow temperatures are required to perform lead-free assembly. Translated, this means higher internal pressure, thereby reducing allowable floor life. In turn, this means the manufacturer must test, retest and reclassify all components as lead-free components.

Studies indicate that these actions result in downgrading components by one to three levels of moisture sensitivity. Hence, some components not considered moisture-sensitive in eutectic assemblies are now considered moisture-sensitive during lead-free assembly. Consequently, the EMS provider must verify the shelf life of these lead-free components. If sensitivity exceeds certain limits, then those components have to be rebaked and retested.

Caveat Emptor 

An OEM should cast a wary eye on the EMS provider's system for separating eutectic components from lead-free components. The last thing an OEM wants is for his boards, supposedly populated with lead-free components, to go through the lead-free reflow oven and come out damaged. This relates back to how QC is conducted for incoming components, how well auditors are trained and the level of professionalism applied to the inventory system. When a traumatic incident like this happens, the EMS provider incurs the cost of its mistakes, but the OEM's delivery time may be significantly delayed as well.

Whoever designs your boards for lead-free assembly must be an expert at lead-free processes; you don't want to risk having a designer inadvertently spec out fabrication using a HASL surface finish or use an SMT component that can only withstand 230[degrees]C on the PCB's bottomside. Some customers have found that they can streamline the process by utilizing a single service provider for lead-free design and manufacturing.

It's vital for PCB designers and manufacturers to be up-to-date and well versed on all design and fabrication areas dealing with lead-free. This includes tracking and implementing lead-free industry developments, understanding how solder base levels are changing, how component vendors are deploying part numbers, which surface finishes are best for given applications, and much more. The lead-free landscape is constantly changing, and the players must continue to educate themselves--and their customers and suppliers--to stay in the game.

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

TABLE 1. Various tradeoffs are involved with selecting the right
lead-free PCB surface finish.

                HASL        OSP        ENIG

Flatness         No         Yes         Yes
Solderjoint     Cn-Sn      Cn-Sn       Ni-Sn
Wirebond         No          No         AI
Cost              $       0.7 X $      3 X $
Reflows           6          2           6
Shelf Life    18 months   6 months   24 months

                 TIN       SILVER

Flatness         Yes        Yes
Solderjoint     Cn-Sn      Cn-Sn
Wirebond         No        Au, AI
Cost           0.8 X $    1.5 X $
Reflows        2 to 3        6
Shelf Life    6 months    12 months

TABLE 2.The chart shows the differences between temperature cycles on
the different zones in the reflow oven for tin-lead versus lead-free
assembly. In a hybrid design, thermal compatibility between the
eutectic and lead-free components is important.

                    TIN-LEAD ASSEMBLY

ZONES       TEMP ([degrees]C)   TIME (sec)

Pre-Heat         100-120          70-90
Soak Time        160-170            90
Ramp                      None
Reflow          Max. 220            60
Cooling           50-60           30-60

                  LEAD-FREE ASSEMBLY

ZONES       TEMP ([degrees]C)   TIME (sec)

Pre-Heat         130-140         100-110
Soak Time        140-170           90
Ramp             170-225           100
Reflow           225-235          15-30
Cooling           50-60           30-60