PCBs

For quality assurance in PCB manufacturing, it is necessary to measure coatings that are just nanometers in thickness. FISCHER offers measurement devices that are specially adapted to the measurement of the thinnest surface coatings on assembled and non-assembled PCBs. We also offer special measurement technology for multilayer circuit boards and flex PCBs.

PCBs

Application notes

Au / Pd Coatings in the nm Range on Printed Circuit Boards

As the electronics industry makes use of ever thinner coatings, manufacturers increase their demands on measuring technologies to provide reliable parameters for product monitoring. One example is the Au/Pd/Ni/Cu/printed circuit board system with coating thicknesses for Au and Pd of just a few nm. For monitoring the quality of these coating systems, X-ray fluorescence instruments have established themselves as the measurement method of choice.

The thinner the coatings, the more important it becomes to select a suitable detector. Table 1 shows a comparison of results from FISCHERSCOPE® X-RAY instruments fitted with a proportional counter tube, PIN diode and silicon drift detector (SDD), respectively.

 

50 nm Au

24 nm Pd

Detektor type

Standard
deviation

Coefficient
of variation

Standard
deviation

Coefficient
of variation

Proportional
counter tube

(0,2 mm Aperture)

2,2 nm

4,3 %

3 nm

13 %

PIN detector

(1 mm Aperture)

0,9 nm

1,8 %

1,2 nm

4,8 %

SDD detector

(1 mm Aperture)

0,2 nm

0,4 %

0,5 nm

2,1 %

Table 1: Various types of detectors and their corresponding achievable standard deviations and variation coefficients

As illustrated in Table 1 above, the SDD’s significantly superior repeatability precision allows for the reliable measurement of even very thin Au and Pd coatings.

The trueness is also better for instruments with SDD because the high energy resolution of the usable signal is less susceptible to influence from the background or adjacent fluorescence lines.

Fig. 1: The FISCHERSCOPE® X-RAY XDV®-SDD measurement system is equipped with SDD, allowing for the quick and repeatable determination of extremely thin coatings

Proper handling of the fluorescence signal generated by the substrate material is also more important with thinner coatings. While a general subtraction of the background signal does improve the repeatability precision, it can also introduce errors into the results. The evaluation software WinFTM® therefore explicitly allows the composition of the substrate material to be taken into account with every measurement.

Your local contact person for FISCHER products will be happy to assist you in selecting a suitable X-ray fluorescence instrument for measuring Au/Pd coatings on printed circuit boards – FISCHERSCOPE® X-RAY XDL® with proportional counter tube, XDAL® with PIN detector, or XDV®- SDD with SDD detector.

Thickness Measurement of Conformal Coatings on Printed Circuit Boards

Conformal coating material is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes. Coatings on assemblies which are too thin or totally uncoated and therefore non-protected board parts could result in damage or malfunction of the electronics.

When electronics must withstand harsh environments and added protection is necessary, most manufacturers of circuit boards coat assemblies with a layer of transparent conformal coating. The coating material can be applied by various methods like brushing, spraying and dipping, or by selectively coating via robot. Different methods of curing/drying are available depending on the conformal coating material.

Coating thickness measurement is important to check the necessary protection level. This measurement can be performed with an eddy current method (DIN EN ISO 2360), using the copper layer as conductive background.

Special properties like the thickness of the copper layer, solder, patch size and coating type might influence the measurements. Therefore FISCHER developed a special probe type FTA3.3-5.6 HF to measure such coatings with the trueness and repeatability typical for FISCHER. For correct measuring a spot/patch size of at least 5 mm is required. Best measurement results are obtained when dedicated measuring spots are integrated into the PCB design.

Special features for the thickness measurement of conformal coatings with the probe FTA3.3-5.6 HF – connected either to FMP portable instruments or MMS PC desktop models:

  • High frequency to avoid influences caused by the variation of copper thickness
  • Large, flat probe-tip prevents indentation of soft coating types
  • Automatic conductivity compensation for base material
  • Measure on Sn, Ag or uncoated copper
  • High accuracy and repeatability

Conformal coating inspection is a critical factor in determining long term reliability of PCBs. The probe FTA3.3-5.6 HF from FISCHER is optimally suited for this application. The probe can be connected to all DUALSCOPE® or ISOSCOPE® FMP portable instruments or FISCHERSCOPE® MMS® PC desktop models.

