Using Surface Mount Technology

 

 

 

Using Surface Mount Technology in the Classroom

 

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This article was originally published by the IEE in their spring 2003 edition of Electronics Education.  Electronics Education is published three times a year and supplied free of charge to every secondary school. It is also available by subscription.  You can download back copies from here

 

Circuit construction has remained essentially unchanged since the advent of printed circuit boards (PCBs); electronic components are mounted on a sheet of insulating material electrically connected by copper foil tracks yet opening up the back of a mobile phone reveals circuitry that would be unrecognisable just 15 years ago.

 

 

Fig 1 Inside the back of two similar sized hand-held products.  On the left a Nokia8210 Mobile phone (c2000) on the right a Sinclair Cambridge calculator (c1974)

 

Electronic components have shrunk in size; many are now too small to handle.  Components can be made smaller by changing the way they connect to the circuit board.  Traditional parts (through hole mounting) have leads which pass through the PCB, modern components are leadless (surface mounting) and connect to pads on the surface of the PCB.  Fitting leads to components limits how small the part can be, as the component-lead junction has to be strong enough to allow the lead to be formed without breaking away from the component body.  A through-hole lead occupies both sides of a PCB, for the hole and the pad around it.  On the other hand the body of a surface mount device can be made smaller because it does not need to mechanically support a lead and because the connection is via a small pad on the board surface, the space under the board can be occupied by another component.

 

The space needed for a hole and surrounding pad on a PCB sets the pitch for pin spacing of integrated circuits.  A lead pitch of 2.54mm (0.1”) is a convenient spacing for ease of insertion into PCBs and IC sockets.  The size of the silicon chip is much smaller than the size of its package, so changing the connection method from through-hole to surface mount makes it possible to shrink the size of the component.

 

Surface mount technology and the integration of more circuitry onto silicon chips have made possible the compact and sophisticated electronic products of today.  The reduction of circuit size not only helps to reduce costs, but it also increases the speed at which digital circuits can operate.  Surface mount technology is essential to produce high performance in computer and communication systems.

 

The assembly of PCBs using surface mount components is mostly by automatic machines.  At the start of an assembly line the unpopulated PCB is screen printed with a paste containing flux and granules of solder.  Paste is applied only to the component pad areas.  Components are picked from component trays or reels and placed onto the PCB by ‘pick and place’ robots.  Each robot can place 30 or more different parts at a rate of up to 4,500 components an hour.  Components are held in place on the PCB by the ‘tack’ of the solder past. Towards the end of the assembly line, the populated PCB travels through an oven where hot air is passed over the board to melt the solder paste fusing components to their component pads.  Components placed on the bottom side of the PCB are held by small dots of glue ‘spotted’ onto the board before the parts are placed.  This stops them falling off when the board is turned over prior to passing through the oven.  Automatic assembly is used for high volume or high value electronic products where the cost of expensive plant can be recovered.

 

Sophisticated assembly methods and the high cost of surface mount handling tools make the use of surface mount components appear to be impractical for students.  However, at Finham Park School we have been using surface mount components successfully for a number of years by improvising methods of handling. 

The main problem with using surface mount components is they move when you try to solder them.  The trick is to ‘tin’ one of the pads first and tack the component to the tinned pad.  The remaining pads can then be soldered normally.  We have found that the larger surface mount components such as those of 1206 outline can be handled easily using inexpensive tweezers.  Smaller parts of 0805 outline can be handled, but they need more care.  (PCBs are more difficult to design using 0805 parts because the pads are too close together for a track to pass underneath the component.)  Most of the integrated circuits used in school are available in a surface mount outline that pupils can handle.  These include CMOS logic, Op-amps, comparators, 555 timers and PICs.  There is a confusing number of package outlines for integrated circuits.  It is best to use ones with gull-wing leads pitched at 1.27mm (0.05”), this is half the spacing of through-hole leads.  The outlines to look for are the SO types.  Do not use SS types (shrink small outline) because they have a lead pitch of 0.025”.  It is a good idea to purchase components before designing the PCB so that they can be matched against the artwork before committing the design to circuit board.

 

When designing pad layouts it is best to extend the pad beyond the component to make it easier to solder.  With automatic assembly, the solder is placed between the pad and the component.  For hand soldering, we have found it better to produce a fillet of solder between the end of the component and the pad.  Solder will wick under the component, but it is easer to check the quality of the joint when the connection is visible.

 

Table 1 gives pad details for useful surface mount outlines.  To design surface mount PCBs using PCBWizard set a custom grid with 40 divisions and turn grid snap on.  From time to time you may need to switch off the intelligent object snap when drawing tracks close together.  If you are designing double sided PCBs remember to select either the component side or the artwork side for placing the pads (or they will appear on both sides).

 

 

Table 1 Pad sizes and pitch for common surface mount components

 

 

 

Fig 2 Screen shot of PCBWizard showing the different pad layouts. (The 1206 style shows three tracks passing underneath)

 

Newcomers to surface mount could start by incorporating 1206 style parts into their designs.  We nearly always mix through hole with surface mount components because some parts are difficult to obtain in small quantity e.g. L293D motor driver.  Considerable reduction in overall size is possible with this approach but be prepared to use thinner track sizes.  There is little point in switching to surface mount to reduce board size and then waste the gain by using track sizes of 0.1”.  We regularly print tracks of 0.01” track and space (by using the minimum necessary UV exposure time and the minimum etch time).  Up to three tracks of this size can pass between the pads of a 1206 component.

 

Fig 3 shows the stages of soldering a surface mount resistor.  It is best to remove the etch resist from the PCB and then either tinplate or spray lacquer the tracking.  Use 26 swg solder to avoid overloading pads.

 

 

Fig 3 Step by step sequence for soldering a 1206 style component

 

 

 

 

 

 

 

 

 

 

Steps 1 and 2

 

Steps 3 and 4

 

Seps 5 and 6

 

Step 7

 

Step 8

 

Fig 4 shows examples of GCSE project work for Electronic Products.  The students concerned had no extra practice or instruction other than the experience of constructing a surface mount project (Christmas decoration) in year 10. The Christmas decoration is constructed by all year 10 students to give them confidence in using surface mount technology and encourage them to consider using it in their year 11 project work.  Most students complete the Christmas decoration without problem, any difficulty they may have tends to be in soldering the PIC.  Short circuits due to the over zealous application of solder (easily remedied with a solder sucker) or open circuits where they have not pressed the IC leads down onto the pads as they solder.

 

 

 

 

 

 

 

Fig 4 Examples of GCSE project work for Electronic Products. 

 

 

Fig 5 Christmas Decoration constructed by year 10 students

 

 

Incorporating surface mount components into their coursework, gives students a sense of working with ‘real’ electronics; the kind found in everyday electronic products.  By reducing the size of their PCBs they can achieve designs with enclosure sizes comparable with commercial products which give them a great sense of achievement.  If you have yet to dabble in surface mount why not give it a try?

 

 

Fig 6 Surface mount adapter for programming PICs with Logicator or PicStart.  (Always test your programme using a through-hole part on a breadboard first.)

 

Further references

 

Farnell semiconductors and passives catalogue

Rapid Electronics (industrial) catalogue

Introduction to Manufacturing CD (TEP)

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Reproduced by permission of The IET