Everything You Ever Wanted To Know About Laminates But .
Everything You Ever Wantedto Know About Laminates but Were Afraid to Ask Arlon-Materials for Electronics, November 2008. All Rights Reserved
Everything You Ever Wanted to Know About Laminates but Were Afraid to AskIntroduction to the 9th EditionDear Reader,It has been over 25 years since the earliest edition of “EverythingYou Ever Wanted to Know About Laminates but Were Afraid toAsk” was pounded out on an old TRaSh-80 Computer. It hasundergone periodic review and editing, including adaption for use onour website. (When I entered the industry Al Gore had not yetinvented the internet.) Before I “retired” in 2004, we did anotherminor revision, but it was largely cosmetic, removing mostreferences to the old military specification and introducing IPC-4101,the “new” specification for laminate and prepreg materials.When I entered the laminates industry in 1983 (when Arlon acquiredHowe Industries) I knew almost nothing about the industry or theapplication of its products. It was a high intensity learning curve forme, with a lot of help from internal gurus and from our customers,who were always willing to share their knowledge and experience. Ihad to ask a million questions (usually several times) before I startedto feel comfortable with it all. The original idea behindEverything ” was to answer the many questions our customersasked about high tech materials – which in large part turn out to havebeen the same ones I had asked for myself in my first few months inthe industry. It wound up being a “best seller” and we “sold out”several early print editions. (The price was right, of course!)The present revision has involved a total review and rewrite. Thereare fundamentals which have not changed in 25 years, but theindustry has moved a long way and Arlon has a much morediversified line of high performance products serving a marketplacewith very different needs and priorities. Yes, we still are the premiermanufacturer of polyimide laminate and prepreg -- and still supply avariety of materials for CTE control. But we also make a family oflow flow materials, a line of thermally conductive materials, haveentered the arena of lead-free and “green” products, to mention afew. What would have been considered a “high tech” multilayerboard 25 year ago is being produced routinely now, and high tech hasbecome altogether different and more complex. We have tried toreflect a changing world in this present edition of “Everything ”It would be impossible to thank everybody who has contributed to“Everything ” through the years, but in particular I want to mentionVince Weis, my mentor when I first entered the industry, and still the“go-to guy” when there is a question to which I don’t have ananswer.I dedicate this 9th Edition of “Everything ” to my many co-workers,industry peers and customers, so many of whom have been generouswith their advice throughout the years, and without whom therewould never have been an “Everything ”Chet Guiles, Rancho Cucamonga, CA2
Table of ContentsI MaterialsIntroductionGeneral Discussion of Resin SystemsIs there an "Ideal" PWB Material or system?Epoxy ResinsBlended Resin SystemsPolyimidesNon-Traditional Resin SystemsTypical Polyimide and Epoxy Products (Arlon Systems)What is High performance?Define Glass Transition Temperature (Tg)What determines "Continuous Operating Temperature" of a systemWhy do epoxies and polyimide turn brown when we expose them to elevated temperature?Do resins require a postcure after laminating?Can we cure polyimide at 360 F without a postcure?What are Dielectric Constant and Loss Tangent?What is Impedance?How do you calculate impedance?Given the normal variability in materials and process, can you design around the laminatedielectric properties you can get from your PWB manufacturer?What is the importance of the copper foil used on laminates?What is the difference between "rolled" and electrodeposited foil? What is Copper FoilTreatment?What is Ohmega-Ply ? What is its application?What is Copper-Invar-Copper (CIC)? Why do we use it? What are its limits?What is HTE Copper? Why do we use it?What is Release Copper? Why do we use it? How is copper affected by Tg, operating temperature, etc.? What factors contribute tocopper bond longevity?What governs the choice of which copper foil type or weight to use in a particularlaminate?ReinforcementsWhat are the differences between the common fiberglass fabric styles used in laminates?What are "Warp" and "Fill?" What is "Weave Distortion?" What problems can it cause?What is S-Glass and why would we use it?What is Kevlar ? Where do we use it?What are the differences between quartz and standard E-glass? What is Thermount ?II PrepregWhat is Prepreg? Briefly, how do we produce it?What is gel time? Why does IPC-4101 make gel time an "optional" test?What is resin content? Why is it important? How do we test for it and control it?3
