Lead-free component compatibility

Lead-Free Component Compatibility Takes Center Stage... 

The industry is also exploring benefits of an approach called backward-compatibility, where the component termination is lead-free, but the on-board solder is Pb containing solder—eutectic SnPb in many cases. Such a joint is neither Pb-free nor RoHS-compliant, but can allow for the component suppher to deliver lead-free components while letting the customer keep the assembly materials and reflow profiles mostly constant. 

A number of lead-free component termination finishes have been evaluated (Ref 13, 55) and used over the years. For passive components (such as chip capacitors and resistors), matte Sn plating has been used for many years with the tin-lead solder, and can be used with lead-free solder as well. For leaded components (e.g., quad flat pack or QFP), plating of matte tin or tin alloys may be used with lead-free solders (forward compatible). The tin whisker concern will be discussed in a later section. Nickel-lead has been used with the tin-lead solder for many years, and Ni-Pd-Au is currently an alternative for leaded components for lead-free soldering Ni-Pd typically does not provide as good wett-ahifity as tin. Area array packages with SAC halls are available and work well with the SAC solder. 

Because there is no single lead-free drop-in replacement for eutectic Sn-Pb, there must be a suitable marking system to indicate the particular alloy (and its melt temperature) used for assembly. This is necessary for an operator to subsequently remove a component and select a metallurgically compatible replacement component (i.e., compatible with the rework alloy, component termination finish, and original assembly alloy). Numerous investigations indicating a variety of lead-free finishes compatible with the Sn-Ag-Cu family of alloys have been conducted [64]. 

Component lead finish is another important aspect that must be considered to achieve total lead-free implementation. The matter of component and PWB finish is discussed in detail in Chap. 12. Both Ni/Pd and Ni/Pd/Au are among the major surface finish candidates for lead-free component leads. Important aspects to consider are the adhesion of a lead finish to the molding compound and process compatibility. The typical thicknesses for these finishes are listed in Table 13. Inadequate adhesion can lead to delamination, followed by moisture ingress and possible popcorn effect. 

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

Resin system CTE values above Tg are much higher than below Tg. Z-axis expansion induces stress on plated vias.The higher temperatures of lead-free assembly result in more total expansion for a given material. Several mature lead-free-compatible materials incorporate inorganic fillers that reduce CTE values. X-and y-axis CTEs are also important for reliability at component and layer interfaces. 

In SMOBC processing, the metal-plated resist is removed to present a flat, clean copper surface for solder mask definition. Tin/lead alloys can be stripped in oxidizing fluoride solutions such as fluoboric acid and hydrogen peroxide or ammonium bifluoride with hydrogen peroxide or nitric acid. (Caution machine construction must be made compatible with fluorides by elimination of titanium and glass components.) Commercial formulations are available to be used inline after the etch machine rinses. Accumulations of spent solution or filtered lead-fluoride deposits must be treated as hazardous waste and have been accepted by solution vendors for treatment and disposal costs. Modern applications usually use lead-free tin plating resists, which can be fluoride containing as previously discussed, or compounds of ferric chloride... 

Parts must be suited for the temperature required by the reflow process. Lastly, ensure that component contact or surface finish is compatible with the solder being used, be it leaded or lead-free. 

Backward Compatibility. On smah PWBs with relatively few components, it is possible to convert ah the components to lead-free assemblies at once. However, in the transition to lead-free assembhes, there will be instances where not ah the components on a complex... 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

Assuming that the components can meet the temperature requirements as discussed above, tin and Ni-Pd-Au platings are generally considered to be forward compatible with lead-free solder, as well as backward compatible with the tin-lead solder. This makes it much easier to manage production lines with the tin-lead solder and lead-free solder in co-existence within the same factory during the transition to lead-free. 

Managing the compatibility issues is critical to lead-free transition. These include materials compatibility (solder, components and PWB), process compatibility (reflow, wave soldering, rework, equipment, and yield), design compatibility, reliability compatibility, and business compatibility (cost, supply chain, and operations). 

