Laser Direct Structuring Resolution By Dave Klitzke and Richard Macary L aser direct structuring (LDS) is the first step of a manufacturing process that produces circuit traces on molded thermoplastic components creating 3- dimensional molded interconnects. Currently the leading technology for producing cell-phone antennas and molded interconnect devices (MID), LDS has been used more widely producing a range of components for use in medical devices, security shields, automotive sensors, and GPS antennas. Anywhere product miniaturization can be achieved by embedding circuits onto a molded component, laser direct structuring should be considered the technology of choice. With LPKF’s Laser Direct Structuring process (LDS) it is possible to produce circuit layouts on complex three-dimensional carrier structures. The laser beam structures the layout directly into the molded plastic part. As a result, weight and fitting space can be effectively reduced. Your design teams enjoy complete 3D capability on freeform surfaces and great freedom for redesigns. Thus LPKF- LDSTM opens up new possibilities. (LPKF Laser & Electronics AG, n.d.) At SelectConnect Technologies, we conducted research to determine how close LDS traces can be structured and metalized on the three most common materials that included LCP, PET/PBT, and PC/ABS without inducing over plating or bridging. Trace widths of 0.010 in (0.254 mm) with a spacing of 0.010 in (0.254 mm) are readily achieved. This article will determine the feasibility of trace widths and spacing below these parameters and the factors influencing resolution. To determine this we produced a geometric pattern with varying trace widths and spacing distances; and structured it onto plaques made from the three different materials using the LPKF Microline® 160 laser system. Following this, the plaques were plated with electroless copper, electroless nickel, and immersion gold. The parts were
then examined for conformance. Trace widths and spaces were measured using video microscopy equipment.Method SelectConnect Technologies employed the following techniques for producing LDS MID’s: CAD files, laser plotting translation, the laser structuring process, and electroless/immersion plating. 1. CAD Files Before a pattern can be structured, the artwork must be generated by CAD software using approved guidelines. Generally, Pro-E or Solid Works work very well because files can be created in either STEP or IGES formats. Virtually any type of geometry can be structured. An important design guideline is all geometry or artwork must be a surface that has a zero height (Z=0 in). This ensures that the artwork will be processed properly by the laser plotting system software. In addition, all modeled surfaces must be continuous. If there are any breaks or discontinuities, these will be reflected in the final structured product. 2. Overview of the Laser Structuring Process Once the laser machine is setup with a plotting program, the pattern is then structured onto the plastic part. It is important for the part to be held firmly by a fixture. This allows the laser’s camera system to identify the fiducials (alignment reference points) speedily. Once the fiducials are found, the laser marks the pattern and a 3-D MID is produced. 3. The Plating Process Care must be taken to ensure LDS components are plated properly. Areas of concern include cleaning solutions, plating bath concentrations, solution temperatures, plating times, and racking among others. The plating can be done with a rack fixture or in a barrel. For fine line resolution, rack plating method should be used because the barrel plating can have a tendency to damage fine traces. Standard plating thicknesses for MID’s are used for this experiment. According to Macary and Hamilton (2010), “The sequence includes electroless copper plating (100 to 600 micro inches), electroless nickel plating (50 to 100 micro
inches), and immersion gold plating (3 to 8 micro inches).” It’s important to maintain these thicknesses throughout the experimentation process so that consistent results are achieved. 4. Measuring Equipment One must accurately measure the width of the traces and the spacing distances. The goal is to discover how narrow the traces can be at the closest spacing possible without the plating bridging or shorting. Inspection and measuring were accomplished using a Scienscope MZ7-PK5-FR-U Dual Arm stand video microscope. It can reliably measure distances in inches through 4 decimal places. Inspection is accomplished through an integrated video system. Once an object is in the lens’ field of view, the image is projected onto a computer monitor. From here one can measure the distance using the microscope’s included software. 5. Process Variables/Influences a. Materials There are a variety of materials suitable for the LDS process. The following list was taken from the LPKF LDS-MID Design Guide Ver. 2.1: LCP PA 6/6T PBT PET (Liquid Crystal (Polyamide) (Polybutylene (Polyethylene Polymer) Terephthalate) Terephthalate) PPA PC PC/ABS PET/PBT(Polyphthalamide) (Polycarbonate) (Polycarbonate/ (Alloy) Acrylonitrile Butadiene Styrene) The three most common material types are LCP, PC/ABS, and PET/PBT which are the focus for this article. LCP displays excellent dimensional stability; PC/ABS has very good mechanical properties; and PET/PBT has very high thermal shape stability (Macary and Hamilton 2010). When a part is molded, it is important that it is designed properly in order to