Measuring the copper thickness in plated through-holes on PCBs

As electronic devices get smaller and smaller, conducting paths must be positioned even more closely together on printed circuit boards (PCBs). This is why, today, most PCBs are multi-layered. In order to transfer electronic signals through to all the layers, these are connected by plated through-holes, also called vias (vertical interconnect access), which are electroplated with an electrically conductive material such as copper. To ensure proper function, the hole lining must be uniform.

For quality control purposes, the thickness of the copper coating lining the through-hole is measured using the eddy current method: A specially designed probe tip, housing a tiny eddy current coil, is simply inserted into the through-hole.

The coil’s special layout causes all of the eddy currents to flow longitudinally along the center line of the through-hole (Fig. 2), such that intervening copper layers exert no influence on the measurement result. Reliable measurements can even be taken despite thin layers of Sn (galvanization) on top of the plating.

Besides its ease of use, another advantage of this probe is that its range of optimum accuracy – that is, without taking any influence from the sample geometry – is for holes from 0.8 to 1.2 mm in diameter, or the typical range of PCB vias; this means that measurements can be made on multiple through-holes of different sizes without having to recalibrate in between.

FISCHER’s needle-like eddy current probe tips are made in different lengths corresponding to typical PCB thicknesses: With the ESL080B and ESL080V probes, a range of board thicknesses from 0.5 to 8 mm is covered. These probes work perfectly with FISCHER eddy-current instruments: e.g. the convenient handheld unit PHASCOPE® PMP10, or the versatile table-top device FISCHERSCOPE® MMS® PC2.

The precise measurement of copper thickness in plated PCB through-holes is made easy with specialized probes (ESL080B / ESL080V) used in conjunction with FISCHER eddy current instruments, such as the PHASCOPE® PMP10 or the FISCHERSCOPE® MMS® PC2. For additional information, please contact your local FISCHER representative.

Controlling the thickness of solder resist in the manufacture of printed circuit boards

To prevent solder from bridging conductive traces and causing short-circuits, while undergoing the soldering process printed circuit boards (PCBs) are coated with a non-conductive lacquer to which solder will not adhere. This ‘solder mask’ also safeguards the board’s circuitry against environmental influences and improves electric strength. With so much depending on this important layer, it is obvious that its quality should be monitored during manufacture.

Traditionally green in color, solder resist (often epoxide resin) was originally developed for facilitating wave soldering processes: not only to prevent unintended solder bridges but also to restrict the solder to just the electrical contacts, thereby reducing overall solder consumption. Today, even when other techniques are used, a solder resist layer is still indispensable for permanently protecting the delicate copper traces from wear, heat and moisture, as well as for insulating the PCB’s circuitry. The thickness of the solder mask is essential to its functionality and must therefore be controlled during production.

Determining the thickness of the solder resist layer implies measuring a non-conductive coating on top of copper – a clear case for using the amplitude sensitive eddy current method. Because the thickness of the covered copper layers can vary widely, one should use a high frequency probe with a low eddy current depth.

For exactly such applications, FISCHER has developed the FTA3.3-5.6HF probe. Its high frequency (20 MHz) makes a 30µm thick copper substrate sufficient to reach optimum results. If measurement uncertainties of 10-15% are acceptable, even coatings on top of copper traces only 18µm thick can be measured with this probe.

Fig.1: High frequency probe FTA3.3-5.6HF

Due to the lateral expansion of the FTA3.3-5.6HF’s eddy current field, the measurement spot must be at least 5-6 mm in diameter to avoid edge effect influences on the measurement results. 

To check the thickness of solder resist lacquers on PCB copper, FISCHER’s high frequency probe FTA3.3-5.6HF is ideal. The probe can be used equally well with the handheld ISOSCOPE® and DUALSCOPE® instruments of the FISCHER FMP family or with the FISCHERSCOPE® MMS® PC2 bench-top unit. Please contact your local FISCHER representative for further information. 

X-ray instruments for standard PCB applications

Determining the coating thickness of standard PCB applications must be fast, precise, non-destructive and cost effective. Ever-higher volumes of standard PCBs are being produced with ever-thinner coatings, often using precious metals and requiring testing on ever-smaller structures. Plus, to be suitable for this purpose any instrument must cope with further sample handling challenges such as flexible or oversized PCBs.

