4.4 PROBLEMS ASSOCIATED WITH THE BASE
4.4.5 Metal Surface Defects
4.4.7 Electrical ... 8
Figures
Figure 4−1 Copper or zinc inclusion in laminate ... 4 Figure 4−2 Etch pits in laminate ... 4 Figure 4−3 Dark spots within laminate ... 5 Figure 4−4 Lint within laminate ... 5 Figure 4−5 Scratches on copper treatment ... 5 Figure 4−6 Shiny spots on copper foil ... 5 Figure 4−7 Laminate voids... 7 Figure 4−8 Foil innerlayer cracking... 7
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Section 4
Base Materials
4.1 GENERALThe raw materials used in the manufacture of printed cir- cuit boards are comprised of three basic building blocks; resin, reinforcement and metal cladding. The materials can be specified in combinations to meet particular applications of electrical, mechanical, thermal or processing parameters. Some of the problems associated with raw materials are chronic, systemic problems caused by the inherent physics of the materials used. Other problems associated with raw materials are commonly caused by an improper assignment of a particular material to an application. It is important to understand the limitations of these materials as well as the needs of the end use.
4.1.1 Resins
Resins are used to impregnate the selected reinforcement. The selection of a particular resin is dependent on the desirable electrical, temperature and process parameters. Epoxy resins are the most commonly used. The difunc- tional base resin may be modified with additives to achieve higher thermal properties or improved chemical resistance. Tetrafunctional, multifunctional epoxy resins or Bismae- limide Triazine (BT) are added to epoxy resins to achieve higher thermal properties. Difunctional epoxy resins have a Tg range of 120 - 130°C. The modified epoxies have Tg ranges of 130 - 150°C or 150 - 190°C. The primary process parameters that are affected when changing between these resins is the lamination cycle and hole cleaning process prior to electroless copper.
Higher temperature resin systems may require more adjust- ments to the printed circuit processes such as: lamination cycle, baking, hole cleaning, drilling and routing. Polyim- ide resin and Cyanate Ester are the most commonly used high temperature resin systems. These resins have Tgs in the 250°C range. Polyimide can be blended to improve processing conditions, however the blending lowers the Tg. PTFE (polytetrafluoroethylene), commonly referred to as Teflon(TM), is used for microwave applications and has significantly different processing conditions than the other resins.
4.1.2 Reinforcements
The most common reinforcement material used is a woven fiberglass cloth. Glass filaments are extruded in different diameters. These filaments are combined together to create yarn bundles of specific filament counts, these yarns are twisted in certain cloth styles. The yarns are then woven on
looms in a standard over / under basket weave. The weav- ing process involves stretching out long yarns in one direc- tion. These yarns are commonly referred to as the ‘‘Warp,’’ ‘‘Grain,’’ or ‘‘Machine’’ running direction of the cloth. The other yarns, perpendicular to the warp, are the ‘‘Fill,’’ ‘‘Woof’’ or ‘‘Cross’’ yarns. These yarns are in the width of the roll of cloth. Once woven the cloth is treated with a ‘‘Finish’’ to make it compatible to accept the resin. There are non-woven cloths available to overcome the dimensional movement associated with the woven cloth. These fabrics are comprised of either random fibers or per- pendicular yarns that are not woven. Kevlar, Aramid or carbon fibers are alternatives to the fiberglass materials. 4.1.3 Metal Foils
Electrodeposited copper foils are available in a variety of weights. Copper is typically manufactured and sold by weight in ounces per square foot or increments thereof. The weight is converted to thickness with a factor determined by the density of the foil, which equates to 0.00135 for one ounce copper. These foils are plated onto a drum which creates a smooth side and the opposite side has a profile of the crystals or copper teeth. A treatment is then applied to one or both sides of the foil. The purpose of the treatment is two-fold, it improves the adhesion of the resin to the copper and creates a barrier layer between the copper and the resin. A coating is then applied to the copper to prevent oxidation during the lamination or baking processes. The copper foil is available with different elongation and tensile properties that need to be considered with the thermal properties of the application and selection of the resin sys- tem.
Rolled copper foil is available which eliminates the profile on the one side of the foil. Some of the thicker copper is only available in rolled form.
Very thin foils are typically provided on a carrier, which enables processing and handling of the foil. The carrier may be either peelable or etchable and it is important to inquire which will be provided.
Specialty metal foils may be required for special applica- tions. Some of the varieties available are: ED Nickel, Tin plated copper foil, coated foils, resistor layer and combina- tions such as ‘‘Copper-Invar-Copper’’ or ‘‘Copper- Molybdenum-Copper.’’
