Homemade PCBs

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ELECTRONICS FOR YOU NOVEMBER 2000

D O - I T - Y O U R S E L F

37

Here is a guide to produce high-

qua lity PCBs quickly and effi-

ciently, pa rt icular ly for profes-

sional prototyping of production boards.

Unlike most other P CB homebrew guides,

emph asis is placed on qua lity, speed, and

repeatability rather than m inimum ma-

terials cost, although t he time sa ved by

getting good PCBs every time usually

saves money in t he long run.

With th e meth od described here, you

can produ ce repea tedly good single- and

double-sided PCBs for thr ough-hole and

surface-mount designs with track densi-

ties of 40 to 50 tracks per inch and 0.5mm

SM pitches. Only photograp hic met hods

have been dwelt in depth because other

methods such as transfers, plotting on

copper, and iron-on toner transfer are

not really suited for fast , repeata ble use.

The problem with toner tra nsfer sys-

tems is t hat the expensive part is the

film, and you can t r eally feed much less

tha n an A5 sheet th rough a laser printer,

so you was te a lot on sm all PCBs. With

photoresist laminate and cheap tr ans-

paren cy media, you only use as much of

the expensive part (the board) as youneed, and offcuts can usually be used

later for small boards.

Artwork generation

You n eed to genera te a positive (copper

black) UV translucent artwork film. You

will never get a good board without good

art work, so it is important to get t he best

possible quality at t his sta ge. The most

importan t t hing is to get a c lear sh arp

image with a very solid opaque black.

Nowadays, artwork is drawn using

either a dedicated PCB CAD program

or a suita ble drawing/graphics package.

It is absolutely essential t hat your PCB

software prints holes in the middle of

pads, which will act as centre marks

when drilling. It is virtu ally impossible

to accura tely han d-drill boards without

these holes.

If youre looking to buy PCB soft-

ware at any cost level and want to do

hand-prototyping of boards before pro-

duction, check th at t his facility is avail-

able. If youre using a general-purpose

CAD or graphics package, define pads

as eith er a grouped object containing a

black-filled circle with a smaller con-

centric white-filled circle on top of it,

or as an un filled circle with a th ick black

line (i.e. a black ring).

When defining pad a nd line sha pes,

the minimum size recommended for vias

(through-linking holes) for reliable re-

sults is 50 mil, assuming 0.8mm drill

size; 1 mil = (1/1000)th of an inch. You

can go smaller with sm aller drill sizes,

but through-linking will be harder .

65mil round or square pads for norma l

components and DIL ICs, with 0.8mm

hole, will allow a 12.5 mil, down to 10

mil if you r eally need t o. Cent re-to-cen-

tre spacing of 12.5mil tr acks should be

25 milslightly less may be possible if

your printer can manage it. Take care

to preserve th e correct diagonal tr ack-

tra ck spacing on m itered corners; grid

is 25 mil and tr ack width 12.5 mil.

The ar twork must be pr inted such

that the pr int ed side is in contact with

th e PCB sur face when exposing, to avoid

blurred edges. In pra ctice, this means tha t

if you design th e board a s seen from the

component side, the bott om (solder side)

layer sh ould be pr inted th e correct way

round, an d th e top side of a double-sided

board must be printed mirrored.

Media

Artwork quality is very dependent on

both the output device and the media

used. It is not necessary to use a t ran s-

parent artwork mediumas long as it

is reasonably tra nslucent t o UV, its fine-less tra nslucent mater ia ls may n eed a

slightly longer exposur e tim e. Line defi-

nition, black opaqueness, a nd toner/ink

retention are much more importa nt.

Tracing paper ha s good enough UV

translucency and is nearly as good as

drafting film for toner reten tion. It sta ys

flatter under laser-printer heat than poly-

ester or acetate film. Get the thickest

you can find a s t hinn er st uff can crickle.

It should be rat ed at least 90 gsm; 120

HOW TO MAKE GOOD

HOMEMADE PCBs?INDRANI BOSE

D o n o t u s e s o d i u m h y d r o x i d e f o r d e v e l o p i n g p h o t o r e s i s t l a m i n a t e s . I t i s c o m p l e t e l y a d r e a d f u l s t u f f

f o r d e v e l o p i n g P C B s . A p a r t f r o m i t s c a u s t i c i t y , i t i s v e r y s e n s i t i v e t o b o t h t e m p e r a t u r e a n d

c o n c e n t r a t i o n , a n d m a d e - u p s o l u t i o n d o e s n t l a s t l o n g


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38

gsm is even better but h arder t o find. It

is cheap an d eas ily available from office

or art suppliers.

