dfx -dfm for flexible pcbs jeremy rygate · total care -ontzorging-全方位服务...

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10-10-2013

© Q.P.I. GROUP

DFX - DFM for Flexible PCBs

Jeremy Rygate

www.qpigroup.com

© Q.P.I. GROUP


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10-10-2013

Jeremy Rygate

30 years experience with Front End in the Electronics industry and PCB manufacturing.

Experience in advanced PCBs, particularly Flex, Flex-rigid and HDI circuits

Work among others on Cost Estimating, Production Methods, Tooling, CAM, Design for Manufacture (DFM), Project based Technical Customer liaison, New Product / Process Introduction (NPI), Cost Reduction exercises

Presented several papers. as “Some Dos and Don’ts of PCB Design”, “New PCB Technologies and Flex-Rigid manufacture” and “Manufacturing PCBs with Stretchable Materials”

www.qpigroup.com

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10-10-2013

Multi-layer - Flex - Flex-rigid - IMS – Back-panels – Stretch – Stretch Rigid – HF

Q.P.I. GroupTechnology Development PCB

Material and process development

EDA PCB-Design

CAM and sales support

� DFM and DFX

PCB supply

Prototype hand assembly

Laboratory

Netherlands – China – Hong Kong - Sweden

www.qpigroup.com

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10-10-2013

Pros of Flexible (and Flex-Rigid) PCB

Flexible

Thin (50µm thick dielectric commonly used with 25µm and 12,5µm as an option)

Light

Can be laser cut with extreme accuracy (25µm) and clean finish

Materials can be supplied in roll format enabling:

� Very long circuits

� Option to use “reel to reel” processing (increased efficiency)

High dielectric strength 6-7 kV/mil (~1kV/mil for FR4)

Consistent thickness and dielectric constant (3.4) as un-reinforced

No reliability issues associated with glass fibre cloth e.g. CAF

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10-10-2013

Pros of Flexible (and Flex-Rigid) PCB

Although flex-rigids can be more expensive than the sum of the individual parts of Flexis and Rigid PCBs, they are already assembled / tested and reliability is higher

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10-10-2013

Cons of Flexible (and Flex-Rigid) PCB

Materials more expensive (order of 10 x FR4 pricing)

PCB generally more expensive

Flexible, possibly needing addition support during assembly

More limited supplier base

Longer manufacturing lead times

Can be reliability issues if some “Golden Rules” are not adhered to

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10-10-2013

Relative cost of various flex dielectrics

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Approximate relative-cost comparison of selected flex circuit laminates

Ref. Fjellstad, J. 2006. Flexible Circuit Technology. 3rd ed. pp.49

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10-10-2013

Comparison of properties flex materialsProperty Polyimid

(Kapton)Fluorinated ethylene

propylene, FEP(Teflon-FEP, Neoflon-FEP)

Polyester

Tensile strength Very High low Extreme high

Manufacturing restrictions

Almost noneAdhesion Problems, low tensile

strength

Sensitive to process chemicals and

soldering temperatures

Fire Hazard Non-flammable Melt High

Moisture absorption High Lower Lowest

Dimensional Stability Unstable Unstable Stable

Flexibility Yes Weak Yes

Possible to connect Yes Yes Yes

Relative cost Moderate Expensive Cheap

Typical useMost commonly used insulation

materialExcellent dielectric Consumer Products

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10-10-2013

How to increase flexibility of flex

Use cross hatched planes (instead full copper plane)

� Note: Impedance values will change

Offset tracks on either side of a core and also on alternate layers. This reduces the strain or compression on the tracks

Use thinner copper. This enables greaterflexibility, but ensure current carrying capability is still met

Do not glue flex layers together. Creasing of inner flex still possible

Reduce overall flex thickness

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10-10-2013

How to increase reliability of flex

Use Rolled Annealed (RA) copper for flex layers.

� More flexible and less prone to cracking than Electro-Deposited (ED) copper

Place flex in the centre of the build.

� This usually enables flex to avoid plating processes, thus avoiding extra copper thickness and plated copper.

Do not use Nickel plating in flexing areas.

� Nickel (ENIG, ENIPIG or Electroplated Ni) is brittle and will crack on bending causing tracks to break.

