Lead-Free Solutions For Ceramic Modules

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Silas Grant
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1 Lead-Free Solutions For Ceramic Modules To support the moves within Europe to remove lead and cadmium from Thick Film products, a new material system of Pb and Cd-free products, known as the LF system, has been launched. A series of new conductors complement existing Pb-free products, and with the exception of hybrid resistors (currently under development), provide a complete system of materials. To qualify as a material system, however, requires demonstration of the compatibility of materials, particularly in demanding applications such as crossovers or multilayers. As well as a comprehensive matrix of planar overlaps of dissimilar materials, compatibility was assessed for 3- dimensional structures such as the multilayer via-fill. Conductor performance using Pb-free solder: solder leach resistance, aged and thermal cycled adhesion, together with heavy wire bonding and fine line resolution, showed that the new Pb-free conductors equal or exceed the performance of current Pb-containing products. Lead (Pb) is one of the materials targeted by the new European environmental legislation (ELV, WEEE & RoHS) and although there are currently exemptions for thick film materials, the industry is moving to implement leadfree products and solders. This has created a demand for Pb-free thick film pastes to meet this trend. Cadmium (Cd) is even more restricted than lead, and is banned from thick film by the ELV and RoHS directives since For some time now, new developments have been Cd-free, hence new products classified as Pb-free are also Cd-free. by A.C. Buckthorpe, J. Cocker, L. Garreau-Iles, D. Greenhill, R. Parr, T. Sweeney, DuPont Electronic Technologies Development of thick film materials requires more than just piecemeal replacement of existing Pbcontaining products with new Pb-free products of similar performance. New products should, wherever possible, not only equal and preferably exceed the performance of existing materials, they should also be designed and tested as a compatible material system. It is often the compatibility of materials in, for instance, a multilayer structure that limits the designer s freedom to translate the functional requirement into a layout manufacturable with high yield. Only in this way will the full potential of thick film hybrids be realised. The introduction of Pb-free materials has been a progressive policy going back at least fifteen years, with those materials that are usually present in the largest volumes, i.e. dielectric and glaze, being the first to be established. Recent developments have lead to the launch of a new system of Pb-free conductor materials (together with Ag-Pd and barrier via-fill materials) for use in multilayer and crossover structures. We take a look at the performance of these materials, how they compare with current Pb-containing materials, and also demonstrate their compatibility as a material system. Figure 1 - Schematic of multilayer build Figure 2 - Schematic of crossover build on doublesided substrate System of materials With the increasing complexity of hybrid applications, layouts based on either crossovers or multilayers have become commonplace. Both these builds require the compatibility of overlapped materials, in particular, dissimilar conductors, and this requires that a system of materials must, in addition to the individual material performance expectations, demonstrate compatibility with the other materials present in the structure whether they are directly or indirectly in contact. For instance, in a typical multilayer, Au bond pads must reliably interface with the main interconnect conductor, e.g. a pure Ag. Figures 1 and 2 show schematics of the two common build structures: a single-sided multilayer and a crossover with double-sided printing and through-holes. Many other combinations are of course possible, but these two figures OnBoard Technology April page 28-6-

Table 1 - System LF - Pb-free system of materials The crossover build uses PtAg for the main conductor, through-hole printing, resistor termination, component/lead attach and heavy Al wire bonding.

AgPd, if preferred, can also be used for resistor termination and component/ lead attach. Au is used for Au/Al wire bonding.

The material system (Table 1) designed to meet these needs is System LF (lead-free material system), which includes a new range of Pb-free conductors together with dielectrics based on the

