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Prototyping with Surface Mount Components

-- By Jim Pearce, Pegasus Technologies


Many engineers express dismay at having to build a breadboard using components that are only available in surface mount (SM) packages. Others will go to great lengths to acquire parts in through-hole versions for their prototypes and then use the SM versions for the production board.

By necessity, I have become very proficient at building prototypes using SM discrete components and integrated circuits. There is one possible disadvantage that I should point out up front: your first prototype must be an actual printed circuit board. I don't find this to be a major problem since I used to go directly from schematic to printed circuit boards even back in the through-hole days.

I find that there are many advantages to doing the initial boards using SM components:

  1. The prototype board is the same physical size as the ultimate product. Since the board itself must be considered a component for many RF boards, this is absolutely necessary.

  2. The surface mount components are physically smaller than their through-hole equivalents, so they are less affected by parasitic effects.

  3. It is easier to change out resistors and capacitors to try different values. You don't have to get solder out of a plated-through hole. Simply bring the hot air soldering hand piece up to the part that you want to remove, grasp the part with tweezers, and lift it off.

  4. You can try different values of discrete components rapidly without solder at all. Just lay the part across the pads and apply a small pressure with your tweezers in the middle of the part so you don't contact either end of the part. Observe the 'scope, meter, or whatever instrument you're using for the change in circuit behavior, and go on to the next value.

  5. The board lasts through more component changes than a through-hole board. I have had through-hole pads lift and break after just one unsoldering operation. This rarely happens with a SM board, though pads with no connections to help dissipate heat will occasionally lift off the board with little provocation!

While a megabuck SM repair station is not necessary (I have gotten along just fine without one), there are a few bench tools that are essential for SM prototyping. They are listed below, roughly in order of importance.

  1. Stereo Microscope. Anyone who is doing serious SM work must have a good stereo microscope for soldering, desoldering, inspection, or probing operations. When selecting a microscope, be sure to pay particular attention to the working distance -- the clearance between the microscope objective lens and the item that it is focused on. I once worked with an otherwise fabulous microscope that had only two inches of working distance. Boy, was it ever easy to burn the plastic ring light on that microscope with a soldering iron! A negative power lens will increase working distance more than it will decrease the magnification. Scienscope supplies reasonably priced stereo microscopes.

    Sciencescope Microscope

  2. Good tweezers. Tweezers are next on my list since they are the tool that you will handle most. I prefer foam cushioned stainless steel tweezers with curved tips like the Erem E7SA. A good pair of tweezers will make your life much easier!

  3. Hot air soldering iron. A hot air soldering iron is useful both for initial soldering and for desoldering. My choice of hot air tools is the Edsyn 971HA. It is one of the less expensive units but is still quite capable. In fact, it has some features (possibly unintended) that make it easier to use than most hot air soldering tools. The most important of these is that the air tubing from the base to the handpiece is flexible and runs at a position where you can squeeze off the air flow with your index finger while moving the iron near to the component that you wish to work on. This prevents blowing off dozens of components while getting everything arranged under the microscope! This soldering station is intended for operation with compressed air and it comes with air handling hookups. I don't have shop air, but rather than pay hundreds of dollars more for a unit with a built in pump I went to WalMart and bought the largest aquarium air pump they had in stock. With very simple and reversible modifications, the soldering iron works quite nicely with this air source. I recommend Howard Electronics as a source for microscopes and soldering supplies.


  4. Stainless steel shim pin lifter. The quickest and least damage-prone method of unsoldering large surface mount integrated circuits is to use a stainless steel shim in an exacto-knife handle. Carefully slide the shim under the pins of the IC while heating with the hot air soldering iron, and they pop right off. I have desoldered many .5mm pitch QFP parts using this technique, and have been able to reuse the parts and have not damaged any of the pads on the board. This method is also quite rapid. A 48 pin part might be removed in as little as 30 seconds. And it doesn't require expensive, complicated special purpose hot air nozzles. This technique is championed by Edsyn, who include a roll of stainless shim stock with their soldering irons. I've not seen this method described anywhere else.

  5. Fine tipped soldering iron. I use a contact (traditional) soldering iron for most initial board building. A well-tinned very narrow tip is a must. Any manufacturer's iron will do but the smaller and lighter hand pieces are very nice for big jobs.  You might try the Edsyn 951SX:

  6. Flux pin. After swapping many components to find the correct values, it's inevitable that the solder on the board will become grainy and laced with impurities. I like to use a Kester low-solids flux pin to clean the board up and make better solder joints. With one of these pins you simply touch the contaminated solder and gently reheat the area with the hot air soldering iron (being careful not to blow parts away), and the solder gets bright and shiny as if by magic. Better still, with a low-solids pin, you don't even have to wash the flux off afterwards.

    Several years ago, many of the no-clean fluxes were useful only for digital circuits since they had relatively high conductivity and would interfere with the operation of any high impedence analog circuits. With the new low-solids, no-clean fluxes that are out now, I have not run across this problem.

In conclusion, I have found that the proper tools make prototyping with surface mount components easier and faster than any of the traditional prototype methods. Certainly it is no longer an onerous task to be avoided at any cost!


This article is copyright © 2000 by Jim Pearce,
Pegasus Technologies. All rights reserved.


 

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