A manufacturable lead-free surface-mount process? NEMI demonstrates that lead-free manufacturing may be possible using existing production tools and equipment.Printed wiring boards (PWBs) assembled with lead-flee solders require higher processing temperatures than boards manufactured with standard tin/lead solders. This fact has been a major source of concern for the electronics industry as the transition to lead-flee assemblies occurs. Component, board and equipment manufacturers are all potentially impacted by manufacturing processes that require reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. temperatures 30 to 40[degrees]C higher than the temperatures used currently. Can existing manufacturing facilities be used, or will new equipment and tools be needed? will lead-free solder solder (sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i. joints be reliable? Can tin/lead components be mixed on boards with lead-free components? How will these tin/lead components work with lead-free solders? These questions, and more, were facing the industry more than three years ago when the National Electronics Manufacturing This article presents a typical manufacturing process of an electronic assembly. Component manufacturing Components such as resistors, capacitors and integrated circuits are generally made by specialized contractors. Initiative (NEMI NEMI National Electronics Manufacturing Initiative NEMI National Environmental Methods Index , Herndon, VA) launched the Lead-Free Assembly Project. Among the project's stated goals were the demonstration of materials and manufacturing processes for lead-flee PWB (Printed Wiring Board) An alternate term for printed circuit board. See printed circuit board. assemblies and the comparison of lead-free and conventional solder joints. Through the cooperation of component, board and equipment manufacturers--along with key original equipment manufacturers (OEMs) and electronics manufacturing services Electronic manufacturing services (EMS) is term used for companies that design, test, manufacture, distribute and provide return/repair services for electronic component and assemblies for original equipment manufacturers (OEMs). (EMS) providers--the NEMI group was able to demonstrate manufacturing processes for lead-free solders using existing tooling. Reliability Test Board Build The project recommended a single lead-flee solder formulation for industry use--Sn3.9 Ag0.6Cu. This tin/silver/copper alloy has a melting point melting point, temperature at which a substance changes its state from solid to liquid. Under standard atmospheric pressure different pure crystalline solids will each melt at a different specific temperature; thus melting point is a characteristic of a substance and roughly 35[degrees]C higher than eutectic tin/lead solder. The project's process group was charged with developing a new reflow process, identifying any new tools needed and transferring the process to a manufacturing location for building reliability test hardware. The group determined a new profile and found that, in most cases, existing manufacturing tools were adequate for handling the higher temperature solder. At that time (early 2000), existing lead-flee pastes were produced only in laboratory testing quantities, and fluxes had not been optimized. The team evaluated five tin/lead (Sn37Pb) and five lead-flee, no-clean, Type 3 solder pastes Solder paste (or solder cream) is a mix of small solder particles and flux. It is used extensively in the automated soldering processes wave soldering and reflow soldering. . One tin/lead and one lead-flee paste were used in the test vehicles (TVs) based on printability, visual solderability and x-ray defect inspection. All test boards were built at a single site, using standard manufacturing equipment. A total of 253 boards were built (Figure 1) using six TV designs: CSP (1) (Certified Systems Professional) An earlier award for successful completion of an ICCP examination in systems development. See ICCP. (2) (Commerce Service P 169; CSP208; TSOP (Thin Small Outline Package) A very thin, plastic, rectangular surface mount chip package with gull-wing pins on its two short sides. TSOPs are about a third as thick as SOJ chips. See gull-wing lead, SOP, SOJ and chip package. 48; 2512 chip resistor resistor, two-terminal electric circuit component that offers opposition to an electric current. Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistance). ; PBGA PBGA Plastic Ball Grid Array 256; and CBGA CBGA Ceramic Ball Grid Array CBGA Central Banks Gold Agreement CBGA Cascade Boer Goat Association 256. Each board featured three distinct test cells: 1) tin-lead solder paste with tin-lead components; 2) lead-free Sn3.9 Ag0.6Cu paste with tin-lead components; and 3) lead-flee Sn3.9Ag0.6Cu paste boards with lead-free components. [FIGURE 1 OMITTED] Immersion silver was used as the board surface finish on all TVs with extra thin small-outline package Thin Small-Outline Packages, or TSOPs are a type of surface mount IC package. They are notably very low-profile (about 1mm) and have tight lead spacing (as low as 0.5mm). (TSOP) and 2512 resistor boards built with a nickel/gold finish. All boards were made from epoxy epoxy Any of a class of thermosetting polymers, polyethers built up from monomers with an ether group that takes the form of a three-membered epoxide ring. The familiar two-part epoxy adhesives consist of a resin with epoxide rings at the ends of its molecules and a curing FR-4 material with a glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). ([T.sub.g]) of 170[degrees]C, except TSOP and 2512 resistor boards, which had a [T.sub.g] of 130[degrees]C. Solder paste volumes were measured during the test board builds for all boards, and representative findings for paste volumes are listed in Table 1. Paste volumes for both the tin/lead and lead-free pastes were found to be acceptable, with lead-free paste having higher volumes using the same printer settings. This difference is due to the good printing characteristics of the selected lead-free paste and the relatively coarse pitch of the components being assembled. The pick-and-place machine placement images were similar for the tin/lead and lead-free components. A 10-zone convection oven convection oven n. An oven having a fan that shortens cooking time by circulating hot air uniformly around the food. , with a nitrogen atmosphere of less than 30 ppm (Pages Per Minute) The measurement of printer speed. See gppm. PPM - Portable Pixmap [O.sub.2], was used for all builds. Temperatures for the lead-free boards ranged from a high of 247[degrees]C on the 2,512 resistor board to a low of 239[degrees]C on the plastic ball grid array “BGA” redirects here. For other uses, see BGA (disambiguation). A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. (PBGA) boards. Table 2 shows examples of the reflow profiles used in building the reliability test hardware. The major difference between boards assembled with tin/lead and lead-free paste was the visual solder joint appearance. Visual images of the tin/lead- and lead-free-soldered ceramic ball grid array CBGA (Ceramic Ball Grid Array) is a type of package design for microcontrollers and IC(Integrated circuit). (CBGA) components are shown in Figure 2. [FIGURE 2 OMITTED] After the builds, the reliability test boards were subjected to electrical test, x-ray inspection, acoustical imaging and automatic optical inspection (AOI AOI Area Of Interest AOI Automated Optical Inspection AOI Art of Illusion (3D modeling software) AOI Associated Oregon Industries AOI Angle Of Incidence AOI Age of Innocence (David Hamilton book, also a band) ). X-ray Inspection X-ray inspection looked for process and solder-related defects. In addition to providing feedback about the TV assemblies, the task allowed the assessment of the suitability of x-ray equipment for inspection of lead-free assemblies. X-ray images for lead-free- and tin/lead-soldered joints were found to be similar in appearance. The equipment required only slight adjustments for measuring solder joint thickness and diameter on lead-free assembled boards. Overall, more defects were observed with the lead-free paste than with the tin/lead paste. Increased voiding for lead-free paste with tin/lead or lead-free ball grid array (BGA (Ball Grid Array) A popular surface mount chip package that uses a grid of solder balls as its connectors. Available in plastic and ceramic varieties, BGA is noted for its compact size, high lead count and low inductance, which allows lower voltages to be used. )/chip-scale pack age (CSP) components was noted. Reduced wetting with lead-free SnAgCu paste and lead-free TSOP components was also indicated, as shown in Table 3. Voiding was attributed to the use of the specific flux in the lead-free solder paste and the reduced solderability of the lead-free SnAgCu ahoy (compared with tin-lead and the elevated temperatures used during lead-free solder paste processing). Accelerated thermal cycle (ATC ATC Air Traffic Control ATC Average Total Cost ATC Certified Athletic Trainer ATC At the Center (Hartford, Maine retreat center) ATC Applied Technology Council ATC All Things Considered ) testing found no reliability problems with the voiding on boards that combined lead-free solder with lead-free components. More development work will optimize the fluxes in the lead-free solder paste at higher reflow temperatures. X-ray inspection found no difference in solder ball In BGA chip packages, it is the tiny globe of solder that provides the contact between the chip package and the printed circuit board. Also called a "solder bump." See BGA. diameters and joint thickness for the various component types across the different tin/lead, lead-free test cells assembled. AOI Inspection The leadframe and chip components were evaluated using AOI inspection to determine the potential differences in visual solder joint appearance and shape of lead-free vs. tin-lead soldered Pronounced "sod-erd." Permanently attached by a hard metal bond. In order to replace a chip soldered to a circuit board, it requires heating the soldering joints until they melt. Contrast with socketed. joints. Lead-free paste with the tin/lead 2512 resistor components were similar in appearance to the solder joints formed by the tin/lead paste with tin/lead resistors. When lead-free paste was used with pure tin resistors, a reduction in wetting on the board pad side and some surface cracking at the solder joint occurred--possibly due to the way the solder joint cooled. Nonetheless, ATC reliability testing found no mechanical integrity issues. Acoustic Microscopy microscopy /mi·cros·co·py/ (mi-kros´kah-pe) examination under or observation by means of the microscope. mi·cros·co·py n. 1. The study of microscopes. 