4.2 PREPREG OR ‘‘B-STAGE’’
Prepreg or ‘‘B-Stage’’ is comprised of a reinforcement that has been impregnated with resin and partially cured. It is
important to understand the characteristics of the prepreg to insure it is matched to lamination processing conditions. Characteristics to be considered are: the ratio of resin to glass (resin content percent or impregnation weight), degree of cure (melt viscosity, gel time, resin flow) and resultant pressed thickness (scaled flow). The value for scaled flow is a measure of material consistency, the actual value in production lamination may differ slightly based on specific lamination process parameters or board designs. Temperature and humidity control of storage conditions for the partially cured material is important to prevent chang- ing the prepreg characteristics and insure consistent perfor- mance during use.
4.3 LAMINATE
Laminate or core material is made of prepreg and metal foil that have been bonded together during a pressing operation and fully cured. The combinations of resins, rein- forcements and metal foils are almost infinite. The printed circuit design and specifications need to be reviewed to insure appropriate selection of material.
4.4 PROBLEMS ASSOCIATED WITH THE BASE MATERIALS 4.4.1 Material Identification
A PROBLEM CAUSE ACTION
Laminate label diffi- cult to remove.
The adhesive backing on the labels is not easily or completely removable.
Use labels with different adhesive. Change requirement to metal impression or ink stamp material.
B PROBLEM CAUSE ACTION
Material does not match label.
Material was mislabeled. Check product identification and work flow procedures. Check stock for additional problems.
C PROBLEM CAUSE ACTION
Material imaged and etched upside down.
Differential foil weights not identified clearly.
Standardize on marking either the heavier or lighter foil. Instruct operators on the standard marking.
Marking of differential foil weights is removed during the preclean operation.
4.4.2 Dimensional Stability
A PROBLEM CAUSE ACTION
Laminate exhibits dimensional change after processing.
Dimensional change parallel to warp direc- tion differs from the fill direction.
Check the identity of the warp direction of the laminate. Characterize the movement and compensate artwork. Material movement due to stress relief after
etching.
Retain as much copper on the panel as possible for mechanical strength. Bake laminates prior to processing. Use thicker laminate.
Designed annular ring is inadequate for manufacturability.
Review design to increase pad diameter. Review design to decrease hole diameter. Contact material supplier to review alternative material options available.
Mechanical scrubbing distorts material. Reduce brush pressure. Evaluate chemical cleaning of material.
Incomplete cure of the material. Verify degree of cure via Differential Scanning Calorimetry (DSC) or Thermal Mechanical Analysis (TMA). Press and/or bake material at time and tempera- ture appropriate for the specific resin system.
Moisture absorption prior to lamination. (This is extremely critical to the higher tem- perature resin systems.)
Verify drying conditions after oxide. Pre-bake prior to lamination to eliminate moisture. Use weight loss to deter- mine optimum time and temperature parameters. Minimize hold times in uncontrolled temperature and humidity envi- ronments between pre-bake and lamination processes. If required, store in conditions similar to those recommended for the particular prepreg resin system.
Weave distortion. Inspect raw cloth for uniform square weave and no bag- ging or sagging. Monitor tension during the treating opera- tion.
Excessive resin flow during lamination. Adjust lamination pressure and/or prepreg flow parameters.
4.4.3 Mechanical Stability
A PROBLEM CAUSE ACTION
Bow and Twist of Laminate Material.
Materials are stacked vertically. Store horizontally on shelves.
Materials with mixed grain directions. Match the grain direction of the glass cloth for all plies within a laminate. Avoid using square sheets or panels where grain direction traceability can be lost.
Asymmetrical glass fabrics in laminate. Review construction options for the desired laminate thick- ness. Change laminate thickness to insure symmetry of glass cloth.
Different copper weights on opposite sides of laminate.
Review laminate construction lay up. Change to thicker glass cloth against thinner copper.
Inconsistent wet out of glass cloth during impregnation and treating.
Cross-section and inspect for complete wet out of glass bundles and uniform resin thickness. Monitor across the web variation of resin content.
Material warped due to exposure to heat operations while panel is not properly sup- ported.
Insure that panels are properly supported whenever exposed to temperatures which exceed the Tgof the resin system.
Material warped due to moisture absorption prior to processing.
Pre-bake material prior to processing at temperatures appropriate for the specific resin system.
Excessively rapid cool down of material after reflow or HASL operations.
Control rate of cooling to avoid too rapid or uneven cool- ing of the panel.
4.4.3 Mechanical Stability (continued)
B PROBLEM CAUSE ACTION
Lifted circuits or low foil bond strength.
Treatment on foil incompatible with resin system.
Specify foil treatment that has been tested as being com- patible.
C PROBLEM CAUSE ACTION
Fractures around drilled holes.
Drilling feeds and speeds are too aggressive for the resin system.
Reduce drilling feed and speed rates. Investigate ‘‘peck’’ drilling part of the way through the board at a time. Drill all holes ‘‘first stage’’, prior to plating, before metal removal.
