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8/14/2019 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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ELECTRONICS FOR YOU NOVEMBER 2000
D O - I T - Y O U R S E L F
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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ELECTRONICS FOR YOU NOVEMBER 2000
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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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ELECTRONICS FOR YOU NOVEMBER 2000
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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
Recommended