Design Guidelines

As a practical guide to considerations involved in fabricating PTFE and Thin Film Microwave Circuits, Filtran Microcircuits Inc. has prepared the following information to streamline your design process and ensure successful production results.

In addition to being awarded the ISO 9001:2000 certification in September 2003, Filtran works to IPC 6018 and can comply with the standards set out in MIL-PRF-31032 (preceeded by MIL-P-55110 which is still in effect for designs created in 1997 or earlier) for GR (non woven PTFE) materials. The ISO 9001:2000 certification implies a similar capability to process all PTFE laminates. Filtran specializes in the fabrication of all microwave materials from such vendors as Rogers, Arlon, GIL and Taconic. Typical structures are microstrip, bonded stripline, mixed dielectric multi-layer and heat-sinked circuitry.

As a leader in the vacuum sputtering industry with several patents, Filtran also sputter deposits thin films onto a variety of hard substrates.

1. Plated-Through-Holes

Difficulty in processing the smallest achievable plated-through-hole diameter is related to the "hole aspect ratio", which is defined as the ratio of material thickness to hole diameter.

As the aspect ratio increases above 5:1 the holes become difficult to drill and plate. Ratios of up to 10:1 require special attention.

Using .050" thick dielectric as an example, a .017" hole represents a ratio of 3:1. If a .010" hole is required, the ratio is 5:1. Similarly, in .100" material, a .020" diameter hole represents a 5:1 ratio.

Not only does the higher aspect ratio create difficulties in drilling and plating, but also small drill sizes can cause yield problems. Drills less than .012" diameter are very flexible and tend to wander, producing holes likely to be oval, ragged and/or poorly located. Drill breakage is also a hazard. If plated-through-holes of less than .010" in diameter are a requirement (regardless of the aspect ratio), please contact Filtran for guidance

Plating thickness in holes must also be taken into account when specifying tolerances on the "diameter after plating". Drill tolerance, plating tolerance and the effects of plating non-uniformity experienced with high aspect ratio holes add together so that an overall tolerance spread is larger than usual. Ideally, Filtran will maintain a range of .0005" to .001" of copper in holes to minimize tolerance effects.

1a. "Blind Hole" Process

Filtran's proprietary "Sputtered Blind Hole" (SBH) process is used when plated-through-holes are required for metal-backed circuits (see Figure #1). This process provides reliable plating to aluminum, copper, or brass heatsinks/groundplanes. This is achieved through an initial thin film copper, "seed" metallization that is then reinforced by electroplated copper. Finally, a surface finish (tin, tin-lead, nickel, gold) is added per customer requirements. The SBH process is used for thick (up to .500") aluminum backings where small vias are required.

Fig. 1.  Anatomy of a sputtered blind hole

Regular plated-through-hole technology is complicated by the requirements of plating on aluminum, as well as PTFE. These complications are severe enough to force some manufacturers to use press-fit pins for grounding purposes. Reliable plated-through-holes may be difficult to achieve because of the typically high aspect ratio of holes in these structures. These technical problems often result in pits, blistering and voids. As a result, manufacturers using a chemical plating approach must mill out the metal backing around vias to reduce the aspect ratio. In many designs this becomes costly and restricts design options.

The SBH process allows designers to specify vias that are as small as .025" in diameter without the need for additional machining of counterbores on the backside of the metal carrier. The process is currently accepted by satellite, military and commercial customers. If you have any questions regarding this process, please call Filtran for further design considerations and information.

2. File Transfer

Filtran accepts files from customers in the following formats: DxF, Gerber, and IGES. Customers can send their files by modem to (613) 737-7262, by diskette or by e-mail (cad@filtranmicro.com). Accurate artwork is critical to the circuit meeting performance specifications. Filtran strives to ensure that the artwork is 5 times more accurate than the end product requirements for circuit definition.

All files should represent the circuit layout as an exact one-to-one showing the end product required by the customer. Hole drilling, circuit layers, routing, silkscreen and soldermask should be represented by separate layers in the file. Filtran's CAD group will apply any necessary etching allowances and stepping of the circuit image to use the laminate materials most efficiently. Notes in files (or hard copy drawings) should clearly depict:

1. Quality specifications.
2. Plating type and thickness.
3. Acceptable tolerances of mechanical shapes and circuit features.
4. Material type.
5. Size and quantity of plated and unplated holes.
6. Special packaging requirements.

It is also recommended that there be one or two reference dimensions for inspection at Filtran and, if necessary, at the customer's incoming inspection operation.

