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Meeting Tough
Requirements for Printed Circuit Boards |
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Electronic equipment manufacturers
benefit from automated assembly, processing, test and packaging systems which include
automatic identification. A bar code label is commonly adhered to a printed circuit board
before soldering to provide for automatic identification. It must withstand harsh thermal and chemical exposures and still meet optical requirements for reliable scanning. This
article details the benefits of barcode labeling and recommends label materials for successful
label use.
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Bar codes facilitate rapid
identification and data entry. Scanning the bar code initiates an appropriate automated action:
Display of Information on A CRT or Data Capture.
Production starts when a bare board is
taken from the Stockroom and a label is affixed to it. At each work center thereafter
the bar code is scanned to identify the board and help to:
- Get information into the database
quickly. Information may include flux density and solder temperature, quantity and lot
number of components used, and test data.
- Update the database in real time so a
summary of process data and correlation to test results can be prepared and
interpreted
rapidly. This helps improve yield.
- Relieve inventory levels as components
are inserted so unused components are available for immediate alternate use.
This reduces
work in process inventory.
- Get information to the work centers via
CRT. Information may include parts picking list, insertion instructions, test
procedures
and individual board test results. This eliminates paper clutter and reduces setup time.
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The information that is to be printed on
the labels is determined by overall automation objectives and strategy. It should be
recognized however, that whatever the information content of the labels, the materials
must withstand harsh environmental
requirements as well.
Labels can sometimes:
- fall off
- curl
- shrink
- absorb chemicals which later leach
out
In order to provide for broader and
more successful bar code use, we investigated the thermal, chemical and optical
requirements for labels adhered before soldering. We also investigated static dissipative properties of
labels that help protect circuits from static discharge. We then identified
and developed candidate materials and tested them for durability. Results follow.
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For through-lead component boards,
labels adhered to the bottom surface contact a solder wave commonly in a temperature range
of 450°F
to 510°F (232°C to 266°C). Occasionally solder temperatures as high as 550°F (288°C) are used. Solder contact time is 2 to 6 seconds. For
example, with a 4 foot per minute
conveyor speed and a 3 inch solder zone, contact time is 3.8 seconds.
Labels adhered to the top surface of
the board may experience considerably different temperatures depending upon whether or not
they are placed near through-hole vias. In order to obtain a worst-case estimate of the
highest temperatures likely to be encountered on the top surface, thermocouples were
adhered to 1.5 mm thick epoxy coated circuit boards which were subjected to 10 second
solder contact times. Measurements showed the top surface temperature is below 350°F (177°C) in a region without through-hole vias. Temperatures near through-
hole vias are
considerably hotter.
Moreover, the heat capacity of the
thermocouple may cause an understatement of the board temperatures. When labels came in
incidental contact with edge holders carrying the board through the wave or when they were
adhered to a metal bracket or handle contacting the wave, temperatures approaching the
solder temperature were observed.
It should be noted the temperature of a
metal bracket may vary significantly from run to run due to variations in the height of
the wave. In the extreme case, the wave may contact the bracket on one run and miss it on
another run. Because top surface temperatures vary substantially, one safe approach is to
select label materials which withstand the solder temperature. Another approach is to
dedicate a through
-hole-via-free region of each board to the label. In the latter case, it
may be advisable to measure the top surface temperature for each board thickness and material in order to assure the selected label material is adequate.
For common surface mount technology
methods, solder paste containing flux and solder powder is screen printed or stenciled on
the precise points of the circuit boards where component terminals will be located.
Components are placed in the solder paste. Then the printed circuit board is heated,
activating the flux, melting the solder and bonding the components to the board. Depending
on the solder alloy, a different temperature and time is required. For example, for
Sn63/37PB Futronix1 preheats 150 to 180 seconds at 302°F
(150°C) and then reflows for 60
seconds with a peak oven temperature of 419°F
(215°C).
For labels applied after soldering,
polyester or tamper evident labels are commonly used. Tamper evident labels help administer warrantees by assuring the serial number cannot be transferred to another
board.
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Labels are sometimes directly exposed
to fluxes, as when they are located on the bottom of the board or on the edge; they are
usually exposed to defluxing fluids.
The purposes of a flux are removing and
preventing formation of oxides and promoting wetting so that complete covering of copper
occurs. An unwanted side effect can be leaving unwanted contaminants on the board.
Thorough removal is essential for certain types including OA, RA, and RSA. Otherwise ionic
contaminants will cause humidity sensitive electrical leakage currents and long term
corrosion.
Common defluxing methods used are
detergent-water washing at 150°F
to 170°F (66°C
to 77°C) and vapor degreasing. Trichloroethane is
rarely used. Methyl ethyl keytone is commonly used to remove conformal coating during
rework of military circuit boards.