2. Inspection The TSOP48, CSP169, CSP208, PBGA256 and CBGA 256 components were evaluated before and after reflow using acoustic microscopy to determine and compare any internal component damage. The 2037 components were evaluated with acoustical imaging prior to board assembly (Phase 1). Component analysis was performed to the guidelines stipulated in IPC/JEDEC JSTD-020 Revision B. Prior to assembly, the components were baked at 125[degrees]C for 12 hours and then stored in a nitrogen cabinet. After the components were assembled and reflowed on test boards using profiles shown in Table 2, the parts were tested again. The TSOP48 components had some internal component damage before assembly. This damage was not sufficient to cause non-acceptance according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. ISTD-020B. After reflow (Phase 2), internal component damage increased, but most of the damage was acceptable according to JSTD-020B criteria. Some parts assembled with the lead-free profile were not acceptable according to these criteria. For the remaining component types, internal damage was observed pre-reflow. No significant increase in damage was found post-reflow for either tin/lead or lead-free processing temperatures. The minor internal damage noted for these components was found to be acceptable and within JSTD-020B criteria. After thermal cycle reliability testing, all parts were again analyzed with no indications of further increases in internal damage/delamination. Rework re·work tr.v. re·worked, re·work·ing, re·works 1. To work over again; revise. 2. To subject to a repeated or new process. n. Evaluation Process steps for the rework of the CSP208 and PBGA256 test boards included component removal, board pad site A pad site is a freestanding parcel of commercial real estate located in the front of a larger shopping center or strip mall. Desirable because of their visibility to consumers, accessibility, and the ease of facilitating drive-thru service, pad sites are typically sought after by preparation and placement/reflow of new components. For lead-free PBGA and CSP component test boards assembled with the lead-free paste, peak solder temperatures of 244[degrees]C and 246[degrees]C, respectively, were used for component removal. After site redressing, paste flux was applied to both component and board surfaces, and new lead-free components were placed and reflowed. For lead-free rework, the peak solder joint temperature was 244[degrees]C for PBGA (time over 217[degrees]C of 66 seconds) and 246[degrees]C for CSP (time over 217[degrees]C of 68 seconds), respectively. The top of the lead-flee component body would normally be around 15[degrees]C higher than the solder joint temperature. To preclude this, a method known as positive air flow was used to apply cooler air over the top of the component to reduce temperature while the component solder joints were heated from all four sides by the heater nozzle An orifice in an inkjet print head through which ink is sprayed onto the paper. Print heads with six thousand or more nozzles are common in today's printers. Nozzle . With this technique, the peak temperature on top of the lead-free PBGA and CSP components dropped to 201[degrees]C and 231[degrees]C, respectively. Despite the developmental nature of the rework and the small sample size, this effort provided a baseline of potential issues encountered during lead-free rework. Conclusions Little difference is found in the manufacture of boards assembled with eutectic tin/lead and lead-free SnAgCu solder paste. The only difference was reflow temperatures. In most cases, existing manufacturing tools and equipment may be utilized for these new processes. X-ray, acoustic and AOI inspection equipment may be used successfully to evaluate lead-free assembly processes. Differences were observed between visual appearance for the lead-free and tin/lead soldered joints, but more work is needed to test AOI inspection tooling. X-ray inspection showed increased voiding with the lead-free paste for CSP/BGA-type components and some reduced wetting with lead-free paste on TSOP components. Components examined by acoustic microscopy showed internal component defects induced by the higher temperature lead-free reflow. Most of these defects, however, were well within component specifications. A method to lower the temperature at the top of the component during lead-free rework of CSPs and PBGAs was developed. More developments in the lead-free assembly and rework processes will be addressed in future projects.
