Laminate material selection for RoHS Assembly, Part 1: success in lead-free assembly is based on matching critical laminate properties with the design constraints.The RoHS deadline has come and gone. Success in preparation for this deadline has varied greatly. For some, it was a relatively seamless transition. For others, it could not have been more painful. And for those with exemptions that continue over the next few years, there is still a great deal of work to be done. The segments that currently have exemptions are generally the more complex applications where product reliability is most critical. In addition, these segments are facing pressure to comply ahead of the deadlines due to component availability issues and constraints CONSTRAINTS - A language for solving constraints using value inference. ["CONSTRAINTS: A Language for Expressing Almost-Hierarchical Descriptions", G.J. Sussman et al, Artif Intell 14(1):1-39 (Aug 1980)]. within the EMS industry. These ongoing transitions will be the most challenging in terms of base material selection and qualification due to the designs of the PCBs, the complexity of assembly, and the requirements for long-term Long-term Three or more years. In the context of accounting, more than 1 year. long-term 1. Of or relating to a gain or loss in the value of a security that has been held over a specific length of time. Compare short-term. reliability and electrical performance. From the standpoint The Standpoint is a newspaper published in the British Virgin Islands. It was originally published under the name Pennysaver, largely as a shopping-coupon promotional newspaper, but since emerged as one of the most influential sources of journalism in the of compliance, most laminate laminate, n a thin slice of porcelain or plastic fabricated in a dental lab, which is cemented to the front of the teeth to cover gaps, whiten stained teeth, or reshape chipped or broken teeth. materials are acceptable. Most materials do not contain lead or the other restricted metals. The brominated flame retardant Brominated flame retardants are produced synthetically in 70 variants with very varying chemical properties. There are several groups:
PCB in full polychlorinated biphenyl Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. design and construction will have a significant impact on the base material properties required. Thin, low layer count PCBs have different requirements when compared to thick, high layer count PCBs. Copper weights, aspect ratios and other design features will also have an impact. End use application and the associated requirement for long-term reliability and electrical performance will impact the decision-making decision-making, n the process of coming to a conclusion or making a judgment. decision-making, evidence-based, n a type of informal decision-making that combines clinical expertise, patient concerns, and evidence gathered from process as well. The requirements for a cell phone, video game or even a computer motherboard Also called the "system board," it is the main printed circuit board in an electronic device, which contains sockets that accept additional boards. In a desktop computer, the motherboard contains the CPU, chipset, PCI bus slots, AGP slot, memory sockets and controller circuits for the will be very different than those for high-end high-end adj. Informal 1. Appealing to sophisticated and discerning customers: a high-end department store; high-end video equipment. 2. servers, telecommunications Communicating information, including data, text, pictures, voice and video over long distance. See communications. gear, avionics avionics (ā'vēŏn`ĭks), electronic instruments used in air or space flight; also the design and production of such instruments. Early planes had few instruments, but as aviation and aircraft became more complex, so did instrumentation. , and critical medical and automotive electronics. Last, not all lead-free adj. 1. not containing the element lead. 2. Not containing a lead additive such as tetraethyllead or tetramethyllead; - of gasoline; as, Most modern cars run on lead-free gasoline s>. Oposite of leaded nt>. 3. assembly processes are the same. Some designs will experience peak temperatures of around 245[degrees]C while others will experience peak temperatures of up to 260[degrees]C, or even higher in a few cases. Some boards may experience 2-3 thermal cycles, others up to 5-6, and more depending on how many reworks are allowed. All of this makes it impossible to recommend one base material for all applications without either under specifying the laminate material and risking defects during assembly or later on in the field, or over specifying the material and paying too much for the material or limiting availability. A very critical point that must be emphasized with respect to these issues is that laminate manufacturers can easily make improvements in one property, but often this can adversely affect other important properties. For example, it is relatively easy to formulate formulate /for·mu·late/ (for´mu-lat) 1. to state in the form of a formula. 2. to prepare in accordance with a prescribed or specified method. a resin resin, any of a class of amorphous solids or semisolids. Resins are found in nature and are chiefly of vegetable origin. They are typically light yellow to dark brown in color; tasteless; odorless or faintly aromatic; translucent or transparent; brittle, fracturing system with a very high time to delaminate de·lam·i·nate intr.v. de·lam·i·nat·ed, de·lam·i·nat·ing, de·lam·i·nates To split into thin layers. in conventional T260 or T288 tests, or to engineer a resin system with a very high decomposition decomposition /de·com·po·si·tion/ (de-kom?pah-zish´un) the separation of compound bodies into their constituent principles. de·com·po·si·tion n. 1. temperature. However, this is often achieved at the expense of other (i.e. mechanical) properties and may make the material more difficult to use successfully in conventional PCB manufacturing processes without