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Quality assurance - Part 1 : During Production

Introduction

Quality is not something that is inspected into your PCB. We build it into your boards from the moment you open the price calculator. Our smart menus guide you towards optimum manufacturability. Then PCB Visualizer checks the manufacturability of your specific data-set. We back the quality of your data by preparing the right tooling, using the right equipment, buying in the right materials, designing and implementing the right processing, and hiring and training the right operators. There’s more on this in our video: “How to make a PCB.” Operator training is critical. It is the duty of every operator to check the boards as they go through their process, and we make sure that they have the training and the expertise.

Of course, our fabrication process also include specific inspection and test steps. We use these to make sure that our processes are running correctly. These steps give you the added re-assurance that the board you receive is correct to your design and will perform correctly over the lifetime of your product. These steps are described below.

Standards

We inspect all boards to IPC-A-600 Class 2 This is the standard used for most PCBs, and is the standard most often specified by our customers. The IPC, or Institute for Printed Circuit Boards, is “a global trade association representing all facets of the industry including design, printed circuit board manufacturing and electronics assembly.” The IPC-A-600 standard “describes the preferred, acceptable and non-conforming conditions that are either externally or internally observable on printed boards”. It divides PCBs into 3 product classes. Class 2 includes “products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical.” Class 3 (where uninterrupted service is critical) is used for aerospace, defence and medical applications. For more information visit www.ipc.org.

Customers, especially those supplying the US market, may also specify UL marking. In this case, we further inspect to UL796. The Underwriters’ Laboratory (UL) is “an independent global safety science company …. dedicated to promoting safe living and working environments, UL helps safeguard people, products and places in important ways, facilitating trade and providing peace of mind.” For PCBs, the most important criterion highlighted by UL marking is flammability. All our FR4 material meets UL 94 V0 plastics flammability test. For more on UL visit www.ul.com.

Inspection steps during production.

Front-end Engineering

The first step is to make sure that the data that we will use to make your PCBs is correct. To find out how we do this, go to our blog “Front-end Data Preparation

Fabrication tests

We run 3 types of test during fabrication, visual, non-destructive measurements and destructive tests. The destructive tests are used to check our processes. They are made on actual PCBs or on the test coupons which we put onto every production panel. After more than 30 years of PCB manufacturing experience, we have developed test coupons on the production panels which provide simple, non-destructive tests for more complex parameters.

Each fabrication step can be seen in our video “How to Make a PCB”. The sequence below is based on a multilayer PCB. Single- and double-sided boards do not use all these steps, but are tested in the same way.

Passport

The results of these checks are summarised for each job in its Passport which contains information on the materials used, measurements made and tests passed. You can access the Passport via the blue book icon with each job under View Running Orders once it has been inspected or under Order repeats/View history.

Traceability

If you need more information on a job, we have full traceability back to material batches etc. Contact euro@eurocircuits.com or your local sales channel for this service.

Step 1. Base material.

This is automatically checked against the order details using a data-matrix. The material data (type, manufacturer, laminate and copper foil thickness) is entered into the job history and will appear in the final Passport.

 

 

2. Print and Etch inner layers.

Visual checks.

This step includes 3 visual checks:

  1. After printing and stripping to make sure that the unwanted etch resist has been stripped away cleanly
  2. After etching to make sure that all the unwanted copper has been etched away.
  3. At the end of the process to make sure that all the etch resist has been stripped from the board.

Sample check.

Each production panel has a specially developed test coupon which indicates that the board has been correctly etched and that the track widths and isolation distances are correct. The type of etch resist used and the values for track width, isolation distances and annular ring are entered into the Passport file.

3. Inspect inner layer copper patterns.

We use Automatic Optical Inspection equipment to scan the inner layer copper and compare it to the design data. The machine checks that all track widths and isolation distances correspond to the design values and that there are no short or open circuits which will cause the finished board to malfunction.

A Pass is entered into the Passport.

4. Multilayer bonding.

Material.

