Jth Edition Of The Famous

Radio Handbook

Tells how to design, build, and operate the latest types of amateur transmitters, receivers, transceivers, and amplifiers. Provides extensive, simplified theory on practically every phase of radio. Broadest coverage; all original data, up-to-date, complete. 848 pg. Order EE-167, only $12.95

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Fig, L The *sandwich1 as seen from the side.

center it. Over this, place a piece of clean glass which will keep the negative pressed against the surface of the copper. You now must pick up this 'sandwich* and walk outside with it. Aim the glass side at the sun and hold steady for one minute. The sun should not be obscurred by clouds or be low in the sky when this is done. After exposing, return indoors and immediately place the copper clad board in a shallow, glass dish of Trichloroethylene (sold in electronics parts stores as a cleaning solvent) being sure the entire board is covered with solution. Gently agitate the dish for one minute then remove the board and rinse it under running water for about 30 seconds. Allowing the light to reflect off the board should reveal the impressions of the resist still left on the board. Check for any imperfections at this time. If some exist, the board must be cleaned and the printing process repeated.

The board is now ready for etching. Ferric Chloride is the etchant used arid is obtainable from chemical supply houses in crystalline form. Varying size tubes can be bought and the size will be determined by the project This is by far the cheapest way to purchase the ferric chloride although it can be had in solution from the local drug

PHOTO RESIST ^COPPER

PHOTO RESIST ^COPPER

Fig- 2. After etching, a cross section of the printed circuit board will look like this. Copper will remain where there is a coating of photo resistl„

Fig- 2. After etching, a cross section of the printed circuit board will look like this. Copper will remain where there is a coating of photo resistl„

store* The crystals which you get are to be mixed in water. The ratio is 8 pounds per gallon of water. Mix until all the crystals are dissolved in the water. Do not use metal containers or utensils with the etchant. You will now find that by dunking a prepared board into the solution it will etch in a couple of hours. There are two methods which will aid in reducing this time to 15 minutes, First of

Submerge board in trichloretkalene developer and agitate for one minute. Immediately rinse in water.

all the chemical reaction which is taking place is enhanced by heat. By elevating the temperature of the mixture to 120° Fahrenheit. a great reduction in time will be noted, [ would recommend doing this by setting the container of etchant inside a container of water placed on a small hotplate. This allows better control, adds a smai bit of safety and allows you to monitor the water temperature instead of the etchant.

Oxygen is also an aid to a faster reaction. A small bubbler with a hose going to the etching tank will speed things up considerably, By suspending the board upside down at the surface of the solution an excellent cutting action is produced as the liquid splashes against the copper. An aquarium type bubbler might be ideal for this and would cost verv little.

A few additional hints at this time might prevent some unpleasant experiences. From the time the Kodak Photo Resist is put on the board until the etching is completed it would he wise not to touch the surface ol the board, if the layer of resist is scratched it will allow the copper to etch where it is removed. Keep your working areas clean, especially the steel wool section, Those nasty little fibers polute chemicals very easily. Avoid inhaling the fumes from the developer (Trichloroethylene) and the etchant (Ferric Chloride), Both are toxic. Keep the developer covered when not in use as it evaporates quickly. While drying the photo resist coating after application store the board in a warm, dark, dust-free place. The dust particles will print as hairlines oil the board. Change developer periodically by observing particles ol contamination floating in it. ( iiange etchant solution when the etch time

The finished product with components mounted.

has increased to twice its original value. Never mix new and used chemicals together. Attempts to restore contaminated chemicals result only in greater quantities of pointed chemicals than you started with.

This is the whole printed circuit process in a nutshell I have used this system with great success on many different projects and am happy to report that it is much easier to do than to say. Below is a list of possible problems which might be encountered in the various stages of production along with their solutions. Also you will find a list of materials fur the layout and printing oi boards.

Problems

All the photo resist comes off in the developer. No print left on board after w ashing in water.

None of the photo resist comes off the board in developer. No print left on board after washing.

Resist turns gummy in developer and peels off board.

Print on board after washing appears and disappears in various places. When held to light print is difficult to see.

Causes

Probably caused by in ider exposure to the light. Also can be caused by improper drying of resist before exposure.

Too much light in room during preparation of board for exposing. Never leave cap off photo resist bottle. Developer bad, Increase developing time,

Coating of resist too thick or not sufficiently dried before continuing.

Coating of resist is too thin. Exposure to light too short. Left in developer too long. Washed board too strenuously or used

Board will not etch at all or just in places leaving big splotches of copper.

Print fuzzy on edges of conductors before etching.

Print fuzzy on edges after etching.

Kntire board etches no copper.

hot water. Water should be cool,

Unexposed resist was not removed by developer (see above). Board not steel wooled enough,

Film negative not held tight against the board during exposure.

Left board in etchant too long. Not left in long enough,

Coating of resist was applied too thin. Too much light allowed to strike board prior to exposure.

List of materials for layout

Quantity

Item

Source

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