sacrificing design flexibility. Therefore, achieving the best balance of properties to meet the needs of the OEM (Original Equipment Manufacturer) The rebranding of equipment and selling it. The term initially referred to the company that made the products (the "original" manufacturer), but eventually became widely used to refer to the organization that buys the products and , EMS and PCB manufacturer is absolutely critical. Critical Laminate Properties The higher temperatures of lead-free assembly have an impact on each of the three main components of laminate materials, the resin system, the glass cloth a woven fabric formed of glass fibers. See also: Glass and the copper foil. TABLE 1 summarizes the key issues for each of these main components. Substantial work describing the impact of lead-free assembly temperatures on the base materials and finished PCBs has been gathered recently (1,2,3,4,5). These works have identified the critical base material properties that must be considered when selecting a material for lead-free assembly applications, and therefore, only a summary is provided in TABLE 2. FIGURE 1 is now a well-known well-known adj. 1. Widely known; familiar or famous: a well-known performer. 2. Fully known: well-known facts. example of what can happen when the wrong choice of base material is made. This is a cross-section cross section also cross-sec·tion n. 1. a. A section formed by a plane cutting through an object, usually at right angles to an axis. b. A piece so cut or a graphic representation of such a piece. 2. of a relatively simple 4-layer board that used a conventional high-Tg FR-4 and was then processed through lead-free assembly with a peak temperature of 260[degrees]C. In this PCB, delamination delamination /de·lam·i·na·tion/ (de-lam?i-na´shun) separation into layers, as of the blastoderm. de·lam·i·na·tion n. 1. A splitting or separation into layers. 2. occurred at the resin-to-internal copper interface as well as the resin-to-glass interface. However, even if a PCB survives assembly without defects, a second key question is, has long-term reliability of the PCB been compromised? For the currently exempt product segments yet to be transitioned to lead-free assembly, this is an even more critical issue. [FIGURE 1 OMITTED] The historical focus on Tg is probably the result of its effect on total Z-axis The third dimension in a graphics image. The width is the x-axis and the height is the y-axis. expansion. However, because of differences in CTE (Coefficient of Thermal Expansion) The difference between the way two materials expand when heat is applied. This is very critical when chips are mounted to printed circuit boards, because the silicon chip expands at a different rate than the plastic board. values, even the relationship between Tg and thermal expansion thermal expansion Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change. requires further analysis. FIGURE 2 illustrates the relationship between Tg and thermal expansion for the products in TABLE 3. [FIGURE 2 OMITTED] Products with the same CTE values (A & C for example; note that above Tg the lines are parallel) will differ in total expansion based on their Tg values (Figure 2). For example, the conventional 175[degrees]C Tg material (C) exhibits less total expansion than the conventional 140[degrees]C Tg material (A) because He onset of the higher post-Tg rate of expansion is delayed by 35[degrees]C. However; the 175[degrees]C material with a lower CTE value (D) exhibits much less total expansion than the conventional 175[degrees]C Tg material even though the Tg values are the same. Furthermore, the 150[degrees]C Tg material with reduced CTE values (B) exhibits approximately the same total expansion (3.4%) as the conventional 175[degrees]C Tg material (3.5%). However; with a decomposition temperature that is significantly higher, this mid-Tg FR-4 material is much more compatible with lead-free assembly than the conventional 175[degrees]C Tg material. Knowing Tg and CTE are not enough. While 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). (Tg) and Z-axis expansion have been a focus of attention over the years, only with the introduction of lead-free assembly has the decomposition temperature (Td) gained significant attention. The decomposition temperature has always been important in terms of reliability, but most people have used Tg as a proxy for material reliability in the days of leaded 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. . One reason for this is that other things being equal, a higher-Tg will result in less total thermal expansion, and therefore less stress on plated vias. What wasn't was·n't Contraction of was not. wasn't was not wasn't be discussed is that it is common for conventional high-Tg FR-4 materials to exhibit somewhat lower decomposition temperatures than conventional 140[degrees]C Tg FR-4 materials. This is highlighted by the fact that most conventional 140[degrees]C Tg FR-4 materials exhibit longer T260 times than conventional high-Tg FR-4 materials. To highlight the importance of Td, examine FIGURE 3. [FIGURE 3 OMITTED] The traditional high-Tg FR-4 materials we have become familiar with have Td values in the range of 290-310[degrees]C. Traditional 140[degrees]C Tg FR-4 materials are generally somewhat higher, with a typical example of a material with a Td of 320[degrees]C shown in FIGURE 2. In the typical tin-lead assembly environment, peak temperatures do not reach the point where decomposition is significant for either the traditional or enhanced products. However, in the lead-free assembly environment, peak temperatures reach the point where a small, but significant level of decomposition can occur for the conventional materials, but not for the enhanced products. This seemingly seem·ing adj. Apparent; ostensible. n. Outward appearance; semblance. seem  ing·ly adv. small level
of decomposition in the conventional products can have extremely
significant effects on reliability, especially if multiple thermal
cycles are experienced.