This is automatically checked against the order details using a data-matrix. The material data (type, manufacturer, pre-preg and copper foil) is entered into the job history and will appear in the final Passport.

Thickness after bonding.

This is measured on each production panel and the result entered into the Passport.

5. Drilling.

The drilling machines automatically check drill diameters to ensure that the size of the holes will be correct. A special test coupon on multilayer boards confirms the position of the drilled holes relative to the (already printed) inner layers.

 

The smallest finished hole size is entered into the Passport.

6. Hole-wall preparation.

We deposit a layer of carbon on the walls of the holes to make them conductive for electroplating. We enter the process into the Passport.

7. Apply plating resist

Visual checks.

After printing and stripping to make sure that the unwanted plating resist has been stripped away cleanly

Type of resist is entered into the Passport.

8. Copper and tin plating.

Non-destructive sample check.

The operator measures the copper thickness in the holes at 5 or more locations on one panel from every flight bar. The result is entered into the Passport .

9. Outer layer etching

Visual checks.

After etching to make sure that all the unwanted copper has been etched away.

Sample check.

Each production panel has a specially developed test coupon which indicates that the board has been correctly etched and that the track widths and isolation distances are correct. The type of etch resist used and the values for track width, isolation distances and annular ring are entered into the Passport file.

10. Soldermask.

During process.

Visual checks:

  1. Each panel is evenly coated with soldermask ink (laquer)
  2. Alignment of soldermask phototool to the PCB

Sample checks:

The operator uses a projection microscope to check every panel to ensure that the soldermask is correctly aligned and that there are no solder-mask traces on pads.

The adhesion of the soldermask to the surface of the PCB is checked by the tape test used after legend printing.

The type of soldermask ink used is entered into the Passport data.

11. Surface finish

Sample checks on all surface finishes:

  1. The thickness is measured using an X-ray scope.
  2. We check the adhesion of the surface finish to the surface of the PCB using the tape-test after legend-printing.

100% visual inspection.

1. Lead-free hot-air levelling.

The surface must be flat and even across the PCB without any non wetting. Component holes must not be narrowed or blocked. A few via holes may be blocked if they are not covered by soldermask.

2. Electroless gold over nickel.

The finish must cover all exposed copper and have the same colour across the PCB. There must be no discolouration even in the holes

3. Chemical silver.

There must be no tarnishing or blackening.

The surface finish used is entered into the Passport, even where the order is for “Any leadfree”.

For gold and silver finishes we also enter the actual values measured.

12. Component legend.

Sample checks after curing:

The operator makes a tape test to check the adhesion of the surface finish, soldermask and legend to the surface of the PCB. We press a strip of pressure-sensitive tape across the test area and then pull it off sharply. There should be no bits of copper, surface coating, soldermask or legend ink adhering to the tape.

Visual check.

The operator checks that the legend on every board is clean and legible without blurring or smudging.

13. Electrical test.

All boards are electrically tested except single-sided boards where electrical testing is an option.

  1. Shorts and open circuits.

We build a netlist from the Gerber and drill data. We use this as a reference netlist to test all nets are tested for shorts and open circuits. A pass is recorded in the Passport. As an extra precaution, if your design system outputs IPC-D-356A netlist format include the file in your data-set. We can then use this to check the Gerber netlist against your design netlist.

  1. Inner layer registration.

A special test coupon allows us to confirm that the inner layer registration is correct.

14. Profiling and milling.

We check the size and position of the board profile and internal milling using special test coupons.

15. Final inspection.

See Part 2.

 

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Quality assurance - Part 2 : After Production

Final inspection steps

1. Check the quantity of boards received.

If we lose boards during manufacture, we immediately launch a top-up order. We push this through production to try to meet the original delivery date. The new order has the extension –E1, so you can see if there’s been a reload on your order by checking the Running orders menu. If we lose boards at final inspection, then we immediately reload. Depending on the type of PCB, a remake is typically completed in 2 working days.

2. Check the dimensions of the PCB against the drawing.

Sample check.