TABLE 1: Solder paste volumes and transfer rates for all board builds
for the project.
Package Paste Avg. Paste Vol. Paste Transfer
(Cu. mils) Rate(%)
256PBGA SnPb 2410 86%
SnAgCu 2677 96%
169CSP SnPb 552 56%
SnAgCu 681 69%
208CSP SnPb 522 53%
SnAgCu 535 54%
256CBGA SnPb 3836 95%
SnAgCu 4037 100%
R2512 SnPb 30908 100%
SnAgCu 32894 100%
TSOP48 SnPb 2312 79%
SnAgCu 2373 81%
Package Avg. Paste Ht.
(mils)
256PBGA 6.7
6.5
169CSP 4.4
5.5
208CSP 5.4
5.1
256CBGA 7.0
6.9
R2512 7.2
7.9
TSOP48 5.4
5.6
TABLE 2: Reflow profiles used in building the reliability test hardware.
Paste/ Specifications Lead-Free Paste/ Lead-Free Paste/
Component Lead-Free Tin-Lead
Type Component Component
256CBGA TaL/Reflow 64-66 sec.
Peak Temp. 240-241[degress]C
256PBGA TAL 61-69 sec.
Peak Temp. 239-243[degrees]C
208 CSP TAL 78-81 sec.
Peak Temp. 244-246[degrees]C
169 CSP TAL 80 sec.
Peak Temp. 244[degrees]C
48TSOP TAL 87 sec.
Peak Temp. 247[degrees]C
2512 resistor TAL 90 sec.
Peak Temp. 247-248[degrees]C
Paste/ Tin-Lead Paste/
Component Tin-Lead
Type Component
256CBGA 84-86 sec.
209-210[degrees]C
256PBGA 84-88 sec.
209-214[degrees]C.
208 CSP 83 sec.
217[degrees]C
169 CSP 88 sec.
215[degrees]C
48TSOP 85-86 sec.
217-218[degrees]C
2512 resistor 87-88 sec.
217-220[degrees]C
Lead-free paste = Sn3.9Ag0.6Cu (mp. 217[degrees]C
Tin-lead paste = Sn37Pb (mp. 183[degrees]C)
TAL = time above liquidus
TABLE 3: X-ray defect summary (percent of total solder joints with a
specific defect).
Test Cell
Component Type SnPb/SnPb SnAgCu/SnPb SnAgCu/SnAgCu
256CerBGA No defects 22% V 3% V
256CBGA 0.1% In 4% V 3% V
208 CSP 0.2% In 0.5% In, 0.8% V 5% V, 0.2% In
169 CSP 5% V 18% V 8% V, 1% In
48TSOP No defects 0.5% In 3% In, 0.3% Ex
2512 chip No defects No defects No defects
In = insufficient solder
Ex = excess solder (bridging)
V = voids evident > 20% of the area of the solderjoint and minimum
void diameter of 10 mil at board-level solder joint slice or center of
ball
V = voids with minimum diameter of 10 mil at board-level solder joint
slice or center of ball
Acknowledgments The author would like to recognize the efforts made by the process group and full project team to make this project a success. For the assembly and inspection work, these include: Len Poch and Maurice Davies (Universal Instruments); Jeff Schake (DEK DEK - Data Encryption Key ); Denis Denis, king of Portugal: see Diniz. Barbini (Vitronics-Soltec); Oliver Bast Bast, in Egyptian religion Bast (băst), ancient Egyptian cat goddess. At first a goddess of the home, she later became known as a goddess of war. The center of her cult was at Bubastis. Her name also appears as Ubast. (Orbotech); Jeremy Jessen (Agilent); Steve Martell (Sonoscan); and Terry Leahy Sir Terry Leahy (born 28 February 1956) is the CEO of Tesco, the largest British supermarket chain. He was knighted in 2002, and was Chancellor of UMIST, his alma mater, from that year until 1 October 2004 when he became co-chancellor of the newly formed University of Manchester. and Mark Walz (SRT/Teradyne). Jasbir Bath is advisory process engineer for Solectron Corp.'s Process Technology Group (Milpitas, CA) and the process group leader of NEMI's Lead-Free Assembly Project; e-mail: JasbirBath@ca.slr.com. |
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