Output devices

Laser printers offer the best all-round

solut ion. These are affordable, fast, and

good-quality. The printer u sed mu st h ave

at least 600dpi resolution for all but t he

simplest PCBs, as you will usually be

working in m ultiples of 0.06cm (40 tracks

per in ch). 600 dpi divides in to 40, so you

get consistent spacing and line width.

It is very importan t tha t the printer

produces a good solid black with n o toner

pinholes. If youre planning to buy a

print er for PCB use, do some test print s

on tra cing paper to check the qu ality first.

If the print er ha s a den sity contr ol, set it

to the blackest. Even the best laser print-

ers d ont generally cover lar ge ar eas well,

but u sually th is isnt a problem a s long

as fine tra cks are solid.

When using tr acing paper or dra fting

film, always use man ual paper feed and

set the stra ightest possible paper out put

path to keep the art work as flat as pos-

sible and m inimise jamming. For small

PCBs, you can us ually save paper by cut-

ting th e sheet in h alf. You m ay need to

specify a vert ical offset in your PCB soft-

ware to make it print on the right par t of

th e page. Some laser print ers ha ve poor

dimensional a ccura cy, which can cause

problems for lar ge PCBs. But as long as

any err or is linear , it can be compens ated

by scaling the pr intout in software.

Print accura cy is likely to be a no-

ticeable problem when it causes mis-

alignment of th e sides on double-sided

PCBsthis can usua lly be avoided by

careful arr angement of the plots on the

page to ensure th e error is the sam e on

both layers; for example , choosing

whether to mirror h orizontally or verti-

cally when reversing the top-side art-

work.

Photoresist PCB laminates

Always use good-quality, pr e-coated pho-

toresist fibreglass (FR4) board. Check

carefully for scratches in the protective

covering and on th e sur face after peeling

off th e coverin g. You dont need dar kr oom

or subdued lighting when handling boards,

as long as you avoid direct sunlight,

minimise un necessary exposure, and de-

velop imm ediately after UV exposure.

Inst agraph ic Microtra k board devel-

ops rea lly quickly, gives excellent resolu-

tion, and is available in thin (0.8mm ) and

heavy copper flavours. On using s pray-on

photoresist, you will always get dust set-

tling on th e wet resist. So it is not recom-

mended u nless you have access to a very

clean area or drying oven, or you only

want to make low-resolution PCBs.

Exposure

The photoresist board needs to be ex-

posed to UV light th rough the a rtwork,

usin g a UV exposure box. UV exposure

units can easily be made using stan-

dard fluorescent lamp ballasts and UV

tubes. For small PCBs, two or four 8-

watt, 30.5cm tubes will be adequate. For

larger (A3) units, four 38cm tubes are

ideal. To determine the tube-to-glass

spacing, place a sheet of tracing paper

on the glass and adjust the distance to

get the most even light level over the

sur face of th e paper.

Even illumination is a lot easier to

obtain with 4-tube units . The UV tubes

you need are sold as replacements for

UV exposure units, black light tubes

for disco lighting, etc. These look white,

occasionally black/blue when off, and

light u p with a light pu rple. Do not use

s h o r tw a v e U V l a mp s l i k e E P R O M

eraser tu bes and germicidal lamps tha t

have clear glass, because these emit

shortwave UV which can cause eye and

skin damage.

A timer that switches off the UV

lamps automatically is essential, and

should allow exposur e times from 2 to 10

minu tes in 15- to 30-second increm ents .

I t is useful if the t imer has an a udible

indication when the timing period has

completed. A timer from a scrap m icro-

wave oven would be ideal.

Use glass sheet ra th er than plast ic

for t he t op of the UV un it, as it will flex

less and be less prone to scratches. A

combined u nit, with switchable UV an dwhite tu bes, doubles as an exposure u nit

and a light-box for lining up double-

sided ar twork s. If you do a lot of double-

sided PCBs, it m ay be worth mak ing a

double-sided exposure unit, where the

PCB can be sandwitched between two

light sources to expose both sides si-

multaneously.

To find th e required exposur e time

for a pa r t icular UV unit a nd laminat e

type, expose a test piece in 30-second

increments from 2 to 8 minutes, develop,

and use the time which gave the best

image. Generally speaking, overexpo-

sure is better t han u nderexposure.