Artwork must accurately show all features. Easily distinguished corner markers shall be included to indicate board outline. All identification, lettering or numbers must be thick enough to exceed the finest circuit features. The thinnest part of any character should be greater than .005", preferably .008".

Filtran uses the latest versions of Gerber and AutoCAD software, along with MasterCAM and the applicable translation software.

3. Photo-Resist

Filtran Microcircuits utilizes a proprietary liquid photo-resist process, suitable for single-sided, double-sided and multi-layer circuits with plated-through-holes. This provides a virtually error-free imaging process. High resolution, dry-film photo-resists are often used for more tolerant applications. Circuit feature size and tolerance and the plating approach being taken will all be considered when determining the correct photo-resist to be used. Some dry film resists react poorly during exposure in the electrolytic gold bath. The resist can lift resulting in poor edge definition.

4. Annular Rings

Plating of dielectric surfaces require a metallic interface to which the additive metal is conductively joined. Plated-through-holes or plated edges must be surrounded by the laminate copper ensuring that a secure and electrically conductive bond between the holes or edge and the circuit pattern is obtained. In this regard, a plated hole needs an "annular ring" of laminate copper around each side of the hole (one side may be a ground plane). For satisfactory performance and manufacturability, it is preferable for annular rings to be a minimum of .010" (i.e. pad diameter to be .020" larger than the drilled holes). Smaller annular rings down to .005" are possible depending on circuit size and materials used. This can affect manufacturability and yield, thus resulting in higher processing costs.

5. Front-to-Back Registration

Accurate front-to-back or layer-to-layer registration of imaged features is important for many microwave applications. Manufacturability is reduced for front-to-back registration tolerances of less than ± .002" and for critical front-to-back registration when many fine features are spread over a large area. If required, registration of less than ± .002" down to ± .001" may be achieved at extra cost.

6. Etching

In general, lines or gaps down to .003" can be included if necessary, and etching tolerances of .0005" may be achieved. Note, however, that thicker copper requires greater allowable tolerance due to the degree of "undercutting" created by the etching process. Filtran recommends using laminate copper foil of 1/2 oz/sq. ft. (17.5 microns thick).

For all standard processing, the thickness of copper required in plated holes will also be plated on the surface and will also have to be etched off. This means that even with 1/2 oz/sq. ft. (17.5 microns) of laminate copper, an extra .0005" to .001" of plated copper is added prior to etching. The etching process must then contend with a total of .0012" to .0017" of copper. If you must have more than .001" of copper in the plated holes, per MIL-P-55110 or IPC-HF-318, and you need tight tolerance etching (fine definition), please ask us about selective plating options. Filtran and other manufacturers are often capable of maintaining a thin copper in high resolution areas while plating up grounding areas.

Features on finished boards will be as faithfully reproduced as the artwork allows, limited only by the photolithography and etching process accuracies. Filtran's special exposure systems and etching process controls assure that the highest standards are obtained. The smallest features (lines and/or gaps, etc.) on a particular circuit will dictate our choice of processes to be applied, using the variety of techniques available.

The design should not include any lines or gaps which are unnecessarily tight, as extra expense can be incurred. An example that is often overlooked is the inclusion of logo, lettering or other patterns which exceed the complexity of the functional circuitry.

7. Plating Finishes

The following finishes on circuit, ground-plane or backing metals can be provided to customers depending upon their requirements.

1. Bare Copper

   Bare copper provides superior circuit definition although, if left exposed to air and moisture, will corrode very quickly. Boards with this finish are shipped in air-tight plastic bags. It is recommended that the circuits undergo minimum handling and storage times be kept as short as possible.

2. White Immersion Tin

   Filtran provides immersion tin processing using Omikron chemistry. Typical thickness will be under 40 microinches (1 micron). This finish is thin and will not affect the circuit resolution as will 300-500 microinches of tin-lead solder. Immersion tin acts as an acceptable solderable coating with a shelf life of 6 - 12 months if stored under ideal conditions. Tarnishing can occur when exposed to moderately hostile environments. In such cases, a mild acid cleaning and/or light abrasive cleaning immediately prior to soldering is recommended.

3. Tin-Lead (Solder)

   We recommend a thickness that is limited to about 200-500 microinches. Electroplated tin-lead on high-frequency circuits is often undesirable due to the loss characteristics of this alloy. Also, the thickness of solder commonly specified (above 500 microinches) can degrade the required circuit definition.

   Nevertheless, if tin-lead is required, Filtran can provide it. Fusing is also available.