For each sample check, the number of samples to be tested is laid down by our QA department. This number is based on the size of the order and more than 30 years of manufacturing experience.

We use Eurocircuits’ standard tolerances unless the customer has requested special tolerances. For standard tolerances see our blog “PCB design tolerances” ( coming soon).

3. Check the board thickness against the drawing.

Sample check.

The standard tolerance based on the laminate manufacturers’ specifications is +/- 10%. If there is a gold edge-connector, the measurement is always taken there

 

4. Check the finished hole diameter.

Sample check.

We measure the finished hole size using a measuring microscope or a tapered probe with a read-out dial. The hole size tolerances depend on the type and size of the hole – see our blog “PCB design tolerances

5. Check hole positions

Sample check

We check the position of the holes relative to the edge of the PCB and to each other using a measuring microscope.

100% visual check.

We check all boards to ensure that there is no drill breakout on outer or inner layers.

6. Bow and twist.

We check where needed.

If any boards are not flat, then we measure the bow and twist. For more information and some hints on designing to avoid bow and twist, go to our dedicated blog http://www.eurocircuits.com/eurocircuits-printed-circuits-blog/bow-and-twist-in-printed-circuits and eC-Glossary.

7. Cosmetic appearance.

Sample tape test.

We use tape tests to test the adhesion of legend ink, soldermask, surface finish and copper. We press a strip of pressure-sensitive tape across the test area and then pull it off sharply. There should be no bits of legend ink, soldermask, surface coating or copper adhering to the tape.

100% visual inspection

The PCB must be clean and free from any damage, scratches, fingerprints, dust etc. All design features must be present (holes, pads, tracks, slots and cut-outs, profiling for panels etc.)

8. Base material.

100% visual inspection

The base material must be correct to specification and show no defects (delamination, measling (white spots), inclusions etc.). More.

9. Copper pattern.

100% visual inspection.

All copper tracks, pads, planes must be present and of correct size to IPC specifications, and not over- or under-etched. To ensure that track widths are correct we use etch-compensation, that is, we increase the track widths on the phototools used to print the boards by the amount that the track will be reduced when it is etched (see further http://www.eurocircuits.com/eurocircuits-printed-circuits-blog/eurocircuits-data-preparation-make-production-panels) . Any nicks (“mousebites”), pinholes or scratches must also be within IPC specifications. More.

The isolation distance track to track (TT), track to pad (TP), and pad to pad (PP) must be correct to IPC specifications. There must be no shorts between copper features or open circuits (these will have been detected by electrical test). The inner layers must be correctly aligned to the outer layers. This has been checked at electrical test. There must be no inner layer copper exposed at the board edge (we normally cut back copper planes to ensure adequate clearance).

10. Copper surface.

Sample check

  1. The principal thickness measuring checks are made immediately after plating, but we check a further sample at final inspection.
  2. The operator makes a tape test to check the adhesion of the surface finish, soldermask and legend to the surface of the PCB. We press a strip of pressure-sensitive tape across the test area and then pull it off sharply. There should be no bits of copper, surface coating, soldermask or legend ink adhering to the tape.

100% visual inspection.

The copper surface under the soldermask must be free of any damage, pitting, oxidisation, staining or burning.

11. Plated through holes.

Sample check

We measure copper thickness using specialist measuring instruments. We have separate probes to measure the copper thickness on the surface of the PCB and through the holes. The copper on the hole walls must be minimum 20 microns thick. The main inspection is immediately after plating (see Part 1). At final inspection we measure a further sample.

100% visual inspection

Plated-through holes must be drilled right through and clear of any obstruction (glass fibres from the laminate, trapped dirt etc.). There must be no defects in the plating through the holes (voids, cracks, plating detached from the wall of the hole etc.). A complete break in the plating will have been detected at the electrical test stage and the board scrapped. More.

12. Non-plated holes.

100% visual inspection.

They must be clean and clear of any obstructions or contamination (glass fibres, plating etc.).

13. Via fill.

100% visual inspection.

Holes specified for via-filling must be completely closed, though the soldermask need not completely fill the hole.

