PCB全制作流程中的激光直接成像技术应用

Graph 1: trend of line & space at DYCONEX over the last 10 years The consequent use of LDI capability over the completePCB manufacturing flowDaniel Schulze, Uwe KramerDYCONEX AGGrindelstrasse 408303 Bassersdorf, SwitzerlandT3.4 PCB QualityOral PresentationAbstractThe systematic implementation of optimized LDI (Laser Direct Imaging) photo resist types, LDI exposure units and fine line AOI (Automated Optical Inspection) equipment enhances the production technology and provides optimized PCB solutions for customers. This standardization results in a significantly faster and easier process flow with very precise registration. For high-reliability medical implants, full traceability can be obtained by adding individual serial numbers, date stamps and 2D barcodes for recording various process parameters.Design requirementsDriven by the assembly technology and analyzing the design requirements of the customers over the last 10 years a clear trend to smaller feature sizes is seen. A typical medical implantable product has afeature size of 50 µm line /space. Typical via diametersbetween 50 and 75 µm andpad sizes between 150 µm and 250 µm are standard tofind on these markets. Toassemble resistors of 0201packagingsizes will increase the requirements to the solder maskalignment as well down to25 µm alignment precision.During the quotation phase a detailed design rule check (DRC) has to be done to analyze customers design requirements. Dependent on these requirements like annular ring, layer to layer accuracy or solder mask to artwork accuracy the right registration systems have to be chosen to define the most cost effective and precise manufacturing flow.The graph below shows that the corresponding alignment accuracies are dependant on the customer design and finally used registration concept within the PCB manufacturing.Graph 3: registration concept dependant on customer design class using the annular ring as exampleGraph 2: typical misalignment failure modes Registration SystemsLaser via LDI Artwork Solder maskMechanical via LDI Artwork Solder maskPlasma via LDI Artwork Solder maskaverage compensationstep & repeat compensationaverage compensationindividual compensationindividual compensationindividual compensationindividual compensationaverage compensationaverage compensationaverage compensationaverage compensationEquipment requirementsThe challenge for PCB manufacturers is to constantly implement new manufacturing technologies to satisfy the customers growing demand in PCB complexity, cost efficiency and shorter delivery cycle times. In addition these requirements from the customers have to be linked to profitability of the supplier. Oneof the trends which has been observedduring the last years is the direct imagingtechnology. The growing market for directimaging tools reflects the increasingvariety of DI (Direct Imaging)technologies and vendors. Specializedsystems for different requirements are available. There are entry level systems,tools with double stage loading for faster throughput, high resolution systems and systems which cope with imaging of solder mask. All these systems offer a great variety of advantages over conventional film exposure which will be described in detail in the next chapters. Before starting with DI technology in production one needs to think about the specifications for the equipment. What should the tools be used for? What is the conversion strategy from film imaging to DI? Are there restrictions from customers which need to be addressed? Should and can the equipment operate in automated mode without operator support? What advantages in detail are of interest for the production environment?Due to the demands of customers the requirements on DI systems are well defined. A film plotter has a resolution of 2.5 µm pixel size in comparison to a modern LDI with coherent light and 1 µm pixel size. Therefore LDI systems can offer resolutions higher than 20 µm line and space. Further requirements of the LDI equipment are registration performance better than 10 µm and programmable compensation schemes to include individual compensation together with step & repeat compensation. Furthermore, the systems need to be able to expose solder masks within a competitive process time. Customers are asking for full traceability during PCB manufacturing. Therefore, implementation of individual serial numbers, bar codes or date stamps during imaging is an important feature.After having defined all these prerequisites the process environment needs to be evaluated for matching the DI properties. Contrary to standard film foil exposure units most LDI systems use only one wavelength. Whenstarting with LDI processing inproduction photo resists are typicallynot optimized for direct imaging. Thismeans they either require longerexposure times compared to filmfoil Graph 4: film exposure and LDI exposure equipmentGraph 5: conventional exposure vs. LDI exposureexposure or they are not suitable at all for DI. Even if the standard