4. Electrolytic Nickel

   Filtran can electroplate nickel per QQ-N-290 as a barrier layer between copper circuitry and a final gold plating surface. Such a barrier is often specified to prevent "migration" or inter-diffusion of copper and gold. This inter-diffusion will occur over time according to temperature and humidity conditions. The severity of environmental conditions, and the thickness and porosity of the gold, will determine whether this is likely to be a problem.

   Nickel is not as conductive as other metals. As a result, it could degrade the electrical characteristics of a circuit beyond tolerable parameters. Accordingly, Filtran suggests that this point be considered in microwave circuit designs. Historically, 90% of all customers with plated gold applications specify a nickel barrier, even at millimeter wave frequencies.

5. Electrolytic Gold

   Filtran provides electroplated gold to SAE AMS 2422 or ASRM B488. The resulting finish is excellent for corrosion resistance, durability and wire bonding. Filtran produces a soft (Grade A) and very pure (Type III) gold finish. It is the highest quality available. For wire bonding applications, Filtran recommends a minimum of 100 microinches of gold over a minimum of 50 microinches of electroplated nickel. For applications that only require solderability and corrosion protection, the gold thickness can be 5-25 microinches over 50-100 microinches of nickel.

   For cost reduction and yield purposes, it is possible to selectively plate thicker gold in the areas of wire bonding and thinner gold everywhere else to accommodate surface mount assembly. This is achieved with multiple photolithographic steps and the cost trade-offs must be examined for each requirement.

   Two methods of electrolytic gold-plating are available. A "Plating bar" approach uses 3-5 mil traces (bars) to electrically interconnect isolated circuitry. The circuitry is first etched in copper and then electroplated with gold. The plating bars are then manually removed from the finished circuits at an additional cost. Filtran has developed a proprietary process for etching of plating bars when the quantity of isolated lands is too large for manual removal. This method eliminates the concerns for "overhang" and exposed copper on edges of the trace.

   A second and more common approach is that of "pattern plating". This process produces exposed copper on conductor edges. It results in an etch profile with an overhang of nickel and gold. The overhang or undercutting must not become too large. If this happens, it could result in an unacceptable variation in line definition (out of tolerance), or slivers that can break off and ultimately cause shorts. In order to deliver acceptable product, a circuit manufacturer must keep tight control on etchant concentration, etching time and etching uniformity.

6. Electroless Gold

   A non-electrolytic or "electroless" plating may be applied if a very thin gold finish (nominally 10 microinches) is required. Electroless gold will cover all exposed copper including conductor edges. Disadvantages include limited thickness availability, lack of adequate durability, and some process limitations. This method is rarely recommended.

7. Plating and Protective Finishes On Aluminum

   Filtran offers zincate followed by copper or nickel and then any of the above plating finishes. Alternatively, Filtran can chromate aluminum per SAE AMS C-5541.

8. Soldermask

   Filtran uses two types of soldermask: liquid photo-imageable and photo-imageable dry film. Although the most common is liquid, dry film is useful in tenting over holes or when three dimensional circuit structures are involved.

8. Machining

Filtran maintains a complete computer-aided machining capability to accurately punch, rout or mill thin substrates as well as thick metal backings. Filtran co-workers have a variety of CNC milling centers, standard PWB CNC driller/routers and dicing equipment at their disposal. Die sets are used on various presses for volume applications. Normal machining tolerances are ± .005" down to ± .001". Tolerances of < .002" should be avoided. Pockets, counterbores, slots and tapped, blind or through holes down to #0-80 are routine operations in aluminum, copper or brass.

Filtran has installed a CO2 Laser Machining facility. Laser machining is often advantageous in processes requiring complex shaping coupled with smaller circuit quantities. Die sets can be very expensive until part counts reach quantities beyond a thousand. Standard PWB routing cannot always meet requirements for accuracy and can cause burring at the cut edges of certain microwave materials. Deburring can be a costly procedure that is often avoided using a laser with the correct settings.

Laser machining can be performed in two ways - by programming the cutting coordinates and following the prescribed cutting path with a focused beam, or by low-power scan of the groundplane side of the circuit. The latter approach, while often taking more machine time, is superior for quick turn and short run requirements because the time for programming is saved. In many microstrip applications, the groundplane is continuous on the backside of the circuit board and this acts as a stencil. With the proper parameters, the accuracy of cut can approach the resolution of the imaged pattern. Microwave materials with a high degree of ceramic content will often require a focused beam approach.

CO2 laser does not cut through copper. As such, customers are advised to recess all traces or copper groundplanes .003" from the edge of cut.

Laser machining is ideal for the following:

1. Dielectric with less than .030" thicknesses.
2. Complex machining.
3. Internal slots with square corners.
4. Circuit sizes smaller than .25" by .25".
5. Slots smaller than .016".
6. Machining tolerances that are tighter than .002".