As long as the temperatures experienced during assembly and 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. do not exceed the temperature at which decomposition begins, there may not be further benefit to higher decomposition temperatures. In addition, the resin system modifications often made to increase Td can often lead to other problems, such as brittleness Brittleness That characteristic of a material that is manifested by sudden or abrupt failure without appreciable prior ductile or plastic deformation. or hardness of the resin system that negatively impacts PCB manufacturability, e.g. drilling, scoring and routing, or fracture toughness In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. in tight pitch PTH PTH abbr. parathyroid hormone Parathyroid hormone (PTH) A chemical substance produced by the parathyroid glands. This hormone is a major element in regulating calcium in the body. areas. As not all lead-free compatible materials use the same resin systems, great care should be used in selecting a material that balances the requirements of the OEM, assembler Software that translates assembly language into machine language. Contrast with compiler, which is used to translate a high-level language, such as COBOL or C, into assembly language first and then into machine language. and PCB fabricator fab·ri·cate tr.v. fab·ri·cat·ed, fab·ri·cat·ing, fab·ri·cates 1. To make; create. 2. To construct by combining or assembling diverse, typically standardized parts: . A simple review of a material data sheet or IPC (1) (InterProcess Communication) The exchange of data between one program and another either within the same computer or over a network. It implies a protocol that guarantees a response to a request. slash sheet is not sufficient to guarantee success. Involvement with the base material supplier to fully understand the capabilities of a material and the required processing to be successful is even more important in lead-free applications. To expand on this, examine FIGURE 4, FIGURE 5 and FIGURE 6. [FIGURES 4-6 OMITTED] First, note the slope of the decomposition curve in Figure 6. By the 5% decomposition definition, this material has a Td of 405[degrees]C. However, if you look at 1% and 2% values, you get 336[degrees]C and 367[degrees]C, which are significantly lower than a reported 5% value of 405[degrees]C. This is not to say that a 1% Td of 336[degrees]C is bad. In fact, this material has exhibited exceptional reliability in lead-free applications. However, it does point to the need to understand decomposition levels other than the 5% level. In contrast, the material in Figure 5 has a 5% Td of 330[degrees]C with 1% and 2% Td's of 326[degrees]C and 328[degrees]C, respectively. In this case, there is a very small difference between the 1%, 2% and 5% values, and this material has also proven to have excellent performance in a broad range of lead-free assembly applications. Last, the standard high-Tg material in Figure 4 exhibits a 5% Td of 305[degrees]C with 1% and 2% values of 297[degrees]C and 301[degrees]C. This is also a small range of values, but in this case the absolute values are lower and begin to approach the range of temperatures that can be experienced, at least locally, in a board that undergoes lead-free assembly and rework. Materials such as this have not performed with adequate reliability to be considered for anything but the simplest of lead-free applications. If PCBs made with these materials are stored in uncontrolled environments and are allowed to absorb even a modest level of moisture, the combination of lower Td's with the higher vapor pressure vapor pressure, pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid. of water at lead-free temperatures can lead to catastrophic defects in assembly, or severely degraded de·grad·ed adj. 1. Reduced in rank, dignity, or esteem. 2. Having been corrupted or depraved. 