reduce knit lines or surface roughness. The following provides more insight in this area: The surface quality of the injection molded part is of high importance for an efficient laser structuring and subsequent metallization process. Target is a smooth surface. Smooth surface means a thin polymer film at the surface of the molding. Therefore, the general design and processing recommendations for engineering plastics have to be considered, e.g.: • Gate and runner dimensions should allow gentle filling of the cavity. • Take flowability of the grade and part wall thickness into consideration to decide on number, location and kind of gates. • Design the tool with sufficient cooling/heating system to achieve uniform temperature distribution in the cavity. • Choose the recommended size of injection unit (metering stroke 1D to 3D). (Radeck 2005) Care must be taken when designing and manufacturing the molded part. The goal is to reduce structuring over extremely rough surface finishes since this can produce unreliable plating results. b. Laser Optimization There are three primary laser settings that are required for successful laser structuring. They are power (watts), frequency (kHz), and speed (mm/s). The settings are unique for each LDS material type. LPKF has conducted in depth research in this area and prepared tables with the appropriate settings for each material type. Radeck (2005) advises, “[…] the use of right laser parameters has a strong influence on achieving the requirements regarding surface roughness, adhesion and selectivity.” Slight modifications to these recommended values are acceptable if structuring and plating integrity are improved as a result.Results Simulated circuit patterns were structured onto plaques made of LCP, PET/PBT, and PC/ABS. The circuit patterns had the following trace widths and trace spaces: 0.006 in (0.152 mm), 0.008 in (0.203 mm), and 0.010 in (0.254 mm). Contact pads were placed on each trace end to test for circuit continuity. Scenario 1 did not compensate for plating creep or expansion. The traces widths on LCP, PET/PBT, and PC/ABS expanded after plating. The expansion of the line widths was due to the plating initiating on debris from the laser structuring adjacent to the lines and
the horizontal growth of the plating circuit. In general, the line increased in width by 0.002 in (0.051 mm) for most patterns. Scenario 2 compensated for the plating growth. The traces were structured at widths and spacing that was reduced to account for the 0.002 in (0.051 mm) plating expansion. Better results were seen. There were no over plated traces at all on all three material samples. The final plated trace sizes for LCP measured very close to 0.006 in, 0.008 in, and 0.010 in. The sizes of the plated PET/PBT trace widths ranged from being under sized ~6% to being oversized by ~22%. The trace results for PC/ABS were also successful. The plated trace widths were undersized by ~8% and oversized by up to ~19%. Appendices A and B contain trace width and spacing measurements for scenarios 1 and 2. The measurements were taken using the Scienscope MZ7-PK5-FR-U microscope. If there was any evidence of over plating along the traces, that was noted as well.Discussion Two different scenarios were tested concerning trace width and spacing. All samples were plated at the following thicknesses: copper (Cu) 250 µin (0.00635 mm), nickel (Ni) 100 µin (0.00254 mm), and immersion gold (Au) ~3-8 µin (0.00008-0.00020 mm). For the first scenario, all plaques were structured at the following trace widths and spacing: 0.006 in (0.152 mm), 0.008 in (0.203 mm), and 0.010 in (0.254 mm). In this first scenario, the plating had a tendency to creep over the edges of the traces thus increasing the trace width and reducing the trace spacing. Over plating was more likely to occur. We attempted to alleviate this problem in scenario two. The plaques were structured using trace widths and spacing that were altered in order to achieve the desired values. The trace widths were structured at values that were narrower to allow for plating creep or expansion. The final goal was to achieve the 0.006 in, 0.008 in, and 0.010 in width and spacing. Scenario 1-Traces and Spacing Structured at 0.006 in, 0.008 in, 0.010 in, No Compensation for Plating Creep 0.006 in Width and Spacing Results
