Global Trends in Lead-free Soldering
BY JOHN H. LAU AND KATRINA LIU
The move toward acceptance of lead-free materials in electronics is coming. Either you jump on the bandwagon or you will be left behind. On February 13, 2003, lead-free became a law in the European Union (EU), with an implementation date of July 1, 2006. From that date on, no electronic products (except those with exemptions) can be made in or shipped to the EU if they contain lead.
Packages such as plastic ball grid array (PBGA) chip scale packages (CSP), flip chip, and wafer-level chip scale packages (WLCSP) are popular in consumer, computer and communication products. Most of these packages use solder as an interconnect material, and will be affected by the lead-free regulations. In this study, the impact of lead-free on PBGA, CSP, flip chip, WLCSP and plastic quad flat pack (PQFP) are examined.
Global Lead-free Regulations
In 1970, the U.S. formed the Environmental Protection Agency (EPA). This is the first time in history that a nation had taken comprehensive stock of the quality of its surroundings. One of the EPA's greatest achievements is banning lead (Pb) additives in gasoline — reducing the concentration of lead in the air by 94 percent from 1980 to 1999.
In April 1993, the Lead Exposure Reduction Act (S. 729) and others were introduced in the U.S. banning lead for plumbing, housing, etc. However, lead in electronics products was exempted!
Figure 1. Senju and ISURF SnAgCuX solder alloys patents.
Figure 2. Classification reflow profile (IPC/JEDEC J-STD-020B, Amendment 1).
In Japan, lead-free laws do not currently exist. However, the control of lead is being strengthened through the review of water quality standards regarding lead, amendments to disposal laws and the enactment of a home appliance recycling law.
Since 1998, Japanese manufacturers have been using lead-free soldering and technology in many popular lead-free consumer products such as MiniDisc players, refrigerators, cleaners, personal computers, notebooks, mobile phones, TVs, VCRs, PCBs and motherboards. The lead-free solder alloys used by some noticeable companies are: 1) SnAgBiIn and SnCu by Matsushita; 2) SnAgBiCu and Sn2Ag4Bi0.1Ge by Sony; 3) SnAgCu by Toshiba; 4) SnAgBi and SnAgCu by Hitachi; 4) SnZn, SnCu, SnZnBi and SnAgCu by NEC.
There are two parts of the Japanese Electronics Information Technology Industries Association's (JEITA) lead-free roadmap. The first part deals with components: 1) Start supplying lead-free terminal components by January 1, 2002; 2) Complete supplying lead-free terminal components by January 1, 2002; 3) Complete supplying of lead-free components by January 1, 2005. The second part of the roadmap pertains to equipment: 1) Start introducing lead-free solders by 2003-2004; 2) Start manufacturing lead-free assemblies by January 1, 2003; 3) Complete lead elimination by January 1, 2006. It should be noted that some companies may be a year ahead of this schedule, while others may be two years behind.
In the EU, there are two lead-free directives or laws on waste electrical and electronic equipment (WEEE), and restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS). In brief, WEEE seeks to increase recycling and recovery of waste equipment. RoHS bans lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (HC), polybrominated biphenyls (PBBs) and polybrominated diphenys ethers (PBDEs). For this study, the focus is the RoHS banning of Pb.
Figure 3. Focused ion beam images taken at various stages of cutting through a matte Sn whisker on a Cu substrate.
In some products, lead is exempted by RoHS. For example, lead in solder for servers, storage and storage array systems; lead in solder for network infrastructure equipment for switching, signaling, transmission as well as network management for telecommunications; lead in electronic ceramic parts such as piezoelectronic devices; lead in high-melting-temperature-type solders such as tin-lead solder alloys containing more than 85 wt percent Pb.
Not all products are covered by RoHS — monitoring and control and medical equipment are not subject to the ban. This equipment is usually complex, reliable and requires long design and life cycles. It is produced in low volume and considered a high capital expenditure product. Large original equipment manufacturers (OEMs) treat this equipment with intensive care and use it as long as they can, or sell the equipment. The chances of this type of equipment ending up in a landfill remain slight. Consequently, this equipment is considered less harmful to the environment and poses a low risk to human health.
