Do you avoid rigid flex design?

It is common to hear, “we avoid rigid flex”,  with the most common objection often being the learning curve to produce a good layout.  Today’s Quick Tip lists the benefits of using a rigid flex construction and situations where this technology makes good sense.

Rigid flex circuits are a hybrid construction consisting of rigid and flexible substrates laminated together into a single package.  They are electrically interconnected by means of plated-thru holes and can be solid flexible or loose leaf flexible construction,  with or without a stiffener.

When to use Rigid Flex vs. Flex

  • When stable area is needed for component mounting and packaging requires flex to fit or flex to install.
  • Used when components are mounted on both sides of the rigid and flex section.
  • Used to solve high-density packaging problems.
  • EMI/RFQ Shielding.
  • Dense Surface Mount Assembly.
  • Controlled impedance with shielding applications.
  • Used to connect rigid boards together.

Benefits of Rigid Flex:

  • Rigid-flex circuits offer enormous advantages in quality, especially with high vibration applications – eliminating connectors, mis-wiring and reducing assembly process steps.
  • Reliability of the assembly is proportionally increased due to the reduction of solder joints.
  • Weight reductions, due to the elimination of connectors and solder joints.
  • The performance of a rigid-flex is dramatically superior to a similar design with the rigid PCB’s and jumpers.  The connector leads and through holes required to join the jumper to the rigid PCB add parasitic inductance and capacitance to the circuitry.  The inductance of one net of the soldered jumper is in excess of 1.5nH, vs 0 of the same net in a rigid flex solution.  Speed, power and clarity of the signal is degraded by the use of the jumper/connector assembly.

Rigid-flex can be differentiated from multi-layer flex construction with stiffeners by having conductors on the rigid layers.  Plated thru holes extend through both the flexible and the rigid areas, with the exception of the blind and buried via construction.

We always recommend involving your supplier in the early stages of the flex or rigid flex design.  An experienced flex circuit engineer will be able to guide you to the correct material stack up and tolerances needed to ensure you receive the product you require.

Please contact us if you have any questions or would like additional information! 

Remember, designing and purchasing printed circuit boards does not have to be difficult!

Polyimide Coverlay and Adhesive Squeezout

When a flexible circuit requires high dielectric or dynamic flexing, an adhesive coverlay film is often the best choice.

This coverlay film is traditionally a layer of adhesive bonded to a layer of polyimide. During processing, heat and pressure are applied to the stack up causing the adhesive to soften and flow.   The adhesive will flow (squeeze-out) slightly beyond the coverlay openings.

This process is necessary for complete encapsulation of the coverlay and to protect the edges of the film from chemicals or abrasion which might cause delamination.

Although this is a desirable result of bonding the coverlay, this “adhesive squeeze-out” also reduces the solderable area of the coverlay opening, and must be accounted for in the design stage.

We are often asked what an acceptable amount of adhesive squeeze-out is. According to IPC-A-600, the coverlay coverage shall have the same requirements as the soldermask coverage in rigid printed circuit boards. The acceptability requirements for coverlay coverage include both the coverlay and the squeeze out of adhesive and are different based on which Class is being built to.

For example, Class 3 requires 0.05 mm (0.00197”) solderable annular ring for 360 degrees of the circumference. Class 2 requires this same solderable annular ring for 270 degrees of the circumference and Class 1 requires a solderable annular ring for 270 degrees of the circumference.

We always recommend involving your supplier in the early stages of the flexible circuit design. An experienced flex circuit engineering will be able to guide you to the correct material stack up and tolerances needed to ensure you receive the product you require.

Please contact us for additional information.  Designing printed circuit boards should not be difficult! 

www.omnipcb.com

Acceptance Criteria for Cap Plating of Filled Vias

We are periodically asked about the acceptance criteria for cap plating of filled holes.

Today’s engineering tip gives the acceptance criteria per IPC A-600.

Target condition:  Copper surface is planar with no protrusion (bump) and/or depression (dimple)

Acceptable condition – Class 1,2, and 3:
• Separation of copper cap to fill material
• No separation of the cap plating to underlying plating
• Cap protrusion (bump) and/or depression (dimple) meets the dimensional requirements in IPC 6012
• Fill material within the blind via shall be planar with the surface within +/- .076 mm (.003”) unless otherwise specified
• When cap plating is specified, fill material within the blind via shall meet the dimple/bump requirements of IPC 6012
• No voids in the cap plating over the resin fill

Please contact us if you have any questions!

Remember, designing and purchasing printed circuit boards should not be difficult!

Ormet Paste – Making Z-Axis connections during lamination

Paste instead of plating ~ something to think about…..
We have been part of several discussions recently regarding Ormet paste and thought others might be interested as well.

Ormet Paste is a product that has been around for a while and it seems that the market is just starting to catch up with the technology.

These products can be used for several different applications, but today we are focusing on using the product to make Z-axis connections during lamination.

In other words, the Ormet Paste 700 series materials allow you interconnect electrically while bonding layers mechanically.

Possible Applications:
Thick boards – layer reduction: 

  • Overall thickness reduction; reduction of aspect ratio by splitting a board into separate builds and joining with Ormet paste which can improve plating and drilling quality.
  • Elimination of back drilling and/or flip drilling

High Speed Cap – Mixed Dielectric Builds:

  • No hole plating of high speed layers.
  • Separate fabrication of high speed layers results in smoother outlayer surface resulting in improved RF performance.

“Any Layer” HDI using Paste:

  • Z-axis conductors applied prior to lamination.
  • Paste interconnects used to connect 2-layer cores in a single process step)

Why is Ormet Paste Different?

Transient Liquid Phase Sintering – Compositions comprising powder metallurgy (90% by weight) mixed in particulate form.