14. Soldermask.

Sample test.

Poor soldermask adhesion or curing is detected by the sample tape test described under Para 10

15. Legend.

Sample test.

Poor legend adhesion or curing is detected by the sample tape test described under para 10.

100% visual inspection.

The colour must be correct and the text legible without smearing. The marking should be cut back from the edge of the soldermask window by 0.1 mm. The legend must be correctly registered. More.

100% visual inspection.

The soldermask must be of the correct colour and free of dirt or damage (some minor repairs are allowed). There must be no soldermask or soldermask residue contaminating pads to be soldered. It must be correctly registered - see eC-Glossary.

16. Peelable soldermask.

100% visual inspection.

The peelable soldermask must be a continuous layer 0.25 mm thick, free from dirt or damage and with no visible separation from the board surface. There must be no residual contamination elsewhere on the PCB.

17. Markings.

100% visual inspection.

Eurocircuits’ order number, UL marking and any other special marking requested by the customer must be in the designated location and on the designated layer (typically the legend layer) as specified in the customer’s order.

18. Surface finishes.

Sample check on all finishes.

Poor surface finish adhesion is detected by the sample tape test described under Para 10.

18.1. Lead-free hot-air solder levelling.

100% visual inspection.

The surface must be flat and even across the PCB without any non-wetting. Component holes must not be narrowed or blocked. A few via holes may be blocked if they are not covered by soldermask.

18.2. Electroless gold over nickel.

Sample checks.

  • Thickness measurements.
  • Solderability test
  • Tape test.

100% visual inspection.

The finish must cover all exposed copper and have the same colour across the PCB. There must be no discolouration even in the holes

18.3. Chemical silver.

Sample check.

  • Thickness measurement
  • Tape test.

100% visual inspection.

Must not be tarnished or blackened. Finished boards are wrapped in “silver save” paper to avoid any oxidisation of the silver.

19. Thermal shock test

Sample test.

A sample is taken from every multilayer job and immersed in molten solder for a defined length of time. It is then checked for any delamination, blistering, solder-mask lifting etc.

20. Solderability test.

Sample test.

A sample is immersed in molten solder for a short time. The surface must be completely coated with solder with no de-wetting or non-wetting.

Calibration

All measuring equipment is regularly calibrated back to national standards

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24 Apr 2014
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Quality assurance -  Microsection analysis

Introduction

Microsectioning or cross-section analysis is a destructive sample testing procedure, used throughout the PCB fabrication industry. We make regular microsections each day, as they allow us to see inside the PCB and make precise measurements to check our production processes and to confirm the quality of the finished PCBs.

We use microsectioning to check the quality of:

  • PCB base material
  • inner structure of multilayer boards
  • plating in plated-through holes
  • thickness and registration of external and internal conductors
  • connection between the layers
  • soldermask cover
  • surface finish thickness

Microsectioning procedure

  1. Select the appropriate PCB or quality-control test coupon
  2. Cut out a piece for sample
  3. Embed the sample in resin
  4. Grind down to a flat surface
  5. Polish and etch back if needed

Multilayer build checking

We check the build-up of the multilayer PCB, the thickness of the cores, copper foils and prepregs, and the effectiveness of the bonding process. We also look for any defects in the laminate after thermal stress (delamination, blistering, voids or cracks etc.)

We inspect the registration of inner copper lands to the holes. The next picture shows the same PCB as the last one, but when we measure the registration of the hole to the inner layer copper we see that there is some misalignment (in this case within tolerance).  We also use a special test coupon on all multilayer production panels to confirm the position of the drilled holes relative to the (already printed) inner layers.

 

 

There should be a robust connection between the wall of the plated through hole and the inner layer copper as shown in the next picture.  A poor or broken connection points to process issues in drilling or hole-wall cleaning.  A broken internal connection on a finished board would be caught by electrical test.

 

Through hole plating

We make 5 non-destructive measurements of the thickness of the plated copper in the holes on every production panel.  We back this up with regular microsectioning to get more information about process quality. For this we use the test coupon which we put on every production panel.

