Welcome to Streamline Circuits Lunch & Learn Design for Reliability & Cost Reduction of Advanced Rigid-Flex/Flex PCB Technology
Accurate PCB data is critical to the tooling process. Here are some key items that you should always include when ordering PCB s
Fabrication drawing that lists the overall & critical dimensions of the PCB graphically. Drawing should also illustrate the layer stack up detail with overall thickness and any critical thicknesses between layers. All other critical information such as Solder Mask color, Silk Screen color, & Final Finish should also be included in your drawing. Indicate on drawing to build per IPC Standards and list the Class & type that best meets your needs. Most PCB S used in consumer products are built to Class II, and most Military, Aerospace, and Medical requiring high reliability will either require Class III or IIIA.
As you all may be are aware, our industry has many data formats with Gerber (RS274X or D) being the most commonly used. However most PCB fabricators prefer the ODB++ format, including Streamline Circuits for it s built in intelligence.
Technology Factors that influence the PCB cost. Material type Layer count Number of lamination cycles Number of drill cycles Lines & Spaces under 3.5 and some fabricators 5 mils will drive cost. Via fill per IPC 4761 Type 6 or 7 Final finish specifications Tolerances required exceed IPC Turn time PCB size Batch size Class type
When High Reliability PCB S are needed Specify Class III plating in the holes. This will increase the amount of copper in the holes by 20% vs. Class II. Add tear drops at all trace to pad junctures for added land area. Via fill your vias per IPC 4761 type 6 or 7 for the best possible via protection. Require your fabricator to show you that they meet or exceed your requirements by providing cross sections & test reports.
When High Reliability PCB S are needed Is there a risk that advice provided can actually increase the price of the PCB? In some cases, yes, especially at the prototype stage where perhaps more attention needs to be focused on lowest total cost for when that product is to be produced in volume, and the real benefits of lowest cost is most important.
Design Strategies Epoxy fill with copper Vias Resin Fill off set Vias Resin fill/stacked Vias All Copper Fill process
HDI Technology
What type of via hole plugging is recommended? The preferred type of plugging for standard product is IPC 4761 type VI filled and covered, with target being complete fill. The image below shows type VI with liquid solder mask coverage. Single sided plugging is not recommended (including type II tented and covered) due to concerns over entrapment of chemistry or likelihood of solder balls being present with HASL finishes.
Key Benefits to Via filling Improved reliability by reducing the risk of trapped air or liquids. Tighter BGA pitches and higher density interconnects by allowing for via-in-pad instead of dog bone designs. Streamline Circuits can support.25mm BGA requirements. Reliable filled and stacked via constructions. Planar copper surfaces above filled via for more reliable surface mounts and increased assembly yields. Enhanced thermal dissipation.
Design Advise Copper thickness of each layer and what is really needed. As a good rule of thumb, the lines and spaces should be a minimum 5x the copper base thickness. Example if you require 1oz finish on outer layers the lines & spaces should be a minimum 3.5 mils. Inner layers can use a multiply of 4.5x if not going through any additional plating processes.
Key factors in specifying materials CTE Z axis (Co-efficient of thermal expansion): This is a measure of how much the base material will expand when heated. Td (Decomposition temperature): This is the temperature at which material weight changes by 5%. This parameter determines the thermal survivability of the material. Tg (Glass transition temperature): The temperature at which the material stops acting like a rigid material and begins to behave like a plastic / softer. T260 (Time to delamination): This is the time it take for the base material to delaminate when subjected to a temperature of 260 degrees C. T288 (Time to delamination): This is the time it take for the base material to delaminate when subjected to a temperature of 288 degrees C. Dk (Dielectric constant): The ratio of the capacitance using that material as a dielectric, compared to a similar capacitor which has a vacuum as its dielectric. CTI (Comparative tracking Index): A measure of the electrical breakdown properties of an insulating material. It is used for electrical safety assessment of electrical apparatus.
Flex & Rigid Flex Tips Don t suddenly change track widths Don t do this Do this instead
Flex & Rigid Flex Tips Added support for pads
Flex & Rigid Flex Tips Cover lay openings should be smaller than the pad width in order to help anchor down the pads.