The Chinese Government recently began formalizing a version of RoHS, with a lead-free implementation date of July 1, 2006. The final law was scheduled to be published at the end of 2003.
What if the EU, China, Japan and U.S. come up with their own versions of RoHS that are all different? How are we going to ship our products? What if the test measurement methods for lead-free solders, components, PCBs and final products are different from different countries? What should we do with our products?
What we need is a uniform definition and solid standards of RoHS and test/measurement methods in the electronics industry to work and communicate using the same language.
Lead-free Solder Paste and Products
Currently, there is no common world standard for lead-free products. However, the stipulation for lead-free content as impurities to lead-free solders is recommended by the Joint Electronic Device Engineering Industry Development Association (JEDEC), Japanese Electronic Industry Development Association (JEIDA) and EU End-of-Life Vehicle Directive (EUELVD) as: JEDEC<0.2 wt percent Pb; JEIDA <0.1 wt percent Pb; and EUELVD<0.1 wt percent Pb.
Lead-free products are those manufactured with the lead-free solder defined by JEDEC, JEIDA or EUELVD, and those that do not allow intentionally added Pb in the products. Lead-free components can be defined as those with less than 0.1 wt percent Pb of the solder.
Figure 4. Tin whisker driving forces and failure mechanisms.
Solder paste consists of solder alloy and flux. The majority of solder alloys in use today are the 63 wt percent Sn 37 wt percent Pb solder alloy, with a melting point of 183°C. Not long ago, there were more than 100 lead-free solder alloys. The leading solder alloys in the U.S. are Sn-3.9 wt percent Ag-0.6 wt percent Cu, recommended by National Electronics Manufacturing Initiative Inc. (NEMI). In the EU, the leading solder alloys are Sn-3.8 wt percent Ag-0.7 percent Cu, recommended by European Consortium (BRITE EURAM). In Japan, the leading solder alloys are Sn-3 wt percent Ag-0.5 percent Cu, recommended by JEIDA. Now, the electronics industry can build infrastructure (design, materials, process, reliability, etc.) around the SnAgCu alloys.
The melting point of SnAgCu alloys is ~217°C, which is 34°C higher than SnPb alloys. Thus, surface mount technology components and printed circuit boards (PCBs) will be subjected to higher reflow soldering temperatures and the impact on cost, performance and reliability are of great concern.
Figure 1 shows the patents, U.S. Patent No. 5,527,628 that is owned by Iowa State University Research Foundation (ISURF), 3.5-7.7Ag1-4Cu0-10Bi0-1Zn; and JP Patent No. 5,050,286 that is owned by Senju, 3-5Ag0.5-3Cu0-5Bi0-5Sb, covering the solder alloys recommended by the U.S., EU and Japan. It is obvious that royalties should be paid to Senju. However, ISURF claims that their patent covers both solders and solder joints. During and after reflow, copper pads on the PCB will dissolve into the solder joints, then the copper content of the solder joints will fall into ISURF's patent coverage. Thus, royalties should be paid to ISURF also. Today, a new company formed by Senju and ISURF, is waiting to collect royalties from paste manufacturers.
Figure 5. Ni-barrier under-plate to block the Cu diffusion into the matte Sn-layer. The stress in the Sn layer, due to the formation of the Ni3Sn4 IMC, is in tension.
The IPC Solder Products Value Council (SPVC) has studied standard tests such as differential scanning calorimetry (DSC) for melting characteristics, IPC-TM-650, 22.214.171.124 for wetting balance response, and IPC-TM-650, 2.4.46 for solder spreading of three lead-free solder alloys — Sn-3 wt percent Ag-0.5 wt percent, Sn-3.8 wt percent Ag-0.7 wt percent Cu and Sn-4 wt percent Ag-0.5 wt percent Cu. The IPC SPCV consists of 16 solder paste vendors: Aima, Alpha Materials and Fry, Advanced Metals, Amtech, EFD, Henkel Loctite, Heraeus, Indium, Kester Northrop Grumman, Koki, Niho Superior, P. Kay Metals, Qualitek, Tai Solder, Senju and Cookson Electronics. The mission of IPC SPVC is to help resolve confusion regarding solder choices and standardize on a single solder.