Upon receipt of files from the customer, Filtran's engineering group will panelize circuitry for optimum use of material. Circuitry is stepped on the panel with appropriate alignment targets, coupons and spacing between circuits. Filtran's standard processing calls for margins at the outside edge of the panel for handling. A minimum outside margin of .500" on one side and .250" on the other three sides of the panel is required. The margins may be increased depending upon the size of panel and the overall thickness of the circuit, including any thick metal backing.

Spacing between circuitry will vary depending on the thickness of the material and the process of machining being chosen. For laser machined parts the spacing will be at .010". Diced parts are spaced between .005" and .009". Filtran chooses spacing in the range of .100"-.125" for circuitry requiring standard PWB routing and where die cutting is used. Finally, for CNC milling, spacing will be between .150" and .300" depending on thickness of the metal backing – assume that the spacing should equal the thickness of the metal backing or a minimum of .150". Spacing and margin guidelines need only be considered by the customer when the customer is supplying materials for processing. In this case, please speak to Filtran engineering.

9. Nomenclature

The most common board identifiers are date codes, cage codes, serial numbers and part numbers. When date-codes are necessary, Filtran will incorporate a 4 digit "YYWW" (year, week) code with our logo. Please indicate the required location. If Filtran is required to position this feature on the circuit, the customer will be notified of the location by way of fax or e-mail. Acceptance by the customer is necessary prior to the start of production. This nomenclature can be defined by etching or by silkscreening. Where component positioning is required, silkscreening is used exclusively.

10. Bonded Stripline and Multilayer PTFE Circuits

Depending on customer requirements, Filtran Microcircuits processes bonded striplines and mixed dielectric multilayer circuitry using a number of commercially available bonding films and techniques. Low and high temperature bonding films originate from Dupont and are supplied by Arlon or Rogers. The choice of bonding film is often defined by the temperature excursions that the circuit board will experience either in assembly or in operation.

Board surfaces may need to be sodium-etched or plasma etched prior to bonding depending on the degree of exposed PTFE surface (dielectric) and the amount of exposure to UV light after etching

Often the dielectric constant of the bonding films do not match the dielectric constant of the core materials being bonded. In a bonded stripline structure, this could have a performance impact. Although Filtran can help, the decision to choose the right materials must be made by the customer's electrical engineers. It is always recommended that a first article be done to confirm a design layout as well as a material choice.

"Fusion Bonding" is a method which takes the multilayer structure to the melting point of the PTFE and fuses layers together without the use of bonding films. If this technique is desired, Filtran will refer the customer to our colleagues at Merrimac Industries in New Jersey.

Customer-supplied files (DxF or Gerber) should include layer-to-layer registration markings and cross-sectional descriptions of the assembly.

Complex machining of pockets and external features are offered using various techniques. Plated edges and plated holes connecting front-to-back and layer-to-layer (blind and buried) are also available. Filtran uses a wide range of masking agents to protect circuitry during subsequent plating or etching cycles.

11. Thin Film Capabilities

Filtran Microcircuits Inc. uses custom Magnetron Sputtering Facilities and Microlithography to achieve the following:

1. Minimum Line Width: .001" (25 microns).
2. Minimum Spaces: .001".
3. Reliable vias (Plated-Through-Holes) as small as .010" in diameter. All hole drilling and scribing on ceramic are processed by in-house laser facilities.
4. Metallized edges including half vias as an alternative for grounding vias.
5. Metallization:
   1. By Sputtering: a wide range of combinations using aluminum, copper, gold, chrome, nickel, titanium, tin, tungsten, tantalum, molybdenum, etc. Resistor Deposition is typically 50 ohm/square using nickel-chromium. Adhesion layers provided are chrome and titanium-tungsten.
   2. By Electroplating: plating to a thickness of 5 microns. (200 microinches for gold, copper, and other metals.
6. Substrates: alumina (white or black), aluminum-nitride, quartz, BeO, glass, silicon, Kapton, etc.
7. Edge metallization available, including one half vias.
8. Proprietary copper metallization for high performance circuits.
9. The following is a capabilities chart that can be used in concert with, or without our Design Guidelines brochure. It can be used as a short form checklist for any designer to confirm that their layout is in conformance with Filtran's manufacturing standards. "Standard" and "Premium" refer to difficulty in processing and are reflected in the pricing structure.

Chart

Please contact Filtran Microcircuits at (613) 737-0706 x243, or E-mail us at fmi@filtranmicro.com, if you would like us to send you a free copy of our new Design Guidelines brochure.

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