Flex & Rigid Flex Tips Transition Zone from Rigid Area to Flexible Area.
Flex & Rigid Flex Tips Should always have a minimum 50 mil clearances from any features in Transition Zone.
Flex & Rigid Flex Tips Nearly all Flex and Rigid-Flex boards are constructed with polyimide inside and this material is highly hydroscopic. It is strongly recommended to bake the boards to reduce the amount of moisture inside the boards before any type of soldering operation. Without such baking there is risk of delaminating, inner-layer separation or cracking of the hole walls.
Flex & Rigid Flex Tips Recommendation for baking per IPC-TM-650 Conditioning 5.1.1 The test specimen shall be conditioned by drying in an oven to remove moisture for a minimum of six (6) hours at 105 to 125 C [221 to 257 F]. This conditioning process is mandatory if this method is used for qualification purposes. This method shall replicate the assembly process. The requirement for conditioning (bake/drying) shall be in accordance with product/process lot acceptance criteria. If conditioning of the PCB is not part of the normal assembly process, and this method is being used for acceptance testing, then conditioning is not a requirement. 5.1.2 Test specimens that are thicker or more complex may require longer baking times to achieve acceptable moisture levels. Record the bake times and temperature if different than those stated in 5.1.1 (see 6.2).
RIGID-FLEX EXAMPLES 6 Layer Rigid Flex 8 Layer Rigid Flex Microvia 6 Layer Rigid Flex with Cavity 10 Layer Rigid Flex Microvia BGA 10 Layer Rigid Flex with Cavity and 4 flex, Microvia, Buried and Blind vias
MANUFACTURING CAPABILITIES STANDARD ADVANCED SINGLE-SIDE FLEXIBLE PANEL SIZE 12"x18" 18"x24" 20"x26" 24"x36" DOUBLE-SIDE FLEXIBLE PANEL SIZE 12"x18" 18"x24" and Up MULTILAYER FLEX PANEL SIZE 12"x18" 18"x24" and Up Layer Count 3 to 12 13+ RIGID FLEX PANEL SIZE 12"x18" 18"x24" and Up Layer Count 2 to 28 28+ Multiple Lamination Copper Foil Weights Internal/ External Kapton Polyimide Stiffener FR4 Stiffener Polyimide Rigid Stiffener 1/4 to 2 ounce Up to 3 ounce.001" to..008" and 007" Up.003" to. 062".063" and Up.003" to. 062".063" and Up Lines, spaces & pad diameters Internal Line Width.0035".001" Internal Spacing.0035".001" External Line Width.0035".001" External Spacing.0035".001" SMT Pitch.010".010" Controlled Impedance 10% 5% Via hole Finish STANDARD ADVANCED Laser Micro Vias.004".002" Blind/buried Vias.004".002" Laser Pads.004".002" Minimum Drilled Hole Size.012".0079" Drilled Hole to Copper.008".007" Castellation Yes Yes Finish surface Tin Lead Plating Thickness.0003" to. 0005" Less.0005" Tin Nickel Plating Thickness 150 Micro Inches 250 Micro Inches Low Stress Nickel 100 Micro Inches 250 Micro Inches Gold Plating Thickness 30 Micro Inches As Specified Electroless Nickel/Immersion Gold Yes Yes Immersion Gold Yes Yes Immersion Silver Yes Yes Entek 106A HT Yes Yes HASL Yes Yes TOLERANCES Plated Hole Tolerances (+/-).002".001" Non-Plated Hole Tolerances (+/-).001".001" Fabrication Tolerance (+/-).005".003" Vision Rout (+/-).003".002" Laser Rout (+/-).002".001"
SINGLE-SIDE FLEXIBLE CIRCUITS Single-Side flexible circuits consist of a single conductive layer on a flexible dielectric film. (see constructions below) FLEX SINGLE-SIDE CIRCUIT CONSTRUCTION SINGLE-SIDED FEATURES: -Very thin construction.003 -.008 (.075mm-. 20mm) -One conductive layer -Reverse bared or back bared pads, provide access from both sides of the part -Support & unsupported finger areas WHEN TO USE SINGLE-SIDED FLEX: _ Dynamic flexing applications _ Unusual folding and forming applications _ Installation/service applications / repair _ Limitations on space / thickness _ Installation / service flexing Unsupported Finger SCULPTURED FLEX CIRCUITS Sculptured flex circuits have variable copper thicknesses within the part. Thin copper is used for the flexible regions & thicker copper is used at the interconnection point. Sculptured flex circuits provide bare metal connection & are a highly reliable alternative to mechanically crimped contact pins Supported Finger