Table 1 in Appendix A shows the measured trace widths and spacing at 0.006 in (0.152mm) for LCP. The laser structured this part, and all of the following parts in scenario 1,at exactly the listed value; in this case at 0.006 in widths and spaces. The tracesexpanded by over .0025 in (0.0635 mm). The average trace width grew to 0.0085 in(0.2159 mm) without any over plated traces.The plating also expanded for the PET/PBT samples (seefigure 1). The traces expanded by about 0.003 in (0.076mm). The average trace width was 0.0090 in (0.2286 mm).This plaque was fairly smooth but this did not stop overplating. A thinner trace must be structured next time toreduce the likelihood of bridging. Figure 1. PET/PBT, 0.006 in.A similar situation occurred on the PC/ABS plaque. A 0.0032 in (0.0813 mm) increase intrace width was evident after plating the sample. The traces structured at 0.006 inaveraged 0.0092 in (0.2337 mm) in width. We theorize that of the three plasticmaterials, PC/ABS has the lowest melting temperature; and that laser splatter is greatercausing additional plating adjacent to the lines.0.008 in Width and Spacing ResultsSample plaques for LCP, PET/PBT and PC/ABS werestructured at 0.008 in (0.203mm). Table 2 in appendix Adisplays the actual trace widths and spacing after plating.The traces structured on LCP appeared defined (see figure 2).The average trace width was 0.0113 in (0.2872 mm), an Figure 2. LCP, 0.008 in.Increase of 0.0033 in (0.0838 mm).The trace widths for PET/PBT expanded as expected. Theyaveraged 0.0122 in (0.3099 mm) (see figure 3). The averagespacing was 0.0041 in (0.1041 mm). This was a 50% increaseover the goal of 0.008 in. Bridging occurred in several areas Figure 3. PET/PBT, 0.008 in.across the trace.The trace width for PC/ABS averaged 0.0119 in (0.3023 mm). The spacing decreased to0.0047 in (0.1144 mm). Over plating was again seen on this material. These traces were
not as defined as LCP or PET/PBT. The plating also expanded almost 50%. The tracesdid not meet expectations when structured over PC/ABS at this width and spacing.0.010 in Width and Spacing ResultsThe final width and spacing was 0.010 in (see table 3, Appendix A). The lines and spaceson LCP appeared very clear and defined. The traces only expanded by about 30%. Theaverage width was 0.0133 in (0.3378 mm) and the average spacing was 0.0066 in(0.1676 mm). No over plating was evident on the sample part. It is expected that goodresults would be seen at 0.010 in widths and spaces since this is our current productionminimum.The trace width on PET/PBT averaged 0.0140 in (0.3556mm), a 40% expansion after plating. The spaces werereduced to 0.0068 in (0.1727 mm). The traces were not asdefined as the LCP sample but there was no bridging (seefigure 4). Figure 4. PET/PBT, 0.010 in.The PC/ABS traces also increased 30% growing from 0.010 in to 0.0138 in (0.3505 mm).Over plating did occur. The corresponding spaces averaged 0.0073 in (0.1854 mm).Scenario 2-Traces and Spacing adjusted, Compensation for Plating CreepThe same materials were analyzed as in scenario 1 but the structured traces and spaceswere undersized by to compensate for the plating creep. The goal was for the finalwidths and spacing to be as close to 0.006 in, 0.008 in, and 0.010 in as possible.0.006 in Width and Spacing ResultsTable 1 in Appendix B shows the measured trace widths and spacing at 0.006 in (0.152mm) for LCP. These traces appeared defined with no signs of over plating. Thestructured trace width was 0.0043 in (0.1092 mm) and spacing was 0.0077 in (0.1956mm). The values were based on previous test results. It was expected that the traceswould expand by about 0.002 in (0.051 mm) after plating. This compensation provedsuccessful, the actual average trace width was 0.0060 in (0.152 mm) with a spacing of0.0059 in (0.1499 mm)-right on target.PET/PBT exhibited similar behavior at 0.006 in width andspacing. The average width of the plated traces was
0.0073 in (0.1854 mm). The average spacing was 0.0048 in (0.1219 mm). The platedtraces were over sized slightly more than expected at 22% (see figure 5). Figure 5. PET/PBT, 0.006 in.Similarly, over plating did not occur with PC/ABS at 0.006 in widths and spaces. Table 1in Appendix B gives an average width of 0.0071 in (0.1803mm) and an average spacing of 0.0048 in (0.1219 mm).The trace widths were structured at 0.0048 in (0.1219mm). Plating creep increased slightly more than expected(see figure 6). In addition, the presence of knit lines wasnonexistent on the PC/ABS samples which most likelyeliminated bridging. Figure 6. PC/ABS, 0.006 in.0.008 in Width and Spacing ResultsThis same idea of narrower traces and wider spacing is carried out for rest of the tracesstructured on LCP. The goal was to achieve a trace having a width of 0.008 in (0.203mm). The average actual trace width was 0.0077 in (0.1956 mm). The spacingmeasured 0.0087 in (0.2210 mm). It was achieved by structuring traces at a width of0.0057 in (0.1448 mm) and a spacing of 0.0103 in (0.2616 mm). The plating expanded,as expected, by about 41% bringing the trace width up tothe desired value.Plating creep was observed on PET/PBT. The traces forthis sample were well defined. No bridging was seen (seefigure 7). The trace width was very close to the targetvalue of 0.008 in It averaged 0.0086 in (0.2184 mm). Thespaces averaged 0.0075 in (0.1905 mm). Figure 7. PET/PBT, 0.008 in.Similar widths and spacing can be seen with PC/ABS.Table 2 in Appendix B shows this data. The averagewidth was 0.0079 in (0.2006 mm) and the averagespacing was 0.0078 in (0.1981 mm). No bridgingoccurred with the traces (see figure 8). Very littleextraneous plating was observed. Figure 8. PC/ABS, 0.008 in.