Versatile label materials must
withstand a variety of fluxes and defluxing solvents and avoid absorbing or otherwise
entrapping them. Laminate, when used, must be tightly joined to the layer beneath so flux
or other liquids do not seep between. The adhesive must resist solvents and it
should be
thick enough so it conforms closely to uneven contours such as printed wires.
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Most printed circuit board bar code
systems use bars as narrow as 0.005 inches (.19 mm) and scanners which have peak spectral
response at 633 nanometers, the wavelength of the helium-neon laser. Widely used standards
(e.g. ANSI MS 10.8-1983 and MIL STD 1189A) require a minimum of 50% reflectance for spaces
and in effect there is a required bar-space reflectance ratio.
A new Automatic Identification
Manufacturers uniform symbology specification is different. It requires a minimum
bar-space reflectance difference of 37.5% and is considered to more closely
characterize scanner operations requirements.
Surfaces which are "matte"
(i.e. not specularly reflective) are preferred as they are said to scan more consistently
with laser scanners.
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Labels can help protect circuits from
static discharge damage by having appropriate surface resistivity. If the surface
resistivity is below about 105 ohms/square, discharge can occur with a large
peak current which can damage the circuit. If the resistivity is between 106 to
109 ohms/square,
charge dissipation is fast enough to prevent charge buildup
and slow enough so any residual charge is safely dissipated; these labels are termed
"static dissipative". If the resistivity is between 109 and 1012
ohms/square, charge accumulation is avoided; these labels are termed
"antistatic". |
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Polyimide, paper and polyester label
materials were wave solder tested. Polyimide labels consist of successive layers of
chemically resistant acrylic adhesive, Kapton© polyimide film, printable coating, a second layer of
adhesive and a second layer of film. Labels were prepared in two ways. Some had wider
laminate than the printable facestock. Some were slit after printing and lamination. In
both cases the over laminate entirely covered the print and the white printable coating.
Extensive testing was done in
through-lead wave solder systems. A wide variety of fluxes were brushed on before
soldering. Solder contact times up to 10 seconds were used to test for "worst
case" conditions. Appropriate post-solder washing was done in
Freon-alcohol azeotropes, detergent water and trichloroethane. Labels on the top and the
bottom of the circuit board were subjected to three passes through flux, wave solder and
cleaning to simulate rework and repair cycles. Polyimide labels withstood the flux, solder
and cleaning cycles, remaining well-adhered and exhibiting no tendency to curl. Moreover,
15 minute exposure to methyl ethyl keystone showed no effect. Label materials to satisfy SMT methods are available.
Optical characteristics of the bar
codes were tested using a verifier having a 633 nanometer light source. The labels had
appropriate bar and space dimensions. The bar reflectance was 4% and the space reflectance
was 64% yielding a reflectance difference of 60% and exceeding the optical requirements
referenced above. Labels scanned readily with wand scanners and a hand-held laser scanner.
Paper labels over-laminated with
polyimide survived the solder contact but they tended to absorb flux and cleaning fluids
through exposed edges. Polyester labels showed promise in top surface applications,
although laminated polyester labels exhibited frequent curling or lifting of their edges.
This effect was attributed to the noticeable shrinkage which occurred. Some reduction in
curl was observed with the use of labels having rounded corners. Photo processed polyester
shrank less and exhibited less label curl. It is available in pre-printed label form from
companies specializing in pre-printed labels.
One new alternative to polyimide works
equally well in many applications and is less expensive. This plastic label handles
the range of temperatures between the 572°F
(300°C) maximum for polyester and the approximately
932°F (500°C) maximum for polyimide.
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The following chemically durable
materials are recommended. ("In-plant" printed labels can be conveniently
printed on the production floor. "Pre-printed" labels are purchased with
incremented serial numbers.)
Recommended materials for in-plant
printing when the label is applied before soldering are polyimide facestock or high
temperature polyimide alternative with matching thermal transfer ribbon.
Recommended material for pre-printed
labels applied before soldering is polyimide facestock with polyimide over-laminate.
Recommended material for labels applied after soldering is
polyester facestock or tamper evident facestock with matching
thermal transfer ribbon.
Printed circuit board label material
selection should be done carefully so thermal, chemical and optical scanning requirements
are matched to the specific application needs. That way, intended automation objectives
can be realistically met.
1) "Step Soldering Aids Intrusive
Reflow", Karl Seelig and Joe Peek, SMT September 1996, P88.
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Paul Henkel is
the Founder of Electronic Imaging Materials Inc.,
Keene New Hampshire. Paul recently retired and his son Alex Henkel
is now the President of the company.
Electronic Imaging Materials Is A Company Specializing
In Label Materials For
Printing Barcodes. |
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