What is flow? How do we test for and control it? Why is it important?What is Scaled Flow? How do we measure it? What does it tell us?Are there other flow test methods that may be more usefulWhat is Rheology?What are "No-Flow" (more accurately “Low-Flow”) Prepregs?What are the most common problems customers will encounter with prepreg?How do we know what the "Shelf-Life" of various prepregs should be?Tell us more about moisture issues in prepreg?What about polyimide prepregs? Is moisture really a bigger issue with polyimides thanwith epoxy systems?Arlon recommends vacuum desiccation of all prepregs prior to use.III LaminateWhat is a Laminate?How Are Laminates Classified?What are the Most Important Laminate Properties?What is MIL-S-13949?What is IPC-4101?How Do I read an IPC-4101 Slash Sheet?How does our part number relate to the IPC-4101 line callout? What testing does IPC-4101require us to do?What is a QPL?What determines the thickness of a laminate? What do we mean by "buildup"?Fabric Style Typical Thickness Per PlyWhat thicknesses can we build and what buildups are typical of standard products?Why don't we recommend the use of single side clad laminate for use as cap sheetsHow does measurement of thickness by cross-section differ from measurement of thicknessby micrometer?Cross-Sectional Thickness RequirementsWhat are the IPC-4101 requirements for thickness tolerance? How do we define them?Special Thicknesses, TolerancesWhat are the "X", "Y" and "Z" axes of a laminate?What is the significance of UL Recognition of polyimide laminates?What is a Cert?What do we include in a Cert/Test Report?How do we control traceability?What is IPC-4103?IV ProcessingHow can a laminator be of assistance to its customers in defining processes or providingprocess information?What are the limitations of laminator-supplied processing recommendations?What are the “critical” process parameters that we need to be aware of?4
At what points in the manufacturing process must changes be made in handling highperformance resins (i.e polyimide) vs. FR-4?What special precautions must be taken with regard to moisture in high performancepolyimide and multifunctional epoxy prepregs?Talk to me some more about oxides?What will be the biggest problems with oxiding?What are “Oxide Alternatives”?What press cycles should we use for MLB lamination?What do we have to do to ensure that our product is properly cured?Why is the rate of heatup critical?What issues might I encounter with out-of-norm pressure?How do I control prepreg flow?Why is vacuum lamination superior to conventional press lamination for high performancemultilayers? How does it differ?We have heard a lot about dimensional stability. How can we get materials that aredimensionally stable?Dimensional Stability CharacterizedDoes Laminate Construction Affect D.S.?What Methods are Available for Enhancing RegistrationWhat determines drill speeds and feed rates?Why can’t I punch my polyimide boards?We hear a lot about Lead-Free Solder processing. What effect will higher soldertemperatures have on my MLB’s?How about processing Woven Kevlar Materials? Are there any special precautions totake while processing aramid fiber reinforced boards?Are there any problems drilling Kevlar ?How about Thermount? What are the process issues we will encounter with Thermountreinforced laminate systems?V Controlled ExpansionWhy is the CTE of a MLB so important?What is CTE (Coefficient of Thermal Expansion)?Can we find CTE-Controlled Products in all Resin systems?What are the CTE's of the materials used in making MLB's?What is the CTE of a typical multilayer board?What are the available solutions to the CTE mismatch problem?Is there a "correct CTE" for a board designed for surface mounting of LCCC's?Why is the coefficient of thermal expansion of the laminate always greater in the Zdirection that in the X and Y axes in the plane of the laminate?Discuss the issue of total Z-direction expansion 50 to 260oCWhat is The Schapery Equation?You mentioned an 0.006" CIC product. Tell us about that. How do we use it? Are thereany limitations?How is Copper-Invar-Copper used?Is CTE the only reason for the use of a metal core in a PWB?5
Sounds like a lot of issues with CIC. Are there solutions for any of them?What is Howefill 50 ? How do we use it?When Would I use a Ceramic Filled Prepreg?Talk to us about woven aramid reinforced PWB's. What are the advantages of this system?Does Use of Woven aramid Reinforced Laminates have any disadvantages?There is still interest in quartz fabrics for CTE control. Why?Are there disadvantages of quartz fabric?Talk to us about Thermount? It looked like the ideal balance of cost/performance forSurface Mount PWB’s. Where does that all stand?And the downside if any?So what is Arlon doing about that?VI AnalyticalWhat is Thermal Analysis?Define Glass Transition Temperature Tg and discuss its significance.Typical Tg's for common resin systemsWhat is DSC and what does it tell us about prepreg and laminate?What is the significance of DSC Testing?What is TMA?What is the difference between TMA and DSC for Tg measurement?What is TGA?What is IR?What is Rheology?What kinds of flow testing are available and what information do they provide?What does rheological characterization mean to you?What is Parallel Plate or Oscillating Disc Rheometer?How does the Parallel Plate Rheometer provide data?What is Cone & Plate Rheometry?How do we test Thermal Conductivity?6