2 Table 1 - System LF - Pb-free system of materials The crossover build uses PtAg for the main conductor, through-hole printing, resistor termination, component/lead attach and heavy Al wire bonding. Where resistors are printed close to a crossover, the PtAg can also be co-fired with the top dielectric and parallel fired with resistors. AgPd, if preferred, can also be used for resistor termination and component/ lead attach. Au is used for Au/Al wire bonding. Two encapsulant options provide a 620 C overglaze for general use with resistors etc, and a high temperature glaze for optimum environmental protection of conductors. The material system (Table 1) designed to meet these needs is System LF (lead-free material system), which includes a new range of Pb-free conductors together with dielectrics based on the well-established QM44 material technology and a 620 C firing overglaze based on QQ620. A Pbfree hybrid resistor series is under development but the existing 7400 surge resistor series is Pb-free, as is its high temperature encapsulant (7401). All the Pb-free materials are also NiO and Cd-free. LEAD-FREE ELECTRONICS ASSEMBLY Table 2 - Overlap compatibility matrix Material compatibility Figure 3 - Ag over Au - normal cosmetics Figure 4 - Au over Ag - unacceptable cosmetics as wrinkled surface indicates poor adhesion serve to illustrate the principal material overlap configurations that a system of materials will need to encompass. The multilayer is based on pure Ag as the main interconnect/inner layer conductor, AgPd for resistor termination and component/lead attach (either gluing or Pb-free soldering) and Au for Au/Al wire bonding. PtAg is an alternative top conductor to AgPd, is heavy Al wire bondable on both alumina and dielectric and could also, in principle, be used instead of Ag as the main interconnect/ inner layer, although intrinsically it has a higher resistivity. For viafill connection between inner Ag layers, and inner Ag to top Ag, PtAg or AgPd, a AgPd via-fill is used, whilst a unique barrier viafill material is used for inner Ag to top Au connection. For a material system to be versatile it is essential that the elements of the system are fully compatible with one another. In general, the biggest problems occur with overlaps of dissimilar conductors, not only due to Ag depletion, which is driven by a difference in the Ag concentration of overlapping conductors, but also due to the physical compatibility of the two materials. The two factors being essentially unrelated. Ag depletion is affected by the conductor thicknesses, geometry, overlap sequence and number of firings, all of which will determine the survivability of the overlap, as evidenced by visible depletion of Ag at the interface and eventual loss of continuity, particularly under heavy current loading. The Q U A L I T Y A S S U R A N C E -7- OnBoard Technology April page 29

most demanding configuration for Ag depletion is the Au-Ag overlap, typically required where Ag tracks are terminated with Au bonding pads.

In reality, overlapped conductors usually have a different appearance, e.g. darker/lighter, mottled etc.

On the other hand, wrinkling or blistering of the surface or edge curl/lift does indicate that the bond between the overlapping materials is suspect and would class the materials as not being fully

soldering or bonding, then these are outside of this remit and would need to be assessed as fit for purpose by the end user.

3 most demanding configuration for Ag depletion is the Au-Ag overlap, typically required where Ag tracks are terminated with Au bonding pads. Physical incompatibility is evidenced by deviations at the overlap from the ideal cosmetic, i.e. no change in the appearance of the overlying material at the overlap. In reality, overlapped conductors usually have a different appearance, e.g. darker/lighter, mottled etc. due to inter-diffusion, but none of which implies a degradation of the long-term reliability of the overlap as an interconnection. On the other hand, wrinkling or blistering of the surface or edge curl/lift does indicate that the bond between the overlapping materials is suspect and would class the materials as not being fully compatible. It should be noted, however, that this assessment of overlaps is purely where the function is as an interconnection; if the intention is to use the overlap for assembly, e.g. soldering or bonding, then these are outside of this remit and would need to be assessed as fit for purpose by the end user. An extensive evaluation of material compatibility was undertaken (Table 2) including conductor overlaps in each of the two possible sequences (i.e. A overlapped on B and B overlapped on A) on both alumina and dielectric. As the market is generally conservative with respect to the use of co-firing, the LF material system was primarily assessed using sequential firing. Only four overlap configurations gave major problems - Au overprinted onto Ag or PtAg and the S1X0 resistor series on Ag or Au. These problems (shown as crosses in Table 2) applied equally to the Pb-containing materials used as controls, and in the case of the Au-Ag/PtAg overlaps, are considered the norm rather than the exception. The incompatibility of Ag with the S1X0 series (blistering at the termination overlap) was confined to the 10Ω/ sq member (S110), whereas when printed onto Au, it was the 100Ω/ sq member (S120) that exhibited bubbling at the termination overlap which, although it could easily go unnoticed, was nevertheless considered a significant concern. Figure 5 - SEM Ag dot map of polished section of 1mm barrier via-fill with Au top connection Figure hr 150 C aged adhesion (SAC solder) Conductor overlaps are frequently only of the order of hundreds of microns, but there are situations where the overlap can cover an extended area and these are often where the cosmetic effects are more evident. Hence, to encompass all possible scenarios, a pattern was used with 250, 375 and 500µm wide lines and pads up to 10x10mm, with parallel and orthogonal overlaps. Figures 3 & 4 illustrate a representative area of the pattern with Au Ag overlaps. They also illustrate the difference in cosmetics as a function of the overlap sequence, with Ag over Au showing no evidence of any significant cosmetic issue, whilst Au over Ag raises a concern about loss of adhesion because of the wrinkling of the Au at the overlap and evidence of the start of edge lift. With earlier material systems, peeling of the Au has even been observed. Au overlapped on PtAg also exhibits the same symptoms, and hence the crosses entered in Table 2 for Au over both Ag and PtAg. The recommendation is to always print the Au first, which also gives better resistance to Ag-depletion and is generally the preferred sequence Figure 7 - Thickness dependence of 1000hr 150 C aged adhesion (SAC solder) OnBoard Technology April page 30-8-