The plating thickness is the average of six measurements taken three on each side of the hole roughly a quarter, a half and three-quarters down.

 

Our standard tolerance for component holes is +/- 0,1 mm. We measure the finished hole diameters at final inspection using a tapered gauge.  Microsectioning backs this up and provides more detailed information on the quality of our processes.  The next picture shows the actual width of a plated hole with nominal diameter of 250 microns.

 

Copper thickness

Inner layers

Inner layers are not plated so the copper thickness is that of the copper foil used.  However, some copper is lost during the cleaning processes.  The IPC A 600 Class 2 standard gives the following values for the minimum acceptable copper foil thickness after processing:

Start copper

Minimum thickness after processing

12 µm

9,3 µm

18 µm

11,4 µm

35 µm

24,9 µm

70 µm

55,7 µm

 

This image show the copper thickness after processing for an inner layer with 35 µm start copper:

 

Outer layers.

Outer layers are plated up when we plate through the holes, so that the final copper thickness is the start copper foil less any copper lost during cleaning plus the plated copper.  IPC A 600 Class 2 gives the minimum acceptable finished copper thickness after processing as:

Start copper

Minimum thickness after processing

12 µm

29,3 µm

18 µm

33,4 µm

35 µm

47,9 µm

70 µm

78,7 µm

 

Microsectioning shows the thickness of an outer layer track with 18 µm start copper:

 

We can also measure separately the thickness of the base copper and of the plated copper.  The base copper foil in this case was 12 microns.

 

Solder mask

The minimum solder mask thickness over the copper conductors should be 8 µm.

 

Surface finish

We can use microsectioning to measure the thickness of Leadfree Hot Air Levelling (HAL). For electroless gold over nickel (ENIG or Che Ni/Au) we can only use microsectioning to measure the nickel thickness (as shown in the picture) as the gold thickness is under 0.1 µm. For measuring the thickness of the gold and for Immersion silver we use non-destructive X-ray measurement.

 

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21 Oct 2014
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What tolerances should I design into my PCB?

Where possible, design to PCB industry standard mid-range tolerances. If you use these tolerances you should be able to source your boards from any fabricator in the world without cost-penalty. Eurocircuits use these specifications and tolerances as the basis of our lowest-cost pooling services. Of course there may be times when component geometry or mechanical constraints mean that you need tighter tolerances. We can usually build boards to meet these requirements but they will cost a bit more as they need special handling or additional process steps (for example for blind or buried vias).

ACTION TIP.

It’s always a good idea to check your data-set and especially any drawings to make sure that they don’t specify tighter tolerances than you need. If they are outside our standard range, we may need to raise an exception, possibly delaying delivery and/or increasing costs.

NOTE

Minimum tracks, gaps and annular rings are defined in the specifications of each service and are not included in this table. There is a complete list in our PCB Design Guidelines p. 7.

Tolerances tables

Specification

Tolerance

Notes

Materials

   

Material thickness

+/- 10%

Based on manufacturers’ specifications

Maximum bow and twist on boards with SMDs

0.75%

See http://www.eurocircuits.com/eurocircuits-printed-circuits-blog/bow-and-twist-in-printed-circuits

Maximum bow and twist on boards without SMDs

1.5%

 

Drilling

   

Production hole oversize – plated

0.10 mm

PCB Design Guidelines p. 8

Production hole oversize – non-plated

0.00 mm

 

Hole size tolerance – plated

+/- 0.10 mm

 

Hole size tolerance – via holes

+ 0.10/-0.30 mm

By default we take all holes 0.45 mm or less to be via holes. If you have component holes with finished diameter 0.45 mm or less, use the box in the Price Calculator marked “Holes <= may be reduced” to indicate the largest hole which can be treated as a via hole. The negative tolerance allows us to reduce via hole sizes to solve annular ring issues and/or to reduce board costs by reducing the number of drilling cycles needed. More.

