Readability Of Directly Marked Traceability Symbols On PCBs

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READABILITY OF DIRECTLY-MARKED TRACEABILITY SYMBOLS ON

PCBs

Christoph Wimmer, Steven Moser, Tim O’Neel, Leah Sun Zhen

Microscan

Renton, WA 98057

cwimmer@microscan.com

Keywords:

Traceability, PCB, DPM

INTRODUCTION:

Compliance with ISO 9001 and lean PCB manufacturing

relies heavily on traceability data to assist with inventory

control, automatic machine setup, warranty claims,

optimization of production capabilities, and reduction of

waste in terms of material handling and material scrap.

Requirements from the military, aerospace, automotive, and

medical device industries make it essential to be able to

identify individual panels, boards, and parts at each

production step. Recent recalls have shown how important it

is to have this data available to reduce recall costs. The most

widely-used method has been manual application of labels.

Keeping up with modern production capabilities can be a

challenge, and most companies are now transitioning to inkjet

or laser marking. The goal is to apply markings that can be

read throughout the overall process and by the end-customer,

and can also be relied upon in the case of recalls and recycling

programs.

BARCODES

It is critically important to choose the right barcode size and

type, since the part number, supplier code, manufacturing

traceability data, environmental information, and

occasionally even country of origin need to be encoded. Due

to limitations of board and component space, several

manufacturers are moving from 1D to 2D symbologies to

take advantage of the higher data density (Table 1). Data

Matrix (DM) ECC 200 symbols are preferred for their robust

built-in error correction, which ensures readability even if as

much as 50% of the symbol is obstructed. There are several

standards concerning PCB identification. The main ones are

ANSI/EIA 706 and EIA PN 3497. These standards describe

how components like PCBs, parts, and populated PCBs need

to be marked. This paper focuses specifically on 2D Direct

Part Mark (DPM) symbols covered in these standards.

Table 1 – Data Capacity of Square DM Symbols

Square

Data Capacity

Symbol Size

Numeric

Alphanumeric

48x48

348

259

52x52

408

304

64x64

560

418

72x72

736

550

80x80

912

682

88x88

1152

862

Table 2 – Data Capacity of Rectangular DM Symbols

Rectangular

Data Capacity

Symbol Size

Numeric

Alphanumeric

8x18

10

6

8x32

20

13

12x26

32

22

12x36

44

31

16x36

64

46

16x48

98

72

MARKING METHODS

Most PCB manufacturers use one of four methods to mark

PCB boards and panels:

 Inkjet printer

 Labels

 Laser marker

 Laser marker with silkscreen pad

Each of these marking methods have advantages in certain

situations. There are still instances of older application

designs using labels for individual marking due to easier

readability, better code quality, or customer requirements.

The comparison (Table 2) doesn’t include the laser silkscreen

pad as it is very similar to laser marking. The pad is normally

applied by the PCB manufacturer and provides better contrast

than direct marks:

Table 3 – Marking Methods

Inkjet

Label

Laser

Easy

Changeover

Yes

No

Yes

Speed

Good

Manual

application

method can

slow down

process

Good

Operating

Costs

Medium

High

Low

Consumable

s

~1.7ct/mar

k

3-10ct/label

~0.03ct/ma

rk

Permanence

Permanent

after wave

solder or

curing

Can be

removed

Will last as

long as the

surface

integrity

Maintenance

Weekly

NA

Monthly

Code quality

A-D

A-C

A-D

Color

Yes

Yes

No

Most new SMT lines use laser marking systems at the

beginning of the production process, as they allow permanent

individual board marking, easy changeover at high speeds,

and low operating costs. PCBs can be marked with either

CO2, YAG, or fiber lasers. Of these types of lasers, fiber

lasers produce the best edge definition and have highly

accurate depth control, but they are also the most expensive.

For the best marking results, it is necessary to optimize pulse

energy, pulse duration, and repetition rate.

VERIFICATION VS. VALIDATION

It is becoming increasingly critical to add a verification step

into the manufacturing system to help achieve the benefits of

data capture and to ensure readability throughout the supply

chain. Automated data capture is crucial for a company’s

success, as the results of decode failures can have a serious

impact. Additionally, verification can be used to monitor how

well the label printer or direct part marking equipment is

performing, and reduce the risk of manufacturing errors or

downtime caused by unreadable codes. Verification catches

deteriorations in the marking process early so that

countermeasures can be employed before unacceptable

product enters the production line. This helps manufacturers

avoid rejections of product by the end customer (Figure 1).