Figure 8-1000 cycle 40/+125 C TCA (SAC solder) Figure 9 - Thickness dependence of 1000 cycle 40/+125 C TCA (SAC solder) to optimise the printing of Au patterning, e.g. bonding pads.

A total of 440 vias on a 2mm pitch with the links formed by buried Ag and, in the case of the barrier via-fill LF182, Au top conductor.

performance. To take advantage of the co-fireability of the LF15X dielectrics, the via-fill was co-fired with the second dielectric print.

4 Figure cycle 40/+125 C TCA (SAC solder) Figure 9 - Thickness dependence of 1000 cycle 40/+125 C TCA (SAC solder) to optimise the printing of Au patterning, e.g. bonding pads. In order to assess conductor compatibility with via-fill materials, a pattern was used with vias ranging in size from 400 to 1000µm square in a multilayer daisy chain configuration. A total of 440 vias on a 2mm pitch with the links formed by buried Ag and, in the case of the barrier via-fill LF182, Au top conductor. Whilst 400µm is a representative via size, the larger sizes create much greater thermal expansion mismatch between conductor and dielectric and are therefore a more demanding test of via-fill performance. To take advantage of the co-fireability of the LF15X dielectrics, the via-fill was co-fired with the second dielectric print. LF182 parts were stressed by temperature cycling (300 cycles, 40/+150 C) and then assessed for resistance increase (changes were less than 0.25%) prior to sectioning to confirm the structural integrity. Figure 5 demonstrates that even after temperature cycling there is no evidence of lack of integrity or delamination at the interface with both the buried Ag and top Au conductors. There was also a complete absence of Ag diffusion upwards into the via-fill, thus supporting previously reported data on the effectiveness of the barrier via-fill material. Full compatibility with the new Pb-free materials was demonstrated for vias up to and including 1mm square. Conductor soldered performance The data presented is for use with Pb-free solder: 95.5/3.8/0.7 Sn/Ag/ Cu (SAC) at a bath temperature of 245 C and with Alpha 611 RMA flux. Adhesion measurements were using the standard DuPont peel test but with Sn-coated Cu wires rather than the standard SnPb-coated wires traditionally used with Pbbased solders. Fired thicknesses, using 200 mesh, were in the range 15-17µm. To represent typical processing, conductors were refired twice at 850 C. Comparative data with 62/36/2 Sn/Pb/Ag solder was also generated but for brevity, has not been included. Although when used on dielectric the new conductors were designed for sequential firing, co-fired data has been included to indicate whether this offers potential performance advantages. It should be noted however, that these materials do not have a vehicle system optimised for printing onto dried dielectric. The format of the data graphs is for the LF materials to be shaded and in the sequence: on alumina, sequentially fired on dielectric LF151, co-fired on LF151. The controls are on alumina. Aged adhesion A matrix of parts, including current Pb-containing conductors as controls, was prepared and then split equally between ageing at 150 C for 1000hrs and thermal cycling at 40/+125 C (typical automotive specification) for 1000 cycles. Ramp rates were 10 C/min with dwell times of 30 minutes. Thermal cycling using the peel test is severe as the 0.8mm diameter Cu wire is soldered directly across three adhesion pads, thus applying significant stress due to the inherent thermal expansion mismatch. However, this method has been used extensively in the past and was able to provide the comparative data required for the purposes of this assessment. It should be noted, however, that established techniques such as window framing (encapsulant printed over the edges of the pad to resist solder attack around the periphery) would give significant improvements in TCA performance beyond that recorded here. LEAD-FREE ELECTRONICS ASSEMBLY Q U A L I T Y A S S U R A N C E -9- OnBoard Technology April page 31

