DFX - DFM for Flexible PCBs Jeremy Rygate

Transcription

1 DFX - DFM for Flexible PCBs Jeremy Rygate 1

2 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 2

3 Q.P.I. Group Technology Development PCB Material and process development EDA PCB-Design CAM and sales support DFM and DFX PCB supply Prototype hand assembly Laboratory Multi-layer - Flex - Flex-rigid - IMS Back-panels Stretch Stretch Rigid HF Netherlands China Hong Kong - Sweden 3

4 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 4

5 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 5

6 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 6

7 Relative cost of various flex dielectrics Approximate relative-cost comparison of selected flex circuit laminates Ref. Fjellstad, J Flexible Circuit Technology. 3rd ed. pp.49 7

8 Comparison of properties flex materials Property Polyimid (Kapton) Fluorinated ethylene propylene, FEP (Teflon-FEP, Neoflon-FEP) Polyester Tensile strength Very High low Extreme high Manufacturing restrictions Almost none Adhesion 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 use Most commonly used insulation material Excellent dielectric Consumer Products 8

9 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 greater flexibility, but ensure current carrying capability is still met Do not glue flex layers together. Creasing of inner flex still possible Reduce overall flex thickness 9

10 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 Nickel finish 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. 10

11 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. 11

12 How to increase reliability of flex (3) 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. 12

13 How to increase reliability of flex (4) Flex and flex-rigids should not be scored - residual material will not snap off on bending Coverlay in rigid area causes high z-axis expansion which leads to barrel cracking of PTHs. It also causes excessive etch-back of the coverlay adhesive during the desmear process Remove coverlay from rigid area but intrude into rigid by 1-2mm 13

14 Flex-rigid showing adhesive removed from rigid area 14

15 Material properties of adhesive (ref Teledyne) Material Acrylate-based adhesive Coefficient of Thermal Expansion CTE (ppm/ C) Glass transition temperature, Tg ( C) Epoxy-prepreg Polyimid film Polyimid-prepreg High Tg epoxyprepreg

16 Material properties Effect CTE in the Z-direction aperture wall Greater tendency to cracks between the inner layer and the hole wall for larger holes Via crack Inner layer Greater tendency to break in the wall of the hole for smaller holes (larger aspect ratio) 16

17 How to increase reliability of flex (5) 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. 17

18 How to increase reliability of flex (6) 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 18

19 How to increase reliability of flex (6) Solution 4 (best): Use mix of C/L and rigid S/R Flex areas with Coverlay on L1 Rigid areas with Solder Resist on L1 Note: Coverlay overlaps Solder Resist onto rigid area by about 2mm 19

20 How to increase reliability of flex (7) Support during assembly Integral support frame 20

21 Support during assembly using fixtures / pallets Aluminium with flex-fixer Examples of materials: aluminium, composite, plastic PEEK (polyetheretherketone) 21

22 Examples of fixtures for two-sided assembly 22

23 Assembly - Humidity & Baking Polyimide absorbs more moisture than Epoxy FR4 ~ 0.13% vs. Polyimide ~ 1-2% Therefore vacuum desiccation or baking of flex and flexrigid 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 23

24 Examples of errors in the circuit board caused by moisture and/or high Temp. (CAF) (CAF) 24

25 Co-operation - the road to success CAD Construction PCB PCB Manufacturing Prototype Manufacturer Industrialisation 25

26 Thanks for your attention Q.P.I. Group B.V. Schootense Dreef HZ Helmond Netherlands Tel.: With thanks to our Swedish colleagues from Q.P.I.-Macer Sweden AB 26