Figure 1: Impact of Symbol Verification

Many manufacturers validate readability by using a reader to

ensure that that a code is readable. But this doesn’t ensure

that it can be read by other readers later in the production

process. True verification provides information about how

well the code can be read and can provide indications about

printing and marking process stability. Verification ensures

that any correctly-functioning reader can decode the given

symbol. Verification is either keyed to an internal standard to

ensure process stability, or to one of the following standards:

- ISO 15415/ISO 29158: 2D Symbols

- AS9132: 2D symbols

Verification is performed by camera-based systems with

specific lighting, and results come in the form of a grade,

from A (perfect) to F (least likely to be decodable by most

readers). ISO 15415 is for printed labels while ISO 29158 is

for directly-marked parts. The following evaluation

parameters are measured specifically for ISO 15415 and ISO

29158:

Axial Non-Uniformity (ISO 15415/ISO 29158)

Axial Non-Uniformity is the amount of deviation along the

symbol’s major axes. In Figure 2 (A), the symbol’s Y-axis

dimension is clearly greater than its X-axis dimension. This

indicates that the marking process is resulting in the Y-

dimensions of individual modules being greater than their X-

dimensions. This inconsistency of X- and Y-dimensions

typically indicates movement of the object as it is being

marked.

Symbol Contrast (ISO 15415)

Symbol Contrast is the value difference between light and

dark symbol elements, and between the quiet zone and

perimeter elements. Figure 2 (B) shows a low-contrast

symbol. The dark elements (etched) and the light elements

(the substrate) are too close in value, which undermines

readability.

Cell Contrast (ISO 29158)

Cell Contrast is the value difference between light and dark

symbol elements, and between the quiet zone and perimeter

elements. Figure 2 (C) shows a low-contrast direct part mark

symbol. The light and dark elements are too close in value,

which undermines readability.

Modulation (ISO 15415)

Modulation refers to the reflectance uniformity of a symbol’s

light and dark elements. In Figure 2 (D), notice that the light

and dark values of some elements are inconsistent.

Cell Modulation (ISO 29158)

Cell Modulation refers to the reflectance uniformity of a

symbol’s light and dark elements. In Figure 2 (E), notice that

the light and dark values of some of the elements in the dot

peen mark are inconsistent.

Decodability (ISO 15415/ISO 29158)

Decodability refers to a symbol’s ability to be decoded per a

standard reference decode algorithm. Figure 2 (F) shows a

high-quality 2D symbol.

Fixed Pattern Damage (ISO 15415/ISO 29158)

Fixed Pattern Damage refers to finder pattern and clock

pattern damage. Notice the missing elements in the clock

pattern and the damaged L-pattern in Figure 2 (G).

Grid Non-Uniformity (ISO 15415/ISO 29158)

Grid Non-Uniformity refers to a symbol’s cell deviation from

the ideal grid of a theoretical “perfect symbol”. The Data

Matrix reference decode algorithm is applied to a binarized

image of the symbol, comparing its actual grid intersections

to ideal grid intersections. The greatest distance from an

actual to a theoretical grid intersection determines the Grid

Non-Uniformity grade. The symbol shown in Figure 2 (H)

exhibits a high degree of Grid Non-Uniformity.

Minimum Reflectance (ISO 29158)

Minimum Reflectance refers to the minimum reflectance of

light by the symbol’s light elements. In Figure 2 (I), notice

that the symbol’s light elements exhibit a minimum

reflectance to ensure contrast against the dark substrate to

allow readability.

Reflectance Margin (ISO 15415)

Reflectance Margin measures how well each element of a

symbol is correctly distinguishable as light or dark in

comparison to the global threshold. Low reflectance margin,

such as that illustrated in Figure 2 (J), may increase the

probability that a symbol element will be incorrectly

identified as dark or light.

Unused Error Correction (ISO 15415/ISO 29158)

Unused Error Correction indicates the amount of available

Error Correction in a symbol. Error Correction is a method of

reconstructing or replacing data that is lost through symbol

damage. 100% Unused Error Correction is ideal, as shown in

Figure 2 (K).

Over- and under-print can also occur, causing symbol

decodability problems.

(A) Axial Non-

Uniformity

(B) Symbol

Contrast

(C) Cell

Contrast

(D) Modulation

(E) Cell

Modulation

(F) Decodability

(G) Fixed

Pattern Damage

(H) Grid Non-

Uniformity

(I) Minimum

Reflectance

(J) Reflectance

Margin

(K) Unused Error

Correction

Figure 2: Verification Evaluation Parameters

Symbol verification immediately following the printing or

marking process ensures proper readability throughout the

application, and can help with the setup of the inkjet or laser

marker. These systems must be properly calibrated with

specialized calibration cards if verifying to the ISO 15415 or

ISO 29158 standard.

1D/2D READERS

Modern barcode readers use a variety of techniques to ensure

that codes can be read properly. These readers can either

come in a handheld or fixed-mount form. Figure 3 shows the

components of a compact fixed-mount reader.

Figure 3: 2D Reader Components

Lighting

The lighting system ensures maximum symbol reflectance.

For optimal performance, proper mounting of the reader is

essential. The reader is normally positioned 30° to 45°

perpendicular to the symbol. The more powerful the lighting

system, the more accurately low-contrast symbols can be

decoded.