For a single-sided PCB, place the

ar twork s toner s ide up on th e UV box

glass, peel off the protective film from the

lamin at e, and place its sensitive side down

on top of the artwork. The laminate m ust

be pressed firmly down to ensure good

cont act all over the a rtwork.

To expose double-sided PCBs, prin t

the solder-side artwork as normal and t he

componen t side mirr ored. Place the two

sheets t ogether with th e toner sides fac-

ing, and carefully line them up, checking

all over th e board ar ea for corr ect a lign-

ment, using the holes in the pads as a

guide. A light box is very handy here,

but exposur e can a lso be done with d ay-

light by holding the sheets on the sur-

face of a win dow.

If printing errors ha ve caus ed slight

mis-registrat ion, align th e sheets t o av-

erage th e errors across the whole PCB,

to avoid breaking pad edges or tracks

when dr illing. When th ese are correctly

aligned, staple the sheets together on

two opposite sides, about 10 mm from

the edge of the board, forming a sleeve

or envelope. The gap between th e board

edge and staples is important to stop

the paper distorting at the edge. Use

the sm allest stapler you can find, so that

th e thickness of th e staple is not much

more than tha t of the PCB.

Expose each side, covering up the

top side with a reasonably light-proof

soft cover wh en exposing the un derside.

Be very careful when tu rning t he board

over, to avoid th e lamina te slipping in-side the ar twork and ru ining the a l ign-

ment . After exposure, you can u sually

see a fa int image of the pattern in the

photosensitive layer.

Developing

Do not use sodium hydr oxide for devel-

oping photoresist lamin at es. It is a com-

pletely and u tterly dr eadful stuff for de-

veloping PCBs. Apart from its caustic-


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ity, it is very sensitive

to bo th tempera ture

an d concent rat ion, and

m a d e - u p s o l u t i o n

doesnt last long. When

its t oo weak it doesnt

develop at a ll, and when

too strong it strips all

the resist off. It is al-

most im possible to get

reliable a nd consistent

result s, especially when mak ing PCBs in

an environment with large temperature

variations.

A much better developer is a sili-

cate-based product th at comes as a liq-

uid concentra te. You can leave the board

in it for several t imes th e normal devel-

oping time without n oticeable degrada-

t ion. This a lso means t hat i t is not tem-

pera tu re criticalno risk of str ipping at

warm er temperat ures. Made-up solution

also has a very long shelf-life and lasts

unt il its used up . You can m ake th e

solution up really strong for very fast

developing. The recommended mix is 1

part developer to 9 parts wa ter.

You can check for correct develop-

ment by dipping the board in th e ferric

chloride very br ieflythe exposed cop-

per should turn dull pink a lmost in-

sta nt ly. If any s hiny copper-coloured ar -

eas rem ain, rinse a nd develop for a few

more seconds. If the board is under-ex-

posed, you will get a th in layer of resist

which isnt removed by the developer.

You can remove this by gently wiping

with dry paper towel, without damag-

ing the pattern. You can either use a

photographic developing tray or a ver-

tical ta nk for developing.

Etching

Ferr ic chloride etchan t is a mess y stuff,

but easily available and cheaper than

most a l ternatives. I t a t tacks any metal

including stainless steel. So when set-

ting up a PCB etching area, use a plas-tic or ceram ic sink, with plast ic fittings

and screws wherever possible, and seal

any meta l screws with silicone. Copper

w a te r p ip e s ma y g e t s p l a s h e d o r

dripped-on, so sleeve or cover them in

plastic; heat-shrink sleeving is great if

youre installing new pipes. Fume ex-

traction is not normally required, al-

though a cover over the tank or tray

when n ot in use is a good idea.

You should always use th e hexahy-

drate type of ferric chloride, which

should be dissolved in warm water u n-

til sat ura tion. Adding a teaspoon of table

salt helps to make the etchant clearer

for easier in spection.

Avoid anh ydrous ferric chloride. It cre-

at es a lot of hea t wh en dissolved. So al-

ways add th e powder very slowly to wa-

ter; do not add water to the powder,

and use gloves and safety glasses. The

solution made from anhydrous ferric

chloride doesnt et ch at all, so you n eed

to add a small amount of hydrochloric

acid and leave it for a d ay or two.

Always take extreme care to avoid

splashing when dissolving either type

of ferric chloride, as it tends to clump

together and you often get big chunks

coming out of the container a nd splash -

ing into th e solution. It can dama ge eyes

and permanent ly stain clothing.