I. MATERIALSIntroductionAlthough this book is entitled “Everything You Ever Wanted to Know About Laminates, ButWere Afraid to Ask,” the real subject matter is Printed Circuit Boards, the materials that are usedto manufacture them, and how you as a designer or manufacturer of MLB’s can benefit by abetter understanding of what we, as laminators, do and supply. Multilayer printed circuit boardsare complex composite structures comprising a series of layers of reinforced resin and copperfoil all put together in a multi-level operation that involves an electrical and electronic designteam, and at the PWB shop level, the integration of a variety of physical and chemical processes.The starting point for the manufacture of those PWBs is the copper-clad laminates and prepregsthat you purchase from Arlon.What are those materials? How are they manufactured? What are their key properties, and howare they processed to give the best results? By the time you’re finished, I hope you’ll have abetter appreciation of the finer points of your raw materials, and through clever use of subliminaltechnology, an irresistible urge to buy them from Arlon.General Discussion of Resin SystemsThe popular names of the materials which comprise the majority of printed wiring boardlaminates have often been generalizations of the chemical names of the principal resin systemsused in each such as "Epoxy," "Polyimide,” "PTFE" and the like. In recent years the search forproducts suitable for evolving high performance applications have resulted in the use of newmaterials and combinations of materials that make the generalizations of the past somewhat moredifficult to sustain. For instance, reference to “Epoxy” or “FR-4” is too general to accommodatethe diverse requirements that characterize Low Flow, Multifunctional, Lead-Free, CAFResistant, High Speed Digital and “Green” epoxy products. This diversification is reflected inthe proliferation of slash sheets in IPC-4101, the laminate and prepreg spec currently in effect inthe industry. (IPC-4101 is discussed in some detail in section III, “Laminates.”) Products canalso be characterized in terms of intrinsic properties, resin system, end application or evensubstrate type.Some of the newer products, such as those based on laser drillable fiberglass, woven Kevlar ,Thermount nonwoven aramid substrate or woven quartz reinforcement are frequently referredto by the generic name of the substrate rather than by the resin utilized, and of course alllaminators including Arlon would like to have you all use our by trade names rather than genericresin designators! (Naturally that would preclude your referring to them by our competitors' tradenames.)Many of the resins in use in the electronic laminates industry are thermosets (which means thatthey "cure" into a hard final product) rather than thermoplastics (which may be melted and remelted). The most notable exception of course is the use of PTFE, or polytetrafluoroethylene(also known by its Dupont trade name as Teflon although DuPont is not the only manufacturer7
of PTFE) in laminates for microwave and RF, where its dielectric properties are ideal for highfrequency applications.Is there an "Ideal" PWB Material or system? If not, what would be the key elements ofsuch a system?Because of the diversity of materials available and the wide range of applications, the concept ofan “ideal laminate” has become fuzzy, to say the least. A material ideal for a high temperatureapplication may be impractical in a microwave or high speed digital design, for instance. As aresult, while we can list properties that are potentially important, each design team needs toestablish a prioritization among them because no one material will have the optimal value for allthe properties which might be considered important. Your laminate “cookbook” does have somecommon “ingredients” (i.e. properties) from which designers will be making choices during thedesign phase:Glass Transition Temperature (Tg): The highest temperature systems are polyimides (such asArlon’s 35N, 33N and 85N) with Tg’s of 250 C or above. Tg is a rough indicator of total Zdirection expansion and hence is considered a proxy for plated through hole reliability. A widevariety of epoxy systems with Tg’s in the 170oC range have found application where eitherprocess temperatures or in-use temperatures (or both) are less demanding. While Tg is a goodframe of reference for traditional epoxy and polyimide materials, it is less reliable for reliabilitycharacterization of highly filled systems and for many of the non-traditional resin systems usedin high frequency/low loss applications whose properties are composites of their variouscomponents.Example: Polyimide Thermount MLB’s will survive 2-3x the thermal cycles of standardpolyimide glass despite having identical resin and Tg because there are no stress concentratorscaused by fiber bundles intersecting the walls of the plated through holes.Example: A filled PTFE product such as Arlon’s CLTE-XT microwave material does not evenhave a Tg in the range of 50 