Printed Circuit Design & Fab - November 2008 - (Page 20)

DFM BaSicS Figure 3. FiGurE 4. When designing a panel, consider the panel’s rigidity to 4. Figure make it more stable on the production machines. FiGurE Figure 3. 3. Alternatives for achieving rectangular shapes. Figure 4. panels. In addition, the individual circuit card margins need to have a component-free area of 3 mm for later routing and fixturing. The circuit card itself is typically rectangular and symmetrical. Circuits whose shapes are not symmetrical strongly hinder the assembly process. In such cases, the card’s stability on the production line is low, and therefore, the reliability and accuracy of the placement of components or smearing of tin are low. With these circuits, it is incumbent upon the designer to perform an artificial supplementation to the measure of a rectangular card to improve stability, as shown in FiGurE 3. This supplementation will be removed during the course of production before shipment to the customer. 3. Panelization Planning Panelization planning can reduce the cost of circuit board production significantly. The panel consists of several cards. By this method, the panel is assembled, and afterwards, the cards are removed and work continues. The length of time that the machines work on a single card while it is in the panel are substantially less than it would be if it was working on an isolated card, even if we take into account the duration of the dismantling after assembly in order to return it to its generic state. When planning a panel, the level of precision in it’s production needs to be examined. In long panels, the precision level (tolerance) of the isolated card is multiplied by all the cards along the length of the panel, creating the potential for more movement of attachment pads along the length of the panel. In a panel with a length or width of more than 12 inches, the cumulative movement can reach 0.03 inch. This is too large of a movement if we want to install a component with 0.02 inch pitch at a high yield. On one hand, proper panel planning needs to preserve the panel’s strength and stability during a procedure that will convey it on the production lines. On the other hand, it needs to ensure that the separation of the cards will be executed as fast and conveniently as possible. There are two principal systems for this: V-CUT and break-away tabs. For V-CUT (also known as V-GROOVE), slots are planned on the panel surface in places intended for separa20 tion. Dismantling the cards from the panel is carried out by a device with two circular cutting blades standing on top of one another creating two V-shapes. A number of parameters must be kept in mind when planning these slots: thickness of the circuit, depth of the slot from both sides of the circuit, thickness of the remaining circuit, slot width and slot angle. In the break-away tabs system, a number of small holes (break-away tabs) are located between two areas intended for separation, and the circuits are dismantled by hand or machine breaking. In this method, a number of parameters needs to be kept in mind: the hole diameter and length, the distance between the holes, the number of holes and the distance of the holes from the edge of the circuit. In order to save production costs in this respect, one can plan the hole diameter according to the diameter of the smaller through (VIA) hole so that a drill change is not necessary to drill in the break-away tabs. It is recommended that one reads STANDARD IPC-2221, detailing how to plan the parameters of the two systems. The V-CUT system is preferable to the Break-Away-Tabs method for several reasons: separation speed, high reliability, minimal manual engagement and low cost. With V-CUT one should pay attention to several factors including; a clearance of 1.5 mm must be maintained from the cutting slot. Likewise, components located close to the cutting slot must be strong enough to resist the mechanical pressure exerted on the area during the cutting process. When the width of the card is greater than 10 cm, one cannot plan a panel with two columns due to the lack of stability at the panel’s center. It is therefore preferred to plan a panel with a single column in order to prevent bending at the center of the panel during its transfer to solder reflow as shown in FiGurE 4. 4. Placement of Critical Components The placement of critical components (e.g. BGA or finepitch) is a highly important aspect in the planning stage that has significant effects on the quality of the card after manufacture BGA thermal Balance. During soldering, the BGA component stores heat. Therefore, it is important to ensure that thermodynamic balance exists on the surface of the board. NOVEMBER 2008 printEd CirCuit dESign & fAB

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

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