Repair and Re-balling stencils 1.5 for SMD components Application Manufacturing in the laser cut and laser Stencil post-processing welding process With the introduction of SMD-technology All laser cut stencils from LaserJob are subjected and the constant drive toward miniaturization LaserJob stencils are produced in temperature- to an automated post-cut brushing process. of components, new procedures and repair controlled production rooms with a fiber laser. The CNC-controlled brushing system removes all processes are constantly being developed in Our custom fiber laser platform produces a exposed burrs on the laser exit side. The brush order to provide reliable repair materials and significantly better beam quality then commonly head travels across the entire stencil surface in safe repair procedures. Despite the low failure used laser systems. The distinct lower cutting horizontal and vertical directions. rates of many of the new integrated circuits, opening (20 µm instead of the commonplace the high cost of these devices, and the subse- 40 µm), with equal depth of focus sharpness, Advantages of this process quent cost of the populated PCB require a allows for reduced heat input into the material. – no enlargement of pad openings reliable repair solution. The high precision of the moderately conical – lowest material removal < 2 µm It is very rare for a new product to achieve a first- apertures increases the process window in the – consistent thickness of stainless steel pass yield of 100 %. Increased component pick and place operation and allows for more material geometries and component leads, particularly efficient solder paste release. with area array components, create the need As a standard feature, the repair stencils are for tighter tolerances and accuracy from rework offered in sizes from 10 mm x 10 mm to systems and rework stencils. Therefore, a fool- 100 mm x 100 mm. Other sizes and geometries proof repair process is necessary. Decreased are available on request. The apertures for the spacing of component leads has also gene- component leads are laser-cut in a stainless rated additional challenges for repair processes steel sheet under high-tension, which is then and repair stencils. With the high demand welded in a frame in the correct position (see on quality and reliability, component repair is picture 1). getting more complex and time consuming. This procedure ensures, that the sensitive prin- For over 10 years, LaserJob has been producing ting mask is protected against external damage high-accuracy laser cut stencils for a variety and is absolutely flat under high-tension. The of rework and repair stations. stiffening frame is also used as a positioning support for the component. Two variations are possible (see pictures 2a and 2b). In picture 2a, the printing mask is welded into the form of a carrier plate. In picture 2b it is welded Picture 2a: Frame welded as carrier plate into the form of a carriage frame. The cut-out section, or nest, is a helpful tool for accurate positioning of the component. Misaligned components are therefore prevented. Advantages – precise aperture geometry – aperture size accuracy ± 3 µm Circlip Toothed washer – aperture position accuracy ± 10 µm within 200 mm – smooth walls – absolute flatness – positioning support for component Picture 2b: Carriage frame Picture 1: Repair stencil welded in frame LaserJob GmbH Liebigstraße 14 82256 Fürstenfeldbruck, Germany phone +49 (0) 8141 52778-0 fax +49 (0) 8141 52778-69 [email protected] www.laserjob.de LaserJob Inc. 55 Fleming Drive, Unit 24 Cambridge, ON N1T 2A9, Canada phone +1 (0) 519 620-1221 [email protected] www.laserjob.com Repair and Re-balling stencils for SMD components Simple printing stencil Custom-fit (adapted) printing stencil QFN / MLF rework with direct print process A printing stencil for manually printing Printing stencil is customized to adapt to Advanced Packages, such as QFNs (Quad Flat connection pads of a printed circuit board the geometry of the printed circuit board. No Lead) or MFLs (Micro Lead Frames), have with solder paste. The geometry of the printing stencil is custo- their connections on exterior sides beyond the The easiest method to repair a component is mized, can be square or rectangular in chip (see picture 4), and have direct contact to manually print the connection apertures form, and can be adjustable to the geometry via side bumps. The QFN-connectors are printed on the printed circuit board by using a printing of the printed circuit board or assembly using a mini stencil, solder paste and stencil together with solder paste. This (see picture 3a). The thickness of the frame is squeegee in a fast and accurate process, thus repair method is only recommended for the between 1,5 mm –2 mm, and provides the minimizing component stress (see picture 5). repair of simple component geometries, required stability required in order to protect The printed component, before being soldered when connection apertures with large pitches the printing stencil from external damage to the substrate, is analysed by optical are available and few components need to be during handling. The thickness of the printing inspection. The mini stencil is removed after repaired with enough available space on the stencil is selectable between 0.020 mm– the reflow process to ensure higher process printed circuit board (see picture 3). 