Area Array Assemblies with Sn-Pb Balls and SAC or Sn-Pb Paste. Another scenario of interest during the transition to lead-free technology is that of conventional area-array components using Sn-Pb balls assembled with SAC paste. This scenario is often described as a forward compatibility situation. In Fig. 11, we show cycles-to-1% failure for Sn-Pb ball area array components assembled with SAC paste versus cycles-to-1% failure for similar components assembled with Sn-Pb paste. The data was gathered from relevant test cells in several independent studies (Ref 3,11,12, 22, 29, 32, 33). Figure 11(a) shows the data for assemblies that were cycled between — 40 and 125 °C (— 40 and 257 °F) (6 data points). Figure 11(b) shows similar test data for thermal cycling under milder conditions 0 to 100 °C (32 to 212 °F) (7 data points) and 15 to 95 °C (59 to 203 °F) (2 data points for 144 Input/Output PEG As assemblies with Ni-Au or Sn-Cu HASL board finish). 

With lead-free materials, process engineers need to understand the specification of every component used on every product manufactured. Assuming that a component s temperature tolerance and lead finish is compatible with a lead-free manufacturing process is a mistake. Many products may be produced that will not pass internal testing or will fail prematurely once they are delivered to customers. Early product life failures are the most costly defects and result in expensive repairs, customer dissatisfaction, and customer loss. 

Developing a lead-free SMT process requires planning and a close-working relationship with all suppliers. Understanding component and board compatibility issues with the use of higher temperatures is essential. Avoiding certain elements, such as bismuth and lead that may impact solder joint reliability also is important. 

THE ROLE OF A SOLDER JOINT IN THE QUALITY AND RELIABILITY OF ELECTRONICS CIRCUITS HAS EVOLVED. HOWEVER, LITTLE ATTENTION IS GIVEN TO SPECIFIC MATERIAL PROPERTIES REQUIRED TO CREATE A VIABLE SOLDER JOINT. WHEN CONSIDERING MATERIALS IN LEAD-FREE TECHNOLOGY, COMPONENT COMPATIBILITY IS KEY. 

Second on the list of concerns and issues is changing process windows of lead-free electronics production. When we look at the materials used in lead-free technology, the compatibility of different components takes center stage. 

In addition, in order to replace lead in a bullet, the selected material should have a large enough specific gravity so that the resulting bullet mass is compatible with commercially available propellants. It is not economically feasible to develop a lead-free round where a special propellant or other component would need to be developed. 

In assemblies utilizing lead-free solder, work must be performed to ensure that the flux, underfill, component, and PCB system is compatible. For an underfill to be effective, the underfill must bond to the die and chip carrier surfaces. Current underfill systems are designed to be compatible with flux systems for Sn-Pb solder, which may not be the case with a lead-free solder. An additional concern for underfilled systems is exposure to reflow temperatures. For example, flip-chip BGA components may experience several reflow cycles after the underfill step. The higher processing temperatures experienced during lead-free soldering can have detrimental effects such as delamination of the underfill material from the chip carrier or die surface. 

Many component suppliers are advertising lead-free finishes on terminations. The compatibility of these finishes with both Pb-free and Pb-based solders must be considered as the transition is made to Pb-free processing. Even the body materials of some components such as capacitors have temperature-sensitive properties that may be altered by the higher lead-free processing temperatures. 

In addition to termination-material or compatibility issues, there are several other component material-related concerns, the most important of which is damage due to moisture uptake when processed at the higher Pb-free assembly temperatures. The most prominent defect is delamination due to warpage or popcoming, a phenomenon whereby the moisture absorbed by a component on the manufacturing floor is converted to steam when exposed to the solder reflow-temperature cycle. Many components are downgraded in moisture-sensitivity-level rating when used with lead-free process conditions. 

In the selection of lead-free solder, it is important to also select compatible board materials, board coatings, temperature-resistant components, and flux to optimize both Pb-bearing and Pb-free solder joints. As noted, there will likely be an interim time period where both Pb-containing and Pb-free components will be populated on the same PCB, depending on the cost and availability of Pb-free components. 

The ability of the supply chain to manage multiple PNs, BOMs for leaded, lead-free, and mixed configurations must be addressed. Given the fact that there is no standard alloy that will be utilized to replace eutectic Sn-Pb solder, and that the many OEMs and contract manufacturers are working to different schedules, this is a daunting task. It will be necessary to implement quality standards considering the nuances of each alloy. Standards must also be established to classify the specifications and limitations of each component as well as the components forward and backward compatibility. 

Compatibility of lead-free solders with a wide range of standard lead-containing and lead-free board and component flnishes has been demonstrated. 

Vianco, P.T. Regent, J. Artaki, I. Ray, U. Finley, D. Jackson, A. Compatibility of lead free solders with lead containing finishes as reliability issue in electronic assemblies. Proceedings of Electronic Component and Technology Conference, 46, Orlando, FL, May 1996 1172-1183. 

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