FLEX DOUBLE-SIDE CIRCUIT CONSTRUCTION DOUBLE-SIDE FLEXIBLE CIRCUITS Double-side flexible circuits consist of two conductive layers & can be with or without platedthrough holes, depending on design requirements (see constructions below) DOUBLE-SIDED FEATURES: WHEN TO USE _ DOUBLE-SIDED FLEX:. Two conductive layers _ Component assembly _ Required when circuit density and layout available on both sides can not be routed on a single layer _ Operating high frequency applications _ Signal or ground / power plane _ Supported and unsupported finger/ applications component areas _ Used for shielding applications _ Dense surface mount assembly _ Controlled impedance applications Adhesive Adhesiveless
MULTILAYER FLEX CIRCUIT CONSTRUCTION MULTILAYER FLEX CIRCUITS The construction that have three or more conductor layers are referred to as multilayer flex. The layers of the circuit are interconnected with plated-through holes, and with or without stiffeners. Adhesive MULTILAYER FLEX FEATURES: _ Three or more conductive layers _ Component assembly available on both sides _ Controlled impedance and shielding possible _ Supported and unsupported finger/ component areas WHEN TO USE MULTILAYER FLEX: _ Required when circuit density and layout can not be routed on a single layer or double layer _ Signal or ground / power plane applications _ Increased circuit density _ EMI/RFI shielding _ Used for shielding applications _ Dense surface mount assembly _ Controlled impedance with shielding applications Adhesiveless
RIGID-FLEX CIRCUIT CONSTRUCTION RIGID-FLEX CIRCUIT CONSTRUCTION: Rigid flex circuits are a hybrid construction, consisting of rigid and flexible substrates laminated together into a single package and electrically interconnected by means of plated-through holes with solid flexible or loose leaf flexible construction, and with or without stiffener (see construction below). Rigid flex boards are normally multilayer design, but double-sided (two-metal layer) constructions are possible as well, and in fact, have been selected for certain microelectronic chip-packaging applications. (see constructions below). RIGID FLEX FEATURES: _ Two or more conductive layers. _ Combined rigid & flex to achieve high-density packaging. _ Eliminate wires and wire harness assemblies. _ Folded/bended/positioned into package size. _ Easy assembly and installation. WHEN TO USE RIGID FLEX: _ Required when circuit density and layout can not be routed on a single layer. _ Used when components are mounted on both sides of the rigid and flex section. _ Used to solve high-density packaging problems. _ EMI/RFI shielding. _ Used for shielding applications. _ Dense surface mount assembly. _ Controlled impedance with shielding applications. _ Used to connect rigid boards together. Expose Finger 6 LAYER RIGID FLEX STACKUP 8 LAYER RIGID FLEX WITH UNBONDED STACKUP 6 LAYER RIGID FLEX WITH SILVER SHIELD STACKUP
SOLID COPPER: Solid copper is the most common method of shielding. Copper shield can be put on one or both sides of the circuit. Solid copper can also cover selective conductors. Solid copper shields increase the rigidity of the circuit, and should be included in thickness to bend radius ratios. CROSSHATCHED COPPER: Crosshatching is an artwork design that relieves much of the copper shield areas by the use of a pattern. Crosshatch shielding can also cover selective conductor. It also helps the circuit to retain its flexibility and can be put on one or both side. SHIELDING Shielding: If the application requires limits in electromagnetic interference/radiofrequency interference (EMI/RFI) shielding on-board or to fabricate low-voltage circuitry, on rigid or flexible substrates. Shields are material around a conductor or group of conductors that limit these factors. CONDUCTIVE SILVER: Conductive silver can be substituted for the copper for shielding purposes in some applications. Silver can be a solid or crosshatched shield and can be put on one or both sides of the circuit. It can also cover selected conductors only. Silver shielding is not recommended for a dynamic flexing application due to its brittle characteristic, and may be prone to cracking in severe bending applications. MINIMUM EDGE OF SILVER SHIELD: The minimum distance edge of silver epoxy to edge of non common electrical feature of pad / trace coverlay openings exposed pads / traces is -.010 and the edge of flex is -.005. (See Clearance picture below). Clearance