photo resist can be exposed with DI the change to LDI optimized resists should be taken into account. Today there is a very big variety of optimized DI resists available on the market. After implementing these resists the cycle time for artwork imaging in comparison to film imaging is significantly reduced.Compared to photo resists conventional solder mask exposure requires much higher exposure energy; up to 800mJ/cm2. When establishing direct imaging processes in manufacturing this fact needs to be taken into account. Depending on other requirements the DI capacity needs to be calculated to be sufficient for conventional solder mask exposure. Optimized direct imaging tools are available on the market which combines both the advantages of direct imaging and the high exposure energy. The other option is to qualify new solder mask materials which are optimized for DI. As mentioned above the need for direct imaging of solder masks is usually given on very thin flex products. However, only a limited number of flexible solder masks are available. These solder masks can be exposed with less than 100mJ/cm2. Since medical customers can not change solder mask during the life cycle of a certified product a qualification of a solder mask for direct imaging is a long-term and strategic decision. Therefore at DYCONEX we have qualified two types of flex solder mask for direct imaging. When applying the solder mask optimized for direct imaging an exposure cycle time of less than 90 seconds per panel is achieved. In comparison to LDI solder mask, conventional solder mask exposure takes 6 times longer.Registration ConceptsSubstrate materials are changing in size during the various manufacturing steps. For example plating, desmearing or any process steps where heat is applied. Therefore a start compensation value is given to the substrates to accommodate for the shrinkage or expansion. Changing the single step artwork exposure to LDI has an influence on the rest of the manufacturing process steps. Conventional film foil exposure with just one single compensation value for the entire manufacturing lot as opposed to single panel compensation value to each individual panel using LDI. Thus the complete registration chain from the beginning of the first drilling step through to the artwork and solder mask exposure and ending up with the final routing has to be reviewed.One of the conventional registration concepts is to build sequential products and align layer to layer referring directly to the inner layer structure. A newer registration system uses x-ray drilling to reference to one or more inner layers at simultaneously. Changing from conventional multilayer registration concept to LDI offers several advantages: less process steps, faster through put time and higher alignment precision.The lower number of process steps in the work flow for LDI registration system is seen in Graph 6. For the conventional registration the reference system needs to be protected with4stickers prior to plating. The punching of the reference system needs to be done. After that the compensation needs to be measured and the films need to be plotted. During exposure the film needs to be aligned to the panel and for good imaging performance vacuum needs to be applied. If required additional sets of films need to be plotted where compensation deviations within the work order occur. The exposure process itself is typically performed simultaneously on front and back of the panel. The overall process time including the supporting processes is 5 - 10 minutes per panel.For the LDI registration process the reference system can be drilled and does not need to be protected. No film needs to be plotted and the individual compensation does not need to be measured in advance. The exposure of front and back is done sequentially. The overall process time including application of resist is less than 2 minutes per panel.Graph 6: comparison between conventional registration and LDI registration systemCompensation conceptsBasically within the film exposure process only an average compensation for the entire lot of multiple panels is possible. Depending on the LDI equipment used a variety of different compensation schemes can be applied:• average compensation• individual panel compensation• step & repeat compensation (sub panel scaling)Step & repeat scaling can help when highcompensation offsets are present within a workorder. Individual compensation is not feasibledue to the following processes where onlyaverage compensation, like final mechanicalrouting or screen printing has been applied.The most accurate and at the same time mostcomplex scaling concept is the step and repeatregistration and compensation.Comparisons between different compensationschemes show the performance improvements.Very thin flex material tends to non linear distortions which occur through the entire manufacturing process. These distortions can not be compensated completely. All remaining distortions which can not be corrected are