If youre making PCBs in a profes-

s iona l envi ronment , where t ime i s

money, you should get a heat ed bubble-

etch tank. With fresh hot ferric chlo-

ride, a PCB will etch in well un der five

minutes. Fast etching produces better

edge-quality an d consisten t line widths.

If you arent using a bubble tank, you

need to agitate frequently to ensure

even etching. Warm the etchant by put-

ting the etching tray inside a larger tra y

filled with boiling wat er.

Tin platingTin-platin g a PCB mak es it a lot easier

to solder, and is pretty much essential

for surface mount boards. Unless you

ha ve access to a roller tin ning ma chine,

chemical tinnin g is the only option. Un-

fortunately, tin-plating chemicals are ex-

pens ive bu t the r e su l t s a re usua l ly

worth it.

If you dont tin-plate the board, ei-

ther leave the photoresist coating on

(most r esists are int ended to act as sol-

dering fluxes) or spr ay th e board with

rework flux to prevent the copper fromoxidising.

Room-temperat ure tin-plating crystals

produce a good finish in a few minu tes.

There are other t inning chemicals avail-

able, some of which requir e mixing with

acid or high-temperature use.

Ensur e that the temperatu re of the

tinning solution is at least 25oC, but not

more than 40 oC. If required, either put

the bott le in a hot water bath or put

the tinning tray in a bigger tray filled

with hot water to warm it up. Putt ing a

PCB in cold tinning solution will usu-

ally prevent tinning, even if the tem-

peratur e is subsequently raised.

For a good tinned finish, strip the

photoresist thoroughly. Although you

can get special stripping solutions a nd

hand applicators, most resists can be

dissolved off more easily and cleanly us-

ing meth anol (methylated spirit) . Hold

the rinsed and dried PCB horizontal,

and dribble few drops of methanol on

th e surface, tilting the P CB to allow it

to run over t he whole surface. Wait for

about ten seconds and wipe off with a

paper towel dipped in methanol.

Rub th e copper su rface all over with

wire wool until it is bright and shiny.

Wipe with a pap er towel to remove the

wire wool fragments and immediately

immerse the board in t he t inning solu-

tion. Dont t ouch th e copper s ur face af-

ter cleaning, as finger marks will im-

pair plat ing. The copper should turn sil-

ver in colour with in about 30 seconds.

Leave the board for about five minutes,

agitating occasionally; do not use bubble

agitation. For double-sided PCBs, prop

the PCB at an angle to ensur e the solu-

tion gets to both sides.

Rinse the board t horoughly and ru b

dry with paper towel to remove any tin-

ning crystal deposits. If the board isnt

going to be soldered for a day or two,

coat it with either a rework flux spray

or a flux pen.

Drilling

If you h ave fibreglass (FR4) boar d, you

must use tungsten carbide drill bits.

Fibreglass eats normal h igh-speed steel

(HSS) bits very ra pidly, although HSS

drills are alright for odd larger sizes

(>2 mm ). Carbide drill bits a re expen-

sive and th e thin ones snap very easily.

When using carbide drill bits below 1

mm, you mu st u se a good vertical drill

sta ndyou will break dr ill very quicklywithout one.

Carbide drill bits are available as

stra ight-shank or thick (sometimes called

tur bo) shank. In stra ight sha nk, th e

whole bit is th e diamet er of th e hole, an d

in thick shank, a standard-size (typi-

cally about 3.5 mm) shank tapers down

to the hole size. The straight-shank

drills are usu ally preferred because they

b r e a k l e s s e a s i l y a n d a r e u s u a l l y

cheaper. The longer thin section pro-


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40

vides m ore flexibility.

Small drills for PCB use usually

come wit h eit her a set of collets of vari-

ous sizes or a 3-jaw chuck. Sometimes

th e 3-jaw chu ck is an optional extra an d

is worth getting for t he t ime it sa ves on

changing collets. For accuracy, however,

3-jaw chucks arent brilliant , and small

drill sizes below 1 mm quickly form

grooves in the jaws, preventing good

grip. Below 1 mm, you should use

collets, and bu y a few extra of th e sma ll-

est ones, keeping one collet per drill

size, as using a larger drill in a collet

will open it out and it no longer grips

sma ller drills well.

You need a good strong light on the

board when d rilling, to ensur e accura cy.