Finish Surface Dielectrics Surface dielectrics are applied to the outside layers of the circuit to insulate the copper conductors. Following are types of surface dielectrics used at all flex. COVERLAY: Coverlay is the layer of insulation film and adhesive that is applied totally or partially over a conductive pattern on the out surfaces of a printed board. This material is normally produced with laser CO2 drill/rout or mechanical drill/rout process. The common via holes are covered with coverlay. The minimum coverlay openings exposed pads for the component holes are. 005 larger than the copper pads. The coverlay openings can be individual barrel pads or gang relief pads depending on area available. LIQUID PHOTOIMAGEABLE SOLDERMASK: Liquid Photoimageable Solder Mask (LPI) is produced by a photo controlled process and used of tight pad spaces. This process enables unique openings to be applied anywhere on the circuit. LPI is usually not used with 2oz copper or above due to the thickness of the copper as it may not conform around the area of some copper features. The via holes are covered with solder mask. The LPI openings expose pads are.003 larger than the copper pads with.003 minimum web spacing. LPI is not recommended for dynamic flex applications.
Even though flex circuits are very pliable and flexible, there are limits to their flexibility. If the bend radius is too tight, the result can be de-lamination and conductor fracture. Bending and Folding Guidelines BEND RADIUS: For single and double-sided flexible printed wiring boards (PWBs), the minimum bend radius should be six times the overall thickness and at least.050 away from the plated through hole (see diagram below). Example: If the overall thickness of the flex circuits is.012, the minimum bend radius should be.072 RADIUSED TRACES: The radiused traces help to alleviate breading during folding and bending BUTTON OR PADS (BARREL) PLATING: The button or barrel plating is a process that allows for the plated through holes to maintain their connection while the traces are not plated, allowing the circuit to have increased flexibility. For multilayer flexible PWBs and multilayer rigid and flexible PWBs (bonded inner layers), the minimum bend radius should be 12 times overall thickness. Example: If the overall thickness of the flex circuit is.030, the minimum bend radius should be.360 FOLD LINES: The fold lines may be designated by tick marks which may be either in the copper layers or silkscreen layers. These features aid in bending and designating bend location. CIRCUIT TRACE WIDTH: The circuit trace width should not change in bend areas the transition should be at least.030 (.76mm) from the fold line.
STIFFENERS/PSA STIFFENERS The common stiffeners require support in areas where connectors or other components are applied. Here are the recommended types of guidelines for stiffeners. FR4/POLYIMIDE RIGID STIFFENER: Come in a variety of thicknesses such as.003 (.076mm),.010 (.25mm) or higher. Can be used to give added rigidity under a component area. Can be used as carrier panel for automated assembly processing. Can be bonded to a flex circuit using a pressure sensitive adhesive or a thermoset adhesive. POLYIMIDE (KAPTON) STIFFENER: Come in a variety of thickness from.001 (.02mm) up to.007 (.14mm) or higher. Can be used to give added thickness under conductors to meet ZIF connector requirement. Can be used to give added strength in high wear areas. Can be blanked at the same time as the circuit outline to meet tight tolerance requires. Can be bonded to flex circuits using a pressure sensitive adhesive or a thermoset adhesive AREA OF STIFFENER: The stiffener and coverlay termination points should overlap a minimum of.030 (.76mm) to void stress points. Eliminating stress points reduce the chance of traces breaking and cracking. COMPONENT HOLES: The holed size in the stiffener is recommended.015 (.38mm) minimum larger than the circuit component holes to allow for registration tolerances. PSA: Pressure Sensitive Adhesive is used to bond flex circuits or rigid circuits without heat requirement. The common PSA is used to bond flex circuits to stiffener to support in areas where connectors are applied. PSA also placed in key location to improve circuit placement and mounting.
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