Printed Circuit Design & Fab - November 2008 - (Page 22)

DFM BaSicS Figure 5. FiGurE Figure 6. 6. AOI – design for inspection. FiGurE 5. BGA components location. Figure 5. 6. Designing According to the Production Process It is important to adjust the circuit board design as much as possible to its production and assembly processes. Proper planning of the card in this framework should enable quick and problem-free production that reduces production costs and increases yields. It is advisable to place irregular components such as through-hole, ICs, fine-pitch, oscillators, as well as high, heavy and sensitive components on the component side. On the print side it is advisable to place passive components such as resistors and capacitors, which are more resistant in these production conditions. In addition, when using TSOP or SOIC components, they should be placed such in a way that applying the solder paste printing can be accomplished lengthwise in the area of the component in order to prevent short circuits near the components’ pins. Asymmetrical spreading of heat can cause distortions in the card, harm to sensitive components and soldering results that are not of a high standard. Placement of BGA components on the edge of the card is not recommended, since these regions tend to be colder during the course of the soldering. Similarly, in the areas close to through-hole components, the mechanical strength of the card is lower; hence, placement in the area of BGA components is not recommended, as illustrated in Figure 6. FiGurE 5. BGA Rework. To enable an optimal dismantling during rework, it is worthwhile to leave a clean area of 3 mm without additional components around the BGA component. In rework machines that work based on IR, keeping a distance of 1 mm is sufficient. X-Ray tests. In double sided reflow cycles it is worthwhile keeping the side parallel to the BGA component free of components to enable a reliable X-ray test to be performed. Components located on the other side of the circuit, parallel to the location of the BGA, are visible during the X-Ray procedure and can distort the results of the component soldering test. Fine Pitch Components. Handling fine pitch components in the assembly line requires meticulous precision. Therefore, it is unadvisable to place theses components near the circuit edges, since excess solder paste may be present. Mictor Connectors. These components are assembled on the SMT lines before putting the card through reflow soldering. In order to facilitate optimal accessibility for placing this component, it should be placed near the circuit edge, approximately 15 mm to the nearest pad. Mictor components longer than 50 mm should be placed along the circuit, in the direction of the card’s motion and inside the reflow soldering. Due to the potential bending of the circuit during the soldering, contact between the component’s center and the PCB might be affected. 7. Designing in X-Ray and AOI Testability The aim is to design the card so that testing will be efficient and fast. This is determined by the electrical and physical design. The physical design adjusts the card to the testing device and locates the testing points. The AOI machine performs visual testing of the card by photographing the circuitry and comparing it to a known, good design. For AOI testing to be efficient and effective, one needs to be meticulous about marking the card consistently, (e.g. fonts identical in thickness and shape, card color, consistent indications, as well as clearly marking the components’ polarity.) Inconsistent marking extends the testing time and unnecessarily increases the database in the AOI machine. Also, for the machine to achieve optimal photography, it is recommended that an open space of 35° to 65° spreading/distribution angle is maintained from each component. This allows the AOI cameras to swiftly and easily identify the component, as illustrated in FiGurE 6. The x-ray machine performs card inspection and is intended mostly for BGA component photography. This testing is sometimes performed automatically, usually in cards with multiple BGAs, after defining the component’s location and formation. For optimal testing, the reference points (fiducial) need to be clear and easily located by the machine. Since x-rays penetrate through the entire card, the area should be clean on both sides. When locating BGA components on both sides of the card, do not place them under one other. This will make it difficult to inspect the soldering quality with the x-ray machine, as shown in FiGurE 7. 5. Space Between the Components When placing components in the printed circuit, keep a minimal distance between the packages. It is recommended that one read standard IPC-2221, which defines the required distances. If components are pressed too close together, this could result in a low production yield, making it difficult to perform circuit rework and repair, as well as hindering the manual and/or automatic visual quality control in AOI machines. 22 printEd CirCuit dESign & fAB NOVEMBER 2008

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - November 2008

Printed Circuit Design & Fab - November 2008 Contents Our Line Market Watch Around the World Happenings ROI Positive Plating Ten Tips to Improve Manufacturability 3D Chip-Package-Board Modeling Improving Circuit Simulation With The Addition Of Real Measurements Ad Index PCB West: Interview with NBS Design Inc. The Influence of Final Finish on Lead-Free Assembly Reliability The Lead-free Soldering Challenges for Peelable Resists Off the Shelf Marketplace BGA Bulletin

Printed Circuit Design & Fab - November 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.