reflected in loss of annular ring and misregistration. Together with the increasing complexity as mentioned in the beginning of this paper the annular ring requirements are also increasing. Today standard alignment requirements are 25 µm for both artwork and solder mask. Typically thin flex material distortions within a workorder can be higher than 300 ppm. This can result, depending on panel size in more than 50 µm loss in annular ring.When film foil exposure is applied no kind of deviation can be corrected. When using individual panel scaling the panel to panel variation can be compensated. This reduces theamount of annular ring failures by a factor of two. The impact of the non linear distortions can Graph 8: material distortion within one single panelGraph 7: material distortion within a work order of multiple production panelsbe reduced when step & repeat registration is applied. Measurements on 25 µm flex material show that the registration within one workorder is improved again by a factor of two. The conclusion of this is that only DI systems can guarantee the high yields to satisfy increasing demands on high density PCB’s.When individual or step & repeat compensation is applied the same compensation methods should be applied to the following process steps. As an example the artwork and solder mask on a 25 µm thin flex panel is imaged using step & repeat compensation. This results in 25 µm registration accuracy. Nevertheless the distortions within a panel are higher than this. Therefore the final mechanical routing also has to be done with step & repeat compensation. Otherwise positioning failure of the PCB structures relative to the routing contour will occur. In other words, the full potential of individual or step & repeat compensation needs to be considered for the entire process chain.Individual and step & repeat compensations in artwork imaging are well established in the PCB industry already. Same holds true for laser processing. Last but not least the solder mask imaging is the process which also needs to be converted to direct imaging.average compensation = x panel individual compensation = 1 panelS&R compensation = 1/x panelaverage compensation individual compensationS&R compensation Graph 10: registration performance depend on used registration conceptTraceability InformationAs mentioned above most customers ask for traceability of their products through the entire manufacturing process. Traceability means a unique numbering of the manufacturing lot, panel, delivery panel and print at least. This can mean bar codes and date stamp of customer specific counters. Since for DI process no film needs to be plotted, these features can be implemented directly during exposure. The individual features are etched in copper. The same can be done when solder mask is exposed with direct imaging. On some products a legend print is included. Even here the individual serial number can be exposed. At DYCONEX all artwork exposures enable individual serial numbers for full traceability.Graph 11: Traceability information with LDI exposure and copper etchedConclusionTo achieve higher yields and faster trough-put times a change from conventional manufacturing technologies like film exposure to advanced manufacturing processes like laser direct imaging is necessary for high-end, high-reliability flexible PCB’s in the medical, avionics or semiconductor market. To use LDI exposure in PCB manufacturing requires several equipment updates. Changing the photo chemistry lines and implementing new photo resists or reference systems are just some examples. Following this path results in a new spectrum of smaller and more miniaturized PCB’s with the advantage of better alignment registration, higher accuracy, faster through put time and a better traceability.Manufacturer IDLot NumberPanel NumberPrin t Position NumberDate CodeCustomer Article Number 2D Barcodecopper etchedBiographyMr. Daniel Schulze studied at the Technical University of Dresden and has a diploma degree in Electrical Engineering. During his diploma thesis and an internship at the Georgia Tech Packaging Research Center he got involved with the work on optical waveguides embedded in PCB’s. In 2005 he started to work as Product Engineer at DYCONEX. Since 2008 he is Engineering Manager at DYCONEX and responsible for the product development of medical imaging PCB’s, hearing aids and cochlear implants, industrial and HF products.Mr. Uwe Kramer studied at the University of Halle/Wittenberg and the Technical University Dresden. He holds a diploma degree in physics. He is specialized in semiconductor and metal physics. The diploma thesis was done in collaboration with Advanced Micro Devises and Fraunhofer Institute for Mechanics of Materials. Thesis of diploma was three dimensional reconstruction and analysis of grain structure by using fine beam techniques. Mr. Kramer joined DYCONEX in 2008 as process engineer. Since 2013 he is manager of the process module artwork and senior process engineer.。