3. Having been reduced in quality or value. long-term reliability. To take this a step further, examine the materials listed in Table 3. These are four FR-4 materials, with material C being the same material as shown if Figure 4, and material D being the same as the material in Figure 5. The impact of decomposition temperature when these materials are exposed to multiple thermal cycles to different peak temperatures is highlighted in FIGURE 7 and FIGURE 8. Figure 7 graphs cumulative weight loss (decomposition) for these materials when cycled repeatedly to a peak temperature of 235[degrees]C. Clearly, there is little impact on resin decomposition when the peak temperature reaches 235[degrees]C. Figure 8 presents the same results when the peak temperature is increased to 260[degrees]C. Obviously, the increase in temperature to 260[degrees]C has a severe impact on resin decomposition for the traditional FR-4 materials as they experience multiple thermal cycles, especially the conventional high-Tg material (product C). The rapid degradation DEGRADATION, punishment, ecclesiastical law. A censure by which a clergy man is deprived of his holy orders, which he had as a priest or deacon. in material C after only a few thermal cycles highlights why this material is not recommended for lead-free assembly applications. Indeed, as more data is gathered, thermal cycling seems to have a much greater impact than even one cycle to a very high temperature, even if it is for a long time. [FIGURES 7-8 OMITTED] Impact on Printed Circuit Reliability Circuit reliability (also time availability) (CiR) is the percentage of time an electronic circuit was available for use in a specified period of scheduled availability. In addition to the works already cited, there have been excellent studies recently on the impact of lead-flee assembly specifically on PCB reliability (6,7,8). These works present statistical analyses showing the impact of lead-free assembly on PCB reliability and reach important conclusions regarding the base materials. While there is not perfect agreement among all published works, the differences typically are the result of a different focus, e.g. whether the focus is on complex, thick PCBs with stringent reliability requirements vs. relatively less complex PCBs with shorter intended field lifetimes or less stringent reliability requirements. Ed--Part 2--"Selecting The Right Base Material for Your Application" will be published in the December 2006 issue of Printed Circuit Design and Manufacture magazine. REFERENCES (1.) Bergum, Erik, "Application of Thermal Analysis Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Techniques include:
(2.) Kelley, Edward, "An Assessment of the Impact of Lead-Free Assembly Processes on Base Material and PCB Reliability," IPC/Soldertec Conference, Amsterdam, June 2004. (3.) Hoevel. Dr. Bernd, "Resin Developments Targeting Lead-Free and Low Dk Requirements," EIPC EIPC Enhanced International Peacekeeping Capabilities (US State Department) EIPC European Institute of Printed Circuits EIPC Egyptian International Programming Center (website) Conference, 2005. (4.) Christiansen, Walter, Dave Shirrell, Beth Aguirre and Jeanine Wilkins Wil·kins , Maurice Hugh Frederick 1916-2004. British biophysicist. He shared a 1962 Nobel Prize for his contributions to the determination of the structure of DNA. , "Thermal Stability of Electrical Grade Laminates Based on Epoxy Resins epoxy resins, group of synthetic resins used to make plastics and adhesives. These materials are noted for their versatility, but their relatively high cost has limited their use. ," IPC Printed Circuits Expo, Anaheim, CA, Spring 2001. (5.) Kelley, Ed, Bergum, Erik, Humby, David, Hornsby, Ron, Varnell, William, "Lead-Free Assembly: Identifying Compatible Base Materials For Your Application," IPC/Apex Technical Conference, February 2006. (6.) Freda, Michael and Furlong furlong: see English units of measurement. , Jason, "Application of Reliability/Survival Statistics to Analyze Interconnect (1) To attach one device to another. (2) A physical port (plug, socket) or wireless port (transmitter, receiver) used to attach one device to another. StressTest Data to Make Life Predictions on Complex, Lead-Free Printed Circuit Assemblies," EPC (1) (Entertainment PC) See HTPC. (2) (Electronic Product Code) A standard code for RFID tags administered by EPCglobal Inc. (www.epcglobalinc.org). 2004, October 2004. Brist, Gary and Long, Gary, "Lead-Free ProductTransition: Impact on printed Circuit Board Design and Material Selection," ECWC ECWC European Cup Winners' Cup 10/APEX/IPC Conference, February 2005. (8.) Ehrler, Sylvia, "Compatibility of Epoxy-Based PCBs to Lead-Free Assembly," EIPC Winter Conference, 2005, & CircuiTree, June 2005. ED KELLEY is director high reliability products at Isola Group; ed.kelley@isola-group.com. Erik Bergum is director global marketing at Isola Group; erik.bergum@isola-group.com.
TABLE 1. Primary lead-free issues for base material components.