0.010 in Width and Spacing ResultsSuccess was achieved for trace widths and spacing at 0.010 in (0.254 mm) on LCP (seeTable 3 in Appendix B). Traces were structured at 0.0075 in (0.1905 mm) and 0.0125 in(0.3175 mm) width and spacing, respectively. This yielded an average width of 0.0093 in(0.2362 mm) and an average spacing of 0.0111 in (0.2819 mm). These averages are veryclose to the desired value of 0.010 in (0.254 mm) making these parameters acceptable.No over plating or bridging was observed.It is intuitive that the wider a trace is and the greater the spacing, the less likely overplating will occur. This was true for PET/PBT. The width averaged 0.0094 in (0.2388mm) with an average spacing of 0.0108 in (0.2743 mm). It should be noted that thewidth was about 6% narrower than the desired value of 0.010 in (0.2540 mm). Thesetraces were structured using a width of 0.0069 in (0.1753 mm) illustrating the necessityfor trace compensation.Plating creep with PC/ABS is similar to that seen on PET/PBT.The trace widths averaged 0.0092 in (0.2337 mm) makingthis about 8% under the target. The spacing averaged0.0110 in (0.2794 mm). There was no noticeable overplating that was evident (see figure 9). Figure 9. PC/ABS, 0.010 inConclusionMultiple factors play a role in the success of plating MID circuitry. Factors includematerial type, surface roughness of the molded part, part cleaning, and platingprocesses to name a few. LCP appeared to be the best material for fine line definition.PBT/PET was the second best and the PC/ABS was a close third.Compensating for plating creep by structuring thinner traces spaced farther apartreduced over plating dramatically. This was the objective in scenario 2. Positive resultswere seen on LCP, PET/PBT, and PC/ABS. No over plating was seen at all for all 3materials on any traces. The plating expanded producing the anticipated results at0.006 in (0.152 mm), 0.008 in (0.203 mm), and 0.010 in (0.254 mm) width and spacingafter plating. The final widths and spaces were close to the expected values. Properplating compensation, adequate plastic surface finish, and sufficient plating practicesyielded positive results when using LCP, PET/PBT, and PC/ABS.
Appendix A Results-Scenario 1Traces and Spacing Structured at 0.006in, 0.008 in, and 0.010 in, No Compensation for Plating Creep Actual Trace Actual Trace Over Material Width (in) Spacing (in) Plating? AVG=0.0085 AVG=0.0027 LCP No AVG=0.0090 AVG=0.0032 PET/PBT Yes AVG=0.0092 AVG=0.0030 PC/ABS Yes Table 1. Trace Width and Trace Spacing at 0.006 in (0.152 mm) Actual Trace Actual Trace Over Plating? Material Width (in) Spacing (in) AVG=0.0113 AVG=0.0046 LCP No PET/PBT AVG=0.0122 AVG=0.0041 Yes AVG=0.0119 AVG=0.0047 PC/ABS Yes Table 2. Trace Width and Trace Spacing at 0.008 in (0.203 mm) Actual Trace Actual Trace Over Material Width (in) Spacing (in) Plating? AVG=0.0133 AVG=0.0066 LCP No AVG=0.0140 AVG=0.0068 PET/PBT No AVG=0.0138 AVG=0.0073 Yes PC/ABS Table 3. Trace Width and Trace Spacing at 0.010 in (0.254mm)
Appendix B Results-Scenario 2Traces and Spacing altered 30-50%, Compensation for Plating Creep Actual Trace Actual Trace Over Material Width (in) Spacing (in) Plating? AVG=0.0060 AVG=0.0059 LCP No AVG=0.0073 AVG=0.0048 PET/PBT No AVG=0.0071 AVG=0.0048 PC/ABS No Table 1. Actual Trace Width and Trace Spacing at 0.006 in (0.152 mm) Actual Trace Actual Trace Over Plating? Material Width (in) Spacing (in) AVG=0.0077 AVG=0.0087 LCP No PET/PBT AVG=0.0086 AVG=0.0075 No AVG=0.0079 AVG=0.0078 PC/ABS No Table 2. Actual Trace Width and Trace Spacing at 0.008 in (0.203 mm) Actual Trace Actual Trace Over Plating? Material Width (in) Spacing (in) AVG=0.0093 AVG=0.0111 LCP No AVG=0.0094 AVG=0.0108 PET/PBT No AVG=0.0092 AVG=0.0110 No PC/ABS Table 3. Actual Trace Width and Trace Spacing at 0.010 in (0.254mm)