Meeting Tough Requirements for Printed Circuit Boards
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.
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.
Label Materials Durability
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
- 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.
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.
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.
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.
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”.
Testing of Labels
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.
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.
About the Author
Paul Henkel is the Founder of Electronic Imaging Materials Inc., Keene New Hampshire. Paul has retired and his son Alex Henkel is now the President of the company.
Electronic Imaging Materials, Inc. is a company specializing in Label Materials For Printing Barcodes.