On Date: 97-01-14 15:14:51 EST
You asked: Please tell me more about electrolytic plating propogation rate
testers.
Propogation Rate Testers are printed circuit board simulations of the ability
of acid copper to form full copper eyelets in circuit board holes processed
through direct metallization.
Please understand that the electroless copper users have always been
accustomed to plating unclad laminate rate panels to simulate the electroless
copper deposits in PWB holes. In the 1980's, the concept of backlighting the
holes in rate coupons was extended to processing the holes through several
pattern plate clean/microetch cycles to give a measure of the assurance that
perfect, void free holes would come from the copper electroplate line. Thus,
shops today run two or three pattern preplate cycles on the holes, cross
section, backlight, and give a numerical reading to the % coverage of copper
in the holes (before copper electroplate.) Various of us electroless copper
vendors gave our customers rating scales - some 1 to 10 where 10 is perfect
and a rating of 6 or 7 after three preplate cycles will still give void free
final product. The beauty of backlights is that a decline of the reading
from 9 to 7 is quite detectable, but void free product will still be produced
while the reason for the decline is corrected. Backlights are frequently
charted and a number of shops run SPC on backlights.
ALONG COMES DIRECT METALLIZATION - NO COPPER IN THE HOLE UNTIL THE BOARD
COMES FROM PATTERN PLATING, AND REWORK IS NEARLY IMPOSSIBLE!!
Propagation rate testers are patterns on prepared circuit boards that show
how well the acid copper will "knit" or "cover" the hole wall in the
electroplate bath. They are also a chartable SPC type of reading so that
drift in process latitude is detected well before voided circuit boards are
produced.
In the oldest MacDermid implementation, a "grid pattern" was etched on single
sided laminate and a hull cell size coupon cut out. The grid lines at first
simulated laminate thicknesses - .031, .062, .093, etc up to .250. The
coupons were passed down the direct metallization line/through the machine.
Coupons were sometimes run through dry film development and always through
the pattern preplate. A copper electroplating bath sample was put in the
hull cell and the plating done for a fixed time such as 5 or 10 minutes.
Rectangles on the grid which "knitted" over were counted and recorded for
SPC. Plating differences between different copper cells on the electroplate
line was obvious. Sometimes, a contaminant in the acid copper is current
density specific and shows up nicely in the Hull Cell.
However, since circuit board fabricators plate holes, not surfaces, a better
test vehicle was needed. The embodiment today uses a series of daisy chain
patterns etched on a two sided board. Drill hole sizes are varied to again
look for aspect ratio problems, dry on contamination, poor acid