to 260oC, yet has a low and consistent Z-direction expansion in thatrange by virtue of which it will have excellent plated through hole reliability.T260, T288 and T300 values represent the length of time that a clad laminate will survive aparticular temperature (respectively 260oC, 288oC and 300oC) before it begins to delaminate orblister. The test is performed in a TMA (Thermo-Mechanical Analyzer) and these values areconsidered good indicators of the short term resistance of products to solder processing; as suchthey have become part of the Lead-Free minimum requirements as defined by IPC.Thermal Decomposition Temperature (Td): This property varies greatly with the chemicalcomposition of materials, from the mid 300oC range for many epoxy systems to over 400oC forsome polyimides. Td is the temperature at which a material begins to degrade thermally. Somedata sheets will list a Td as the temperature at which a material has lost 5% of its original weightdue to decomposition. A better indicator of performance would be the onset temperature, at8
which significant weight loss begins to occur. By the time a material, has lost 5% of its weightto decomposition it may well be unusable in many applications.Dielectric Constant and Loss Tangent: Dielectric constant determines the speed at which anelectrical signal will travel in a dielectric material. Signal propagation speed is expressedrelative to the speed of light in a vacuum, which is roughly 3.0 x 1010 cm/sec. The dielectricconstant of a hard vacuum (space) is defined as 1.00. Higher dielectric constants will result inslower signal propagation speed. Loss Tangent is a measure of how much of the power of asignal is lost as it passes along a transmission line on a dielectric material.These are determined by the inherent properties of the components of any specific resin system,although we normally refer to a “relative dielectric constant” because it is dependent on testmethod and frequency as well as material per-se. No one value of Dk is “ideal” for allapplications. For many high speed digital applications, modified epoxy systems with dielectricconstants in the range of 3.0 to 3.5 have been found to offer good cost-performance value. Inother applications such as critical antennas, low noise amplifiers, etc., a very low loss materialsuch as PTFE is optimal. Choice of specific material is as much an issue of loss characteristicsunder use conditions as it does of Dk per-se. For microwave and RF applications, pure PTFE ona fiberglass substrate, with a Dk of 2.1 and Loss of 0.0009, is the best available in widecommercial use. Consistency of dielectric constant is important for maintaining designedcharacteristic impedance (Zo) values.Registration (aka “Dimensional Stability”): All laminate materials shrink to some degree onetching, and the key to proper registration is consistency both of product and process. The mostsuitable material would be one that moves minimally when it is etched, and has consistent andreproducible movement that would allow predictable factors for artwork compensation. The“ideal” material would not require artwork compensation at all, and would always registerproperly so as not to require drill compensation. The IPC test referred to as “dimensionalstability” has at best a tenuous connection to the actual registration of a specific board design.Registration continues to be a key fabrication concern for high layer count and HDI designs.Coefficient of Thermal Expansion (CTE, expressed in ppm/oC): Should be roughly matched tothe expansion requirements of claddings, devices to be mounted on the surface and thermalplanes buried in the interior. Current thinking says that for leadless ceramic chip carrierattachment, 6.0 ppm/ C is ideal. (At present, Arlon’s 45NK woven Kevlar reinforced laminateswith relatively low resin contents come closest to the 6.0 CTE ideal.) Other materials such asquartz reinforcement, Copper-Invar-Copper distributed constraining planes and nonwovenaramid reinforcement achieve values as low as 9-11, a substantial improvement over 17-18 forconventional epoxy or polyimide laminates and have proven acceptable and consistent in avariety of SMT designs.Thermal Conductivity (Tc): As the density of components on a board increases due to demandfor increasing functionality, and the overall surface areas of PWBs decrease, the watt-density ofpower being generated on a PWB increases. Critical devices exhibit failure rates that double forevery 10oC of temperature increase, and so there is a
Everything You Ever Wanted to Know About Laminates but Were Afraid to Ask Introduction to the 9th Edition Dear Reader, It has been over 25 years since the earliest edition of “Everything You Ever Wanted to Know About Laminates but Were Afraid to
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It has been almost 30 years since the earliest edition of “Everything You Ever Wanted to Know About Laminates but Were Afraid to Ask” was pounded out on an old TRS 80 (Trash 80) Computer. It has undergone periodic review and editing, including adaption for use on our
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