Hole size tolerance – non-plated

+/- 0.05 mm

 

Aspect ratio

1:8

Ratio of board thickness to production drill tool

Hole positional tolerance

0.10 mm

Hole to hole

Minimum hole to hole distance

0.25 mm

Measured from production hole to production hole.

See PCB Design Guidelines p. 9 and technical blog

http://eurocircuits.com/eurocircuits-printed-circuits-blog/the-smallest-possible-distance-between-two-holes

Minimum non-plated production hole to copper

0.25 mm

 

Hole wall copper

   

Minimum copper thickness

20 μm

 

Surface finish thickness

   

Leadfree hot-air levelling

1 – 30 μm

 

Electroless gold over nickel

Ni:3 -6 μm;

Au: 0.05 – 0.10 μm

 

Immersion silver thickness

0.2 – 0.4 μm

 

Plated hard gold over nickel

Ni: 3 – 6 μm; Au: 1- 1.5 μm

 

Soldermask

   

Minimum soldermask to pad clearance = Mask Annular Ring (MAR) – plated holes

0.10 mm

This depends on the copper pattern classification – see PCB Design Guidelines p.15

Minimum soldermask track cover = Mask Overlap Clearance (MOC)

0.10 mm

On tight layouts there may need to be a “trade-off” between MAR and MOC – see PCB Design Guidelines p. 16

Minimum soldermask web = Mask Segment (MSM)

0.10 mm

 

Minimum soldermask to pad clearance = Mask Annular Ring (MAR) – non-plated holes

0.125 mm

 

Maximum tented finished via size

0.25 mm

To ensure that the via hole is plugged with soldermask use ViaFill – see PCB Design Guidelines p. 16 & 20

Soldermask thickness on top of conductors

>15 μm

For more information see eC-Glossary.

Soldermask thickness on conductor edge

>7 μm

 

Legend

   

Minimum line width

0.17 mm

 

Minimum height for legibility

1.00 mm

 

Legend to soldermask cut-back (clipping)

0.10 mm

After clipping we also remove any bits of line smaller than 0.17 mm

Breakrouting

   

Minimum clearance board edge to copper tracks/pads – outer layers

0.25 mm

Copper planes can extend to the board edge. Select “copper to board edge” in the Price Calculator

Minimum clearance board edge to copper tracks/pads – inner layers

0.40 mm

 

Minimum slot finished width

0.50 mm

 

Profile dimensional tolerance

+/- 0.20 mm

 

Positional tolerance profile/cut-out to hole

+/- 0.20 mm

 

Slot dimensional tolerance

Width: +/- 0.10 mm

Length: +/- 0.20 mm

 

Minimum copper around plated and non-plated slots

As annular ring for plated and non-plated holes

 

Scoring/V-cut

   

Maximum board thickness for scoring

2.00 mm

 

Minimum board thickness for scoring

0.80 mm

 

Minimum clearance board edge to copper pattern – outer and inner layers

0.45 mm

This to allow for the V-cut. If copper pattern is nearer to the board edge, use breakrouting

Profile dimensional tolerance after separation

0.30 mm

 

Rest material

0.45 mm +/- 0.10 mm

 

Positional tolerance upper to lower score

+/- 0.25 mm

 

Minimum score depth

0.15 mm

 

Edge bevelling

   

Nominal bevel angle

30° +/- 5°

See eC-Glossary

Rest material

0.25 mm

 

ViaFill

   

Maximum ViaFill finished hole size

0.25 mm

 

Peelable Mask

   
   

See PCB Design Guidelines p. 19

Carbon

   
   

See PCB Design Guidelines p. 18

Heatsink Paste

   
   

See PCB Design Guidelines p. 21

Electrical Test

   

Minimum test pitch

0.10 mm

 

Smallest testable pad

0.05 mm

 

Test voltage

up to 1000V

 

Test current

100 mA

Adjustable

Continuity test

Capacitance/

resistance 1 Ohm – 10 KOhm

 

Isolation test

Capacitance/

resistance up to 10 GOhm

 
Posted under:
PCB quality
Posted on:
24 Apr 2014
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