Figure 10 - Solder leach resistance with SAC solder Figure 12 - LF171 (PtAg) - 175, 150 & 125µm Lines at 45 plus detail of 150µm line/space pattern The 150 C aged adhesion results (Figure 6) show

The AgPd and PtAg also have excellent aged adhesion on dielectric, both sequential and co-fired, whilst the Ag retains adequate adhesion on dielectric, particularly when cofired.

In effect, this is a measure of robustness of the LF materials, particularly the adhesion on dielectric.

5 Figure 10 - Solder leach resistance with SAC solder Figure 12 - LF171 (PtAg) - 175, 150 & 125µm Lines at 45 plus detail of 150µm line/space pattern The 150 C aged adhesion results (Figure 6) show that the LF conductors perform as well as, or better than, existing materials. The AgPd and PtAg also have excellent aged adhesion on dielectric, both sequential and co-fired, whilst the Ag retains adequate adhesion on dielectric, particularly when cofired. Whilst adhesion data is traditionally generated with 200 mesh, this may be restrictive in use, and hence comparative data (Figure 7) has been included for 325 mesh (9-11µm fired). In effect, this is a measure of robustness of the LF materials, particularly the adhesion on dielectric. LF121 AgPd is the best performer, being insensitive to thickness on dielectric as well as alumina, closely followed by LF171 PtAg. Even the Ag, LF131, has adequate adhesion on dielectric (particularly co-fired) if the fired thickness is kept at ~ 16µm. Thermal cycled adhesion The TCA data (Figure 8) is shown in two different formats, the first showing the data conventionally, whilst the second presents the data as the measurement point at which 50% of the mean initial adhesion was retained, i.e. how long they survived. This enables a more complete picture of the material performance to be obtained, as what is really of interest in thermal cycling data is not if the parts failed, but when they failed. Figure 9 shows the TCA thickness dependence of the LF materials. The new LF materials perform as well as, or better than the controls, and also survive 1000 cycles on alumina. For TCA performance on dielectric, the options are PtAg (LF171) or Ag (LF131), preferably co-fired with the dielectric. Of these two, LF171 has the better overall performance. Solder leach resistance SLR is assessed by recording the numbers of 10 sec dips in SAC solder at 245 C before the resistance of a 200 square 500µm serpentine track reaches or exceeds 2W (above 2W, the resistance rapidly goes open circuit). Figure 10 shows that the best SLR performance with Pb-free solder (4 dips), whether on alumina Figure 11 - Line resolution capability or dielectric, was obtained with the high Ag-bearing conductors, namely PtAg and Ag. The SLR performance of both the reference and LF AgPd was lower at 2-3 dips. Line resolution capability A 4x4 inch pattern was used which had a comb pattern interdigitated with a serpentine, with equal line width/spacing in 25µm increments from µm, orientated parallel, orthogonal and at 45 to the print direction. Also included were circles at fixed 200µm spacing. A standard 325 mesh 30µm wire screen with Ulano CDF3 film emulsion was used, print speed of 200mm/sec, printing with a flood blade. In order to simulate the initial deterioration in print definition normally seen during a print run, 25 parts were printed without interruption, and parts numbered were then assessed for line spreading (Figure 11). The matrix was LF materials (shaded, as in previous graphs) on alumina and sequentially fired on dielectric LF151, controls on alumina. The LF materials performed as well as or better than the controls with LF121 and LF131 showing, as is normally the case, deterioration on dielectric. LF121 at 150µm resolution performed well for a AgPd but the outstanding performer was LF171, as not only having previously demonstrated line resolution better than the conservative rating of 150µm reported here (Figure 12), but also showed minimal deterioration on dielectric. OnBoard Technology April page

Table 1: Pb-free solder alloys of the SnAgCu family

Table 1: Pb-free solder alloys of the SnAgCu family Reflow Soldering 1. Introduction The following application note is intended to describe the best methods for soldering sensors manufactured by Merit Sensor using automated equipment. All profiles should