 During thermal processing:

  • The alloy becomes molten and reacts with metal to form new alloy compositions and/or intermetallic compounds
  • This reaction continues until one of the reactants is fully depleted (reaction starts at 150C, normal lamination temperatures).
  • This is unlike most silver pastes which are held together by the polymer.
  • This also forms a metallurgical bond with metals it comes in contact with.

Ormet does not cure, it sinters into a metal mass.

This is very basic information taken from the Ormet literature.  If you are interested in more detailed information, please let us know.  Contact information is included below.

Remember, designing and purchasing printed circuit boards does not have to be difficult!
Tara Dunn – tarad@omnipcb.com – 507-332-9932
Elizabeth Foradori – elizabeth@omnipwb.com – 856-802-1300

Pad Cratering: The Basics

Pad cratering, what is it?  Pad cratering is defined as a separation of the pad from the PCB resin/weave composite or within the composite immediately adjacent to the pad.  It triggers failures only if the crack propagates into a copper trace or conduction pad makes the circuit open or intermittent.

Pad Cratering Drivers:

  • Finer pitch components
  • More brittle laminates (lead-free compatible)
  • Stiffer solders (SAC vs. Sn Pb)
  • Presence of large heat sinks
  • Location
  • PCB Thickness
  • Temperatures and cooling rates

Issues with Detecting Pad Cratering

  • Companies are frequently unaware until failure occurs
    • Recalls can be frequent and painful 
  • Potential Warning Signs
    • Excessive BGA repair rates
    • High percentage of “defective” BGA’s
    • High rate of “retest to pass” at in circuit test (ICT)
  • Difficult to detect using standard procedures of X-ray, generally must use destructive testing such as dye n pry, ball shear, and ball pull
  • Expensive advanced testing methods now available to screen for pad cratering

“Band Aid” Solutions for Pad Cratering

Solutions to prevent costly damage are labor intensive, costly, or have marginal impact on the problem.

  • Expoxy fillets at BGA corners
    • Ineffective and do not prevent board flexing
  • Increase board thickness to eliminate flexing
    • Expensive
  • Changing BGA and PCB pad design
    • Limited effectiveness, limits design and sources of components, can be expensive
  • Underfill of BGA components
    • Labor intensive, prevents rework of BGA parts after assembly

Zeta Cap as a solution

 

How does it work?

  • Zeta Cap requires no special processing or equipment.  It simply replaces the outer layer foil in the PCB construction
  • When used as a cap layer, it becomes the interface between the copper pad and the rest of the PCB
  • The more pliant cap will prevent or block fractures and protect copper connections (traces) the the pad

Why does it work?

It serves as a barrier to block pad cratering with a unique combination of properties:

  • High strength without being brittle
  • Modulus allows for some flexibility
  • Very high Tg and Td
  • Very low CTE (close to copper and solder)
  • Halogen free
  • Lead free compatible
  • Used on PCB surface with convention materials

If you are interested in learning more, please contact us for additional information.  Material spec sheets and test and reliability data are readily available! 

 

Purchasing and designing printed circuit boards does not have to be difficult! 

 

A special thank you to Insulectro for working with us and compiling this information! 

 

  

PCB Material Data Sheets

For all of our friends that feel just a little uncertain when material data sheets are discussed and words like dielectric constant, Tg and  coefficient of thermal expansion are thrown into the discussion, we have compiled a short cheat sheet just for you.

Dk – Dielectric Constant – The way the electrical signal moves through the material from front to back.
Dissipation Factor – Loss Tangent – How much the signal dissipates or “leaks” out of the substrate.  The higher the speed, the higher the losses.
Coefficient of Thermal Expansion – Thermal fractional change in material for unit change in temperature.
Resistivity:  How strongly the material resists a current.
Tensile Strength:  Relates to the rigidity of the material as well as the peel strength.  How far can the material be stretched before permanent damage occurs.
Dimensional Stability:  The tendency of material  to change in the x and y dimensions when copper is etched away from the dielectric material or when exposed to temperature extremes.
Tg – Glass Transition Temperature – Temperature that material exhibits a change in physical characteristics. The resin changes from rigid or hard to a soft, rubber like material.
Td  – Thermal Decomposition Temperature – Temperature at which the material weight changes by 5%.  This parameter determines the thermal survivability of the resin material.
 
As always, if you have questions or would like additional information, please contact us!
Purchasing and designing printed circuit boards does not have to be difficult!

Benefits of a fine line additive PCB process

The traditional subtractive etch process for manufacturing printed circuit boards becomes much less reliable when working with features that are sub 1 mil.  There has been a new additive process developed that uses a precursor ink to break through these barriers and meets four market needs:

Image

1.  Fine line (< 25 micron) Additive (Rigid and flex PCB)

  • Precursor ink makes the additive process practical
  • Additive process makes the fine lines practical
  • This process works well with existing manufacturing equipment
  • Cost savings over conventional processing can be 35%-60%

2.  Via excellence (Rigid PCB)

  • Precursor ink is the key advantage
  • Reduces electroless usage by 60% or more
  • Reduces the use of water by 60% or more
  • Lower cost than conventional processing

3.  Stacked microvia excellence (Rigid/Flex PCB)

  • Better results than conventional processing
  • Precursor ink is the key advantage
  • Eliminates the need for sequential lamination
  • Cost savings can be 20%-30%

4.  Selective via plating (Multilayer rigid PCB)

  • Precursor ink and ink blocker are the keys
  • Eliminates the need for sequential lamination processing
  • Cost savings can be 20%-60%

This process can be used as a stand-alone technology or in conjunction with the traditional subtractive etch process.  Please contact us if you are interested in learning more!

www.omnipcb.com