0.300 mm and should be adjusted to transfer stability and to avoid solder paste sloughing The printing stencil is custom-sized and the required amount of solder paste. and smearing. should not extend beyond the dimensions of To guarantee a perfect release of the solder 600 mm x 600 mm. In tensioned conditions, paste and to avoid smearing of the solder Advantages of the simple method of the the apertures are cut and followed by a brushing paste during the release of the stencil, a release direct print process of the QFN connectors process to remove any burrs from the tool, in the form of four mechanical ball – fast and simple print process laser exit side. The quality of the laser cut is pens, was developed. The ball pens guarantee – accurate and stress-free process in-spected afterwards by measuring the a defined snap off from the stencil to the – cost effective rework aperture with a CCD camera with transmitted printed circuit board. In addition, a cut-out – single-step reflow process light. Stainless steel material (1.4301) is area in the frame can be cut to allow avoi- used with a tensile strenght > 1100N / mm² in dance of existing components. order to guarantee minimal aperture toleran- ces. The printing stencil is delivered without a frame welded on the stencil, and can be any standard thickness between 70 µm and 300 µm. The printing stencil thickness would be selected based on required paste volume. Picture 3: Connector with few apertures Picture 3a: Repair stencil with adapted Picture 4: QFN component and simple geometry geometry Picture 5: Example of a QFN rework stencil Picture 6: Re-balling stencil Picture 7: Re-balling stencil with runoff channel Weld seams Repair stencil Re-balling of BGAs or CSPs with Repair of BGA with a solder paste print Quality control a re-soldering process process on the PCB Quality assurance is paramount at LaserJob. The repair or re-soldering of BGAs is a common It is also possible to print solder paste direct Effective quality control starts with incoming process in the electronic production. The con- onto the printed circuit board (PCB) with the inspection of stainless steel sheets and stencil nections of the de-soldered BGAs or CSPs are support of a stencil in lieu of the re-balling frames. A thickness measurement instrument quite often damaged to the point that they process. After de-soldering the component, controls every stainless steel sheet with an are no longer able to be re-soldered to the prin- the residual solder is removed with an extractor accuracy of ±0.5 µm. Pad size and pad geome- ted circuit board. After the residual solder or with solder braid. Solder paste is applied, tries are inspected immediately after the is removed from the land area, a flux pen is via a stencil, onto the PCB (see picture 8). laser cutting process. The aperture shape is used to wet the BGA-connections. With the Afterwards, the component is placed onto the measured, via CCD-camera with transmitted aid of a suitable mask with the corresponding PCB and reflowed. light, to a precision of 1.0 µm. The tolerances pitch of the BGA balls, the balls are distri- of the laser cut opening are illustrated buted over the mask (see picture 6). The excess in picture 9. balls, which have not dropped into the stencil apertures, are diverted back into the container utilizing a runoff channel (see picture 7). squeegee side ± 3 µm With the recommended reflow process, the balls b x on the BGA connections are reflowed. 1 / 2 a stencil assembly side ± 3 µm thickness x = reference value b = value measured by x = a + b transmitted light 2 a = value measured by reflected light a = b + ≤ 12 µm a – b = ≤ 12 µm b = x – ≤ 6 µm a = x + ≤ 6 µm Picture 8: Direct solder paste printing process Picture 9: Tolerances of a laser cut opening on the PCB (company Finetech) Design proposal for the print process of Variation 1: round shape of Thermal Die Pad Area QFN-components (Stencil thickness: 100 µm) Stencil opening: circular pads: For connecting apertures of all 4 sides 390 µm x 220 µm (–10 µm reduction in length and width), pitch 0.5 mm of the package, we recommend the use of a Stencil opening for Thermal Die Pad Area: Ø 950 µm, pitch 1.65 mm.