COMPONENT LEAD-FREE ASSEMBLY IMPACT POTENTIAL SOLUTIONS
Resin System 1. Peak assembly 1. Formulate resin
temperatures can system with higher
reach point where decomposition
resin decomposition temperatures.
begins.
2. Higher temperatures 2. Formulate for lower
result in increased coefficients of
thermal expansion thermal expansion.
and stress on plated
holes as a result.
3. Vapor pressure of 3. Evaluate materials
absorbed moisture for moisture
much higher at absorption/release
lead-free assembly characteristics;
temperatures; can drying processes in
lead to blistering/ PCB fabrication
delamination. and/or assembly.
4. "Phenolic" lead-free 4. Evaluate non-dicy/
compatible materials non-phenolic
often not as good laminate materials.
for electrical
performance,
especially Df.
Fiberglass Cloth Thermal & mechanical Cleanliness and choice
stress on of proper coupling
resin-to-glass bond. agent more important.
Copper Foil Thermal & mechanical Copper nodularization
stress on and treatments for
resin-to-copper bond. improved adhesion.
COMPONENT RELATED CONSIDERATIONS
Resin System 1. Reformulation can adversely affect
electrical properties and
manufacturability.
2. Can also impact mechanical
properties and manufacturability.
3. PCB storage conditions prior to
assembly are much more important,
especially humidity conditions.
4. New lead-free compatible material
now available with improved
electrical performance and
attractive cost/performance.
Fiberglass Cloth Loss of resin-to-glass adhesion
through thermal cycling could impact
CAF resistance.
Copper Foil May impact conductor losses,
especially at high frequencies .
TABLE 2. Critical base material properties for lead-free
assembly applications.
PROPERTY DEFINITION
Decomposition Measures weight loss from resin degradation
Temperature, Td as a function of temperature. Td is typically
defined as the point at which 5% of the
original mass is lost to decomposition, but
other levels can also be reported, e.g. 1,%,
2% or "onset."
Glass Transition Thermodynamic change in polymer from a
Temperature, Tg relatively rigid, glassy state, to a softened,
more deformable state.
Z-Axis Expansion Change in physical dimension (in Z-axis) as a
function of temperature, expressed as a
"coefficient of thermal expansion" (CTE) or
percentage expansion over a temperature range.
Moisture Absorption Tendency of a material to absorb moisture from
the surrounding environment. Can be assessed
by more than one method, including water soak
or in an increased pressure & humidity
environment.
Time to Delamination While not a fundamental property, measures the
time for delamination to occur at a specific
temperature, e.g. 260[degrees]C (T260) or
288[degrees]C (T288).
PROPERTY ISSUE
Decomposition Resin decomposition can result in adhesion loss
Temperature, Td and delamination. A 5% level of decomposition
is severe, and intermediate levels are
important for assessing reliability since peak
temperatures in lead-free assembly can reach
onset points of decomposition. A high Td by
the 5% definition does not guarantee
performance. Conversely, a low Td by the 5%
definition is not necessarily bad if the onset
temperature of decomposition is high enough.
Glass Transition Several properties change as the Tg is
Temperature, Tg exceeded, including the rate at which a
material expands vs. temperature. Modulus also
decreases significantly as Tg is exceeded.
Z-Axis Expansion CTE values above Tg are much higher than below
Tg. 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.
Moisture Absorption Vapor pressure of water is much higher at
lead-free assembly temperatures. Absorbed
moisture can volatilize during thermal cycling
and cause voiding or delamination. PCBs that
initially pass lead-free assembly testing may
exhibit defects after storage in an
uncontrolled environment, as a result of
moisture absorption. This should be considered
when evaluating materials and PCB designs.
Time to Delamination Related to decomposition temperature and
adhesion between material components. Thermal
expansion and moisture absorption can also
influence results. In multilayer PCBs, the
treatment of the internal copper surfaces is
also critical.
TABLE 3. FR-4 base material properties
PRODUCT DESCRIPTION GLASS TRANSITION
TEMP., [degrees]C
A Conventional 140[degrees]C Tg FR-4 140
B Improved Mid-Tg FR-4 150
C Conventional High-Tg FR-4 175
D Improved High-Tg FR-4 175
PRODUCT DECOMPOSITION % EXPANSION, 50-260[degrees]C
TEMP., [degrees]C (40% RESIN CONTENT)
A 320 4.2
B 335 3.4
C 310 3.5
D 335 2.8
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