A dichroic halogen lamp, u nder-run at

9V to reduce brightness, can be m ounted

on a microphone gooseneck for ea sy po-

sitioning. It can be useful to raise the

working su rface about 15 cm above the

normal desk height for more comfort-

able viewing. Dust extraction is nice,

but not essentialan occasional blow

does the trick! A foot-pedal control to

swit ch t he drill off an d on is ver y con-

venient, especia lly when frequently

changing bits. Avoid hole sizes less than

0.8 mm unless you r eally need them.

When ma king two identical boards,

drill them both together to save time.

To do this, carefully drill a 0.8mm hole

in the pad near each corner of each of

the two boards, getting the centre as

accura te a s possible. For larger boards,

drill a hole near t he centre of each side

as well. Lay the boards on top of each

other and inser t a 0.8mm t rack pin in

two opposite corners, using t he pins as

pegs to line the PCBs up. Squeeze or

hamm er the pins into the boards, and

then in to the rema ining holes. The two

PCBs are now nailed together accu-

rately an d can be drilled together.

Cutting

A small guillotine is t he easiest wa y to

cut fibreglass laminat e. Ordinar y saws

(bandsaws, jigsaws, and hacksaws) will

be blunted quickly unless these are car-

bide-tipped, and t he dust can cause sink

irrita tion. A carbide t ile-saw blade in a

jigsaw might be worth a try. Its also

easy to accidentally scrat ch thr ough the

protective film when sawing, causing

photoresist scratches an d broken tra cks

on the finished board. A sheet -meta l guil-

lotine is also excellent for cut tin g boar ds,

provided th e blade is fairly sharp.

To make cut-outs, drill a series of

small holes, punch out the blank, and

file to size. Alterna tively, use a fretsa w

or small hacksaw, but be prepared to

replace blades often. With practice its

possible to do corner cutout s with a guil-

lotine but you have to be very careful

th at you d ont over-cut !

Through-plating

When laying out double-sided boards,

give some thought to how top connec-

tions will be made. Some component s,

such as resistors and unsocketed ICs,are m uch easier to top-solder th an oth-

ers (radial capacitors). So, try to make

the top conn ection to t he ea sier compo-

nent . For socketed ICs, use tu rned-pin

sockets, preferably the ones with t hick

pin section under the socket body. Lift

th e socket s lightly off the board , solder a

couple of pins on th e solder side to ta ck

it in place, and adjust so th at t he socket

is str aight. Solder all th e solder-side pins,

and then the required top-side pins by

reheat ing the joint on t he solder side.

For vias, holes which link sides with -

out components, use 0.8mm snap-off

linking pins. These are much quickerthan using pieces of wire. Just insert

the bottom of the stick into the hole

and bend over to snap off the bottom

pin. Repeat t he pr ocess for other h oles

and t hen solder both sides.

If you need proper through-plated

holesfor example, to connect to inac-

cessible top-side pins, or for und ernea th

surface-mount devicesMulticores

Copperset system works well, but the

kit is very expensive. It uses ba il bars

consist ing of a r od of solder, with a cop-

per sleeve plated on the outside. The

sleeve is scored at 1.6mm int ervals, cor-

responding to the PCB thickness. The

bar is inserted into the hole using a

special applicat or and bent over to snap

off the single bail in t he hole. It is th en

pun ched with a m odified aut omatic cen-

tre-punch, which causes the solder to

spray over the en ds of the plated sleeve

and also pushes the sleeve against the

side of the hole. The pads are soldered

on side to join the sleeve to the pads,

and then the solder is removed with

bra id or a solder sucker to leave a clear

plated hole.

Fortu nat ely, it is possible to use th is

system for platin g standa rd 0.8mm holes

without bu ying the full kit. You can buy

the bail bars separately as refills. For

the a pplicator , use a 0.9mm a utomatic

pencil that works much better than the

original a pplicator, as you get one bail

for every press of the bu tton. Get a small

automatic centre-punch an d gr ind th e

tip off to make it completely flatthis

works fine for pun ching the ba ils. For an

anvil, use a thick flat piece of metal. Plat e

all the holes before fitting any compo-

nent s so that the bottom sur face is com-pletely flat. Holes must be drilled with a

sharp 0.85mm carbide drill to get the

hole size right for th e platin g process.

Note tha t i f your PCB package

draws pa d holes of th e same size as th at

of drill, the pad hole can come out

slightly larger than the drilled hole,

causing connection problems with the

plating. Ideally, the pad holes should

be about 0.5 mm, r egardless of hole size,

to make an accura te centre ma rk. t

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Homemade PCBs