Lens System and CCD/CMOS Sensor

The lens system, in combination with the CCD/CMOS

sensor, ensures that codes as small as 2.5 mil can be decoded

at the required reading distance. Readers typically come with

specification charts that show field of view, depth of field,

and minimum readable symbol size. Additional features such

as autophotometry may also be available, allowing you to

adapt gain and shutter speed automatically. Autophotometry

optimizes image exposure and allows readability of codes on

variously-colored PCBs.

Processor

The processor runs the decoding software with advanced

algorithms, allowing the reader to decode difficult-to-read

symbols.

Memory

The reader’s memory can be used to store no-read images for

further analysis.

Interface

RS-232, USB, or Ethernet are most commonly used. Various

industrial protocols such as EtherNet/IP and PROFINET can

be supported by modern barcode readers.

Configuration Software

Traditionally, users must download and install specialized

configuration software to make reader parameter

modifications. Modern readers can leverage web browser

technology for configuration and testing. By simply entering

the IP address on any web-enabled device, all reader

parameters can be configured. This means that no software

needs to be installed. Browser-based configuration programs

support different operating systems (Windows, MacOS, iOS,

Android) and run on any web-enabled device (PC, tablet,

phone). Intuitive user interfaces limit the amount of clicks

necessary to start decoding (Figure 4).

Figure 4: Web-Based Configuration Software Results

RESULTS:

We tested 2D readers on different colored boards that were

laser marked with 5mil (0.127mm) codes:

(A) Black PCB

(B) Black

Silkscreen

(C) Blue PCB

(D) Copper on

Green PCB

(E) Green PCB

(F) Red PCB

(G) White

Silkscreen

Figure 5: Tested Boards

First we used a UID DPM verifier and verified the codes

according to ISO 29158. As expected, these were high-

quality codes, the only exception being the code on the red

board which was Grade B due to lower cell contrast and fixed

pattern damage. We tested comparable, standard, off-the-

shelf, second generation miniature readers from different

vendors. The first test was to unpack the reader, download

and install software, connect, and power up the reader.

Setup Time

Reader 1

2 min. (Web-based

configuration)

Reader 2

19 min.

Reader 3

28 min. (6 min. to find the

right software to install)

Placement of the code was slightly easier with reader 1 as it

had a larger field of view (21.7 cm2 vs. 10.6 cm2 for reader 2

and 4.2 cm2 for reader 3). Additionally, the web-based user

interface was much more intuitive. The next step was to try

to read the codes on the PCBs without any additional

adjustments.

Grade

Reader 1

Reader 2

Reader

3

Black PCB

A

Yes

Yes

Yes

Black

Silkscreen

A

Yes

Yes

Yes

Blue PCB

A

Yes

Yes

Yes

Copper on

Green

PCB

A

Yes

Yes

Yes

Green

PCBs

A

Yes

Yes

Yes

Red PCB

B

Yes

No

Yes

White Silk

Screen

A

Yes

Yes

Yes

Two of the readers were able to read all the codes right out of

the box while reader 2 couldn’t read the code on the red

board. This was very likely due to insufficient illumination

provided by the reader. After full adjustment, reader 2 was

still unable to decode the symbol on the red PCB.

CONCLUSION

Modern traceability systems in SMT production lines rely

heavily on proper board marking and advanced readers.

Adding verification directly after the marking process

ensures symbol decodability throughout the application, and

helps with the setup and testing of inkjet or laser markers. As

tests have shown, most readers are capable of handling small

codes on different colored boards. This is essential to

ensuring that proper production data is stored, communicated

throughout the production line, and used in production to

initiate job changes. Tests also show that web-based

configuration software has an immediate advantage as no

software has to be installed, there is no need to worry about

version conflict, and any computer can run the web-based

configuration user interface.

REFERENCES

[1] Understanding 2D Verification, Microscan 2013

[2] Direct Product Marking for PCB Identification, Jetec

2004

[3] Laser Marking Technologies, Sintec Optronics,

[4] Ink Jet Systems, Jetec Corp.

[5] J. Sercel and U. Sowada, Why excimer lasers excel in

marking, Lasers & Optronics, Vol. 7, No. 9 (1988), 69-72

[6] T. J. McKee, How lasers mark, ElectroTechnology, Vol.

7, No. 2 (1996), 27-31

[7] S. J. Parnas, Indelible coding with lasers: variable

information marking on-the-fly, Surface Mount Technology,

Dec. 1989, 20-28

[8]J. F. Higgins, Laser marking of passive components,

hybrids and semiconductors, Proc. of SPIE Vol. 611 Laser

Processing of Semiconductors and Hybrids (1986), 40-47

[9]Cucu, T.C., 1D and 2D solutions for traceability in an

Electronic Manufacturing Services company, Electronics

Technology, 2008. ISSE '08.

[10] Amit Grover, Parameters Effecting 2D Barcode

Scanning Reliability, Advances in Computers Volume 80,

2010, Pages 209–235