� Solution 1: Change finish to a non Nickelfinish e.g. Immersion Tin, Immersion Silver or HASL.

� Solution 2: Remove finish from flexing area by applying coverlay or flexible solder resist in this area (prior Nickel finish). In example above move left edge of coverlay to right to meet pad.

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10-10-2013

How to increase reliability of flex (2)

Remove sharp angles in tracks near bend points. can cause a CRACK while bending

Use “button plating” if flex is plated.Thus avoiding extra copper thickness

Use elastic fillet e.g. Eccobond along Flex / Rigid interface. This will help prevent chaffing at the rigid edge

Avoid geometries that would encourage stress concentration.

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10-10-2013


Keep PTHs and Surface Mount pads away from F/R interface.

� As a rough rule keep PTH edge 3,18mm from F/R edge and S/M pads 0,5mm away from edge

Minimum radius of internal corners.

� The minimum radius of internal corners of the flex profile should be 1,5mm.

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10-10-2013


Coverlay in rigid area causes high z-axis expansion which leads to barrel cracking of PTHs. It also causes excessive

etch-back of the coverlayadhesive during the desmearprocess

� Remove coverlay from rigidarea but intrude into rigidby 1- 2mm

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Flex and flex-rigids should not be scored - residual material will not “snap off” on bending

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10-10-2013

Flex-rigid showing adhesive removed from rigid area

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10-10-2013

Material properties of adhesive(ref Teledyne)

Material Coefficient of Thermal Expansion

CTE(ppm/°C)

Glass transition

temperature, Tg(°C)

Acrylate-based adhesive

400-600 40

Epoxy-prepreg 85 130

Polyimid film 50 390

Polyimid-prepreg 55 210

High Tg epoxy-prepreg

70 170

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10-10-2013

Inner layer

Via

aperture wall

crack

Material properties

Effect CTE in the Z-direction

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Greater tendency to cracks between the inner layer and the hole wall for larger holes

Greater tendency to break in the wall of the hole for smaller holes (larger aspect ratio)

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10-10-2013


Avoid coverlay or high spots in support frame

� The support frame will be thicker than the rigid PCB, the gap leads to excess solder causing shorts.

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10-10-2013


Coverlay on flex or outer layer flex of Flexi-rigid. This will be difficult to resolve small surface mount apertures in coverlay in component areas.

� Solution 1: Apply coverlay all over and then laser ablate apertures. Allow for misregistration at least 25µm smaller apertures

� Solution 2: Increase apertures (see fabricator for sizes)

� Solution 3: Use flexible photoimageable S/R (liquid or dry film) all over but is less reliable than bonded Coverlay

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10-10-2013


Solution 4 (best): Use mix of C/L and rigid S/R

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Flex areas with Coverlay on L1

Rigid areas with Solder Resist on L1

Note: Coverlay overlaps Solder Resist onto rigid area by about 2mm

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10-10-2013

Support during assembly using fixtures / pallets

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Aluminium withflex-fixer

Examples of materials: aluminium, composite, plastic PEEK (polyetheretherketone)

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10-10-2013

Assembly - Humidity & Baking

Polyimide absorbs more moisture than Epoxy

� FR4 ~ 0.13% vs. Polyimide ~ 1-2%

Therefore vacuum desiccation or baking of flex and flex-rigid prior assembly is essential

The baking discussion would form another presentation in its own right

QPI has extensive knowledge, experience and data on the effects of moisture and baking of PCBs

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10-10-2013

Examples of errors in the circuit board caused by moisture and/or high Temp.

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(CAF)

(CAF)

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10-10-2013

Co-operation - the road to success

www.qpigroup.com

Construction

CAD

PCB Manufacturing

Prototype Manufacturer

Industrialisation

PCB

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10-10-2013www.qpigroup.com

Thanks foryour

attention

Q.P.I. Group B.V. Schootense Dreef 27 5708 HZ HelmondNetherlandsTel.: +31 492 590059E-mail: [email protected]

With thanks to our Swedish colleagues from Q.P.I.-Macer Sweden AB

dfx -dfm for flexible pcbs jeremy rygate · total care -ontzorging-全方位服务...

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