These tests are designed to determine if there are significant differences in process performance between the three SnAgCu solder alloys. Test results have been published in a white paper report by the IPC SPVC. Based on the statistical analysis of the test data, as long as process parameters are concerns, there is no significant difference among these three alloys.
Reliability tests of the three lead-free alloys with various components are ongoing. Three hundred and twenty test boards have been assembled (160 test boards with 72 components per 6-layer PCB at 93-mil thick by one company, and 160 boards with 260 components per double-sided PCB by another company). For each company, 40 test boards for each lead-free solder and the 63Sn37Pb solder were used as controls. Thermal shock and thermal cycling tests are also ongoing. Results should be available to the electronics industry in 2004.
Many in the technical community have recently said that Sn-4Ag-0.5Cu, Sn-3.9Ag-0.6Cu and Sn-3.8Ag-0.7Cu are not as reliable as Sn-3Ag-0.5Cu. One of the reasons is that more and larger Ag3Sn platelets are presented in the higher Ag-content solder joints. These platelets degrade solder joint reliability, especially for smaller solder joints (such as flip chips) under isothermal fatigue, mechanical and shock and vibration conditions. However, this does not mean Sn-4Ag-0.5Cu, Sn-3.9Ag-0.6Cu and Sn-3.8Ag-0.7Cu solder joints are unreliable. More reliability test results, such as those conducted by IPC SPVC, of real lead-free solder joints are needed.
Besides solder, the other element of solder pastes is flux. Each paste vendor has its own flux and, unfortunately, it is proprietary. Flux is the substance (chemical/material) that prepares and protects the surface to be soldered by removing surface oxides. This provides a clean, metallic surface and prevents further oxidation during the soldering process. Flux assists in the transfer of heat from the molten solder to the joint area so that the base metals reach a high enough temperature to be wetted by the solder. Flux also reduces the interfacial surface tension between the solder and the base metal, enabling the solder to flow over and metallurgically wet the solderable surface.
Since the wetting characteristics of SnAgCu alloys are generally not as good as those typically exhibited by SnPb alloys at lower process temperatures, the selection of flux and solder paste is important to ensure good solderability.
Lead-free will change the world of the electronics industry, especially for component suppliers. Many suppliers are trying to keep their customers. Most of them will provide both tin-lead and lead-free components, but it will be expensive. Most likely, when the tin-lead components comprise less than 10-20 percent of the lead-free components, they will stop making tin-lead components. The lead-free roadmap for component suppliers is by the middle of 2004, when all lead-free components should be qualified because the system houses need approximately one year to qualify their lead-free products.
There are four groups of components: discrete (chip resistors) packages; SMT leaded (plastic quad flat pack) packages; balled (plastic ball grid array) packages; and plated through-hole (PTH) packages. For SnPb-leaded components, surface finishes mostly have SnPb plating. For lead-free components, the leading surface finishes are matte-Sn plating for SMT leaded components for short life-cycle (5 years or less) applications and matte-Sn plating with Ni-barrier under-layer for long life-cycle products; Sn3 wt percent Ag0.5-1 wt percent Cu for solder-balled components; and either Sn-plate or Sn-dip for PTH components.
The qualification specifications for lead-free plastic (moisture-sensitive) components are provided by IPC/JEDEC J-STD-020B. To be more in line with JEITA standard uses, the first amendment of J-STD-020B was recently published (Figure 2, and Tables 1 and 2). Peak temperatures (Tp) increase and a more complex matrix of temperature, thickness and volume for lead-free components are added. Also, the time within 5°C of actual peak temperatures is changed to 15 seconds. It should be emphasized that the balloting will only reflect these changes, but not the whole standard. If "no" votes are cast in the next few months, then the current 020B will continue to be the de facto guideline to qualify component suppliers. Most of the system houses, however, are already requiring that all the lead-free components be able to withstand 260°C for lead-free soldering.
