Frequently Asked Questions

Designing surface mount footprints Back

Introduction

These guidelines are not intended as a substitute for manufacturers recommended footprints, nor are they intended to be regarded as authoritative. They are intended to assist those users with less experience in this area to produce footprints which will not be too far removed from the optimum, and should enable users to avoid the commonest mistake of making pads too small.

Technique

It is assumed that the required device will be a DIL or quad footprint, most likely for a gull-wing device, though many of the comments are equally applicable to other device types. Dimensions are discussed in thousandths of an inch (approx 0.025 mm). If the typical copper foil is used, its thickness is just under 1 thousandth of an inch, so allowing for the slight over etching always required, it is reasonable to assume that one thou of copper will be lost during etching. Unfortunately chemical reactions generally don't care which way is down, so the same amount is lost from each exposed copper edge, therefore all finished tracks, pads, text, etc. are approximately 2 thou smaller than designed, having lost one thou from each edge.

To design the footprint, set the working grid to be the same as the pitch between the device pads. Now take two dimensions. The first is the maximum overall package width including device pins of maximum permitted length. The second is the nominal package width excluding pins. Call these A and B respectively. Calculate ½(A+B+1) - this is the required pad spacing. Calculate ½(A-B) +1 - this is the required pad length. Take the maximum pin width from the manufacturer’s data sheet, and add 2 thou. This is the absolute minimum pad width. Ideally you should also add half the maximum pin misalignment as well, if design clearances allow. Create and name an appropriate pad style using these figures.

Place one pad on grid using this style. Now the symbol and a coincident relative origin are to be placed. Put the cursor in an arbitrary position near what will be the centre of the footprint. Press the cursor keys once horizontally and once vertically. Without touching the mouse, press 'O'. This sets the relative origin on grid. If there are an odd number of pads on a side, the grid can be left, but if there is an even number, press 'H' and select Half of Grid. Press a cursor key in the same direction as the line of pads then press 'O' again. (For quad packs, press a cursor key in the direction of even rows of pads only. If both are even, press once in each direction.) Press 'H' again and re-select Grid. This has correctly positioned the relative origin so that the pads fall on grid. Don't worry at present about the spacing between rows. Highlight the 'S', right click and select properties. Tick the relative box, and set position to (0,0). The symbol origin will move to the reference origin. Now position pin 1 so that it's displacement from centre is approximately right, and position the pin name and number appropriately. Note that it may be necessary to use [View], [Display] to make them visible. You may need to select a smaller grid interval whilst doing this, but be sure to restore Grid afterwards. Remember to leave space for the external silk screen outside the pad.

Once the pad is positioned, use 'D' and click to create the remainder of the row. You will need to reposition the pin names and numbers for the first pad in the next row. Rotate the pad before this if necessary. Continue in this fashion until all the pads are placed. Note that if the pads are not central either side of the origin (e.g. in an eight pad row, the symbol origin doesn't lie centrally between pads 4 and 5, but perhaps between pads 3 and 4), that it is possible to drag a whole row by using and dragging any of the highlighted pads. Highlight each row of pads in turn, right click on one, and select Properties. There will be one common co-ordinate and one '(Different)'. Edit the common one so that the relative value is exactly half the pad row spacing calculated earlier. This may be positive or negative depending on the row. When [OK] is pressed, the entire row will move to the correct position. When this has been completed, all the pads will be correctly positioned.

Next create an appropriate silk screen legend. This should lie outside the pad pattern to prevent the silk screen legend acting as a stand-off. Be aware that some manufacturers will not put silk screen where solder resist is not present, so allow adequate pad clearance. You also need to add a reference origin, to define where the component identifier will be. It is good practice to put this outside the pad area as well. Turn off the pin names and numbers if not required, and save the symbol to library. Note that it was necessary to place these legibly, as they are needed during component creation.

Back | Contents KB020029 / 02-Apr-2010 / Keywords: SMT footprint library