D PROBLEM CAUSE ACTION
Laminate thickness too thin at cross section.
Material thickness specified did not account for the differences between mechanical mea- surements with micrometer, foil profile and cross section measurements.
Increase the nominal laminate thickness specified to account for the difference. Specify purchase of material by cross section measurements. Specify low profile copper to minimize difference.
4.4.4 Foreign Material / Inclusions
A PROBLEM CAUSE ACTION
Copper or Zinc embedded in the laminate resin. (See Figure 4-1.)
Usually caused by a large nodule (tooth) that has broken off the copper foil and become embedded in the resin.
Evaluate for intended application. Review the profile of the copper foil for consistency and dimension. (Use low or very low profile foil, if required for application.)
B PROBLEM CAUSE ACTION
Etch pits in the lami- nate. (See Figure 4-2.)
A large copper nodule is etched away leav- ing a pit or white spot in the laminate.
Evaluate for application. Review copper profile. Specify low or very low profile foil.
C PROBLEM CAUSE ACTION
Treatment Transfer. Imbalance in the process that generates the treatment on the foil allowing separation.
Evaluate for acceptability. Contact material supplier. Figure 4−1. Copper or zinc inclusion in laminate. Figure 4−2. Etch pits in laminate.
4.4.4 Foreign Material / Inclusions (continued)
D PROBLEM CAUSE ACTION
Dark spots within the laminate or prepreg.
Contamination of dirt from raw materials or laminator’s plant.
Evaluate for intended application. Perform audit of opera- tion for cleanliness. Instruct supplier to review quality of incoming raw materials.
Darkened resins from reaction vessels or treater ovens.
Evaluate for intended application. Review preventative maintenance frequency for cleaning of treater ovens. Lint, fiber or hair in the laminate or prepreg.
(See Figures 4-3 & 4-4.)
Evaluate for intended application. Review clean room practices. Review static electricity levels in the prepreg sheeting and lay up operations.
4.4.5 Metal Surface Defects
A PROBLEM CAUSE ACTION
Pits and dents in the metal surface.
Dirt or contaminant on press plates or foil. Foil conforms to defect during lamination leaving a dent.
Specify appropriate ‘‘Grade’’ of material. Review plate cleaning process. Minimize exposure of plates and foil to environment during lay up operation.
B PROBLEM CAUSE ACTION
Scratches in the metal surface or treatment. (See Figure 4-5.)
Handling during the manufacturing process or storage.
Review equipment, conveyors, shelves and tables for sharp edges that could scratch laminate. Review proper handling procedures with operators.
Figure 4−3. Dark spots within laminate. Figure 4−4. Lint within laminate. Figure 4−5 Scratches on copper treatment. Figure 4−6 Shiny spots on copper foil.
4.4.5 Metal Surface Defects Metal Surface Defects (continued)
C PROBLEM CAUSE ACTION
Wrinkles in the metal surface.
Improper handling of the foil during lay up. Review proper handling procedures with operators. Movement of the laminate stack during press
loading.
Inspect handling and loading equipment for proper opera- tion. Instruct operators in proper method for transporting stacks of laminate.
D PROBLEM CAUSE ACTION
Resin spots on metal surface or areas of unetched metal.
Resin dust on metal surface during lay up operation.
Vacuum or heat seal edges of prepreg to minimize dust during lay up.
Pinhole in metal foil allows resin to bleed through to surface during lamination.
Check incoming foil for pin holes. Check for proper han- dling of foil during process. Minimize kinking of foil that could create micro cracks in the foil allowing bleed through. Check sheeter for dirt on rollers that may punc- ture foil.
E PROBLEM CAUSE ACTION
Shiny spots on the copper clad lami- nate. (See Figure 4-6.)
This is caused by a high resin spot on the prepreg due to broken filaments or chafed ends etc. in the glass cloth. This condition is not known to cause defects in the inner layer process, unless a wrinkle in the copper foil is present. Severe conditions can abrade the treatment on the back side of the foil.
Determine acceptability within the process. Inspect prepreg for resin lumps caused by broken filaments. Select glass styles and resin contents to minimize condition. Work with glass manufacturer for long term elimination of broken filaments.
F PROBLEM CAUSE ACTION
Haze on foil surface after cleaning or lack of imaging resist adhesion.
Improper control of the laminate pre- cleaning process.
Review process variations by chemical analysis. Modify bath make up frequency or incorporate feed and bleed sys- tem.
Laminate pre-cleaning process does not include chemistry to remove oxidation inhibitor.
Consult supplier to determine proper pre-cleaning chemis- try for foil.
Uneven application of the oxidation inhibitor on the foil surface.
Verify process control of the foil treatment process.