Tin whiskers cause critical reliability issues (to shorten the leads) of lead-free leaded components (Figures 3 and 4). The most likely tin whisker mechanisms are shown in Figure 3. The major driving force for tin whiskers is the compressive stress in the Sn layer. The causes of this stress are due to:
- The diffusion of Cu into Sn and the formation of Cu6Sn5 intermetallic compound (IMC) can generate compressive stress in the Sn layer;
- The thermal loads;
- The mechanical loads;
- The shock and vibrational loads;
- The self-diffusion of Sn along Sn grain boundaries to the root of a whisker will supply more Sn atoms to push the Sn whisker upward (dislocation loop climbing effect on the grain boundaries);
- The weak spots on the SnOx layer.
The reactions 1 and 5 are ongoing. A combination of 2, 3, 4 results in compressions that will enhance the initiation and growth of the tin whisker. However, if the combination of 2, 3, and 4 results in tensions, then the initiation and growth of the tin whisker may be reduced. This is very important for designing the acceleration tests. Some methods to mitigate the tin whisker reliability risk are:
- To use matte Sn with a larger grain size to reduce the whisker growth rate;
- To use a thicker matte Sn-plating (8 µm minimum, 10 µm preferred);
- To reflow components right after matte Sn-plating to form a thicker IMC, which acts as a barrier layer and reduces further compressive stress building up in the Sn layer;
- To anneal the components (150°C for 2-3 hours, or 170°C for 1 hour) right after matte Sn-plating to achieve the same effects of 3 (above);
- Use a second element in matte Sn-plating (eg., Bi) as a substitute for Pb (most of the Japanese companies use this method);
- Use a Ni barrier under-plate (0.2-2 µm) to block the Cu diffusion into the matter Sn-layer to form the CuSn IMC for high-end products (Figure 5). It can be seen from Figure 5 that because of the formation of the Ni3Sn4 IMC and the chance for the Cu to diffuse in the Sn is slim.
The tin-whisker acceleration tests released by SolderTec/ JEIDA/NEMI are:
- High Temperature/High Humidity Test: 60°C and 93 percent +2/–3 percent RH;
- Thermal Cycling Test: high temperature = 85°C, low temperature = –40 or –55°C, and the dwell time, heating and cooling rates are to be determined;
- Ambient Conditions: at room temperature = 20 to 25°C or 15 to 35°C are to be determined, and at specific conditions, if necessary, will be identified.
The potential JEDEC tin-whisker acceleration tests proposed by NEMI are:
- High Temperature/Humidity Test: 60°C+/–5°C and 93 percent +2/–3 percent RH for 1,008 hours, minimum;
- Thermal Cycling Test: –55°+0/–10°C to 85°C+10/–0°C for 1,000 cycles, minimum;
- Storage Test at Ambient: 20 to 25°C and 30 to 80 percent RH for 1,008 hours, minimum./ol>
For all these tests, there are no bias voltage, pass/fail criteria, and no SnPb baseline enforced even if it is recommended. What the industry needs is whisker-free matte Sn-plating chemical solutions, acceleration test methods and the corresponding acceleration factors and acceleration models — in addition to temperature and humidity, mechanical and shock and vibration driving forces should be included.
For a complete list of references, please contact the authors.
Look for Part II in the February 2004 issue. Part II provides an in-depth look at Lead-free PCBs, Lead-free SMT Assembly Processes, Lead-free Solder Joint Reliability, Lead-free Transition Issues and Lead-free Substitutes.
JOHN H. LAU, Lead-free technical program manager, may be contacted at Agilent Technologies Inc., 5301 Stevens Creel Blvd., Santa Clara, CA 95052; (408) 553-2358; email@example.com. KATRINA LIU, senior manager, may be contacted at Top Touch Consultants Ltd., A-17C, Block A, YueHai Bldg., Nanyou Rd., Nanshan Shenshen, Guandong, 518054 China.