The x-ray fluorescence (XRF) method has been well established for its reliability and accuracy in measuring metallic coatings on PCBs. FISCHER offers with its dedicated X-Ray PCB family an extensive array of XRF products for analyzing and determining thickness of all the standard coatings used in practical applications on PCBs. Even complex multi-layer and (precious) alloy coating systems can be tested easily and accurately.

 

 

Fig.1: FISCHERSCOPE® XULM® PCB with extensions for flexible PCBs

Various models and options address a range of needs and challenges, whether to accommodate difficult positioning requirements or small structures: the customer can choose from “top-down” or “bottom-up” instruments, manual handling or motorized XY-stages, as well as extended sample supports.

 

 

1st Layer

2nd Layer

3rd Layer

4th Layer

Measuring application

Typical Range

Typical Range

Typical Range

Typical Range

Au/Ni/Cu

0.3 - 0.7

3 - 15

10 - 40

--

Au/NiP/Cu

0.02 - 0.08

1 - 6

10 - 40

--

Ag/Cu

0.1 - 0.5

10 - 40

--

--

Sn/Cu

0.5 - 9

10 - 40

--

--

SnPd/Cu

2.5 - 10

10 - 30

--

--

Au/Pd/NiP/Cu

0.02 - 0.08

0.03 - 0.1

1 - 6

10 - 40

Tab.1: Typical PCB applications and coating thicknesses [µm]

With its focus on precision and trueness, FISCHER also provides a wide selection of calibration standards, produced in its own accredited calibration laboratory. To measure PCB layer thicknesses below 100 nm it is mandatory to calibrate the instrument with standards of similar thickness. For coatings below 50 nm FISCHER offers precise instruments with semiconductor detector.

The specially designed and developed FISCHER X-Ray PCB instruments perfectly meet the quality control needs of PCB manufacturers: Fast and easy-to-use, and equipped with matching calibration standards, they allow for highly precise, reliable and non-destructive measurements. For further information contact your local FISCHER sales representative.

Simplifying quality control on PCBs with automatic pattern recognition

Modern printed circuit boards (PCBs) are furnished with a huge number of contact points for electrical connections, all of which are coated with metal. The metrological monitoring of these coated areas is imperative for precise process control. But especially for large-scale boards, manual positioning on these tiny measuring spots is simply unfeasible.

For the metrological monitoring of the thickness and material composition of coated contact points on PCBs, X-ray fluorescence (XRF) has been established as a very effective method. FISCHER offers several different models of its XRF systems, the FISCHERSCOPE® X-RAY series, which are specifically optimized for the measurement of contact pads on PCBs. As the demands for automation have steadily grown also for process control, the next logical step was to develop a solution for fast and effective quality control that requires minimal operator effort.

To minimize operator involvement during measurement, the WinFTM® analysis software (version 6.30 and up) now offers automatic pattern recognition. This allows accurate and precise positioning of the measuring spot on very small structures in all XRF instruments with programmable XY tables. Especially in automated processes, pattern recognition can be used effectively, for example when testing large circuit boards and measuring repeatedly at the same positions.

While is not uncommon for a small offset from the originally programmed measuring points to occur when the device is loaded with the next PCB, the true measurement position on structures in the micrometer range can only be found accurately using the fine adjustment capabilities of pattern recognition technology.

With WinFTM® , image details or patterns can be defined using the image recognition menu, which lets the user freely select the measuring position within the image frame. Then, before measuring starts, the software compares the measuring spot (in the focus of the crosshairs) with the picture detail – and automatically derives a more accurate positioning, locating and targeting the next contact pad in the row. It is also possible to define several image details or patterns in order to perform automated measurements on a variety of structures.

Using the pre-sets in the pattern recognition menu, it is easy to run this software function even without prior knowledge. In addition, a variety of search algorithms can be chosen, as well as allowing for minor deviations from the target image (pattern) via error-checking.

The new pattern recognition feature built into the WinFTM® software enables excellent automated quality control of printed circuit boards when used with the XRF measurement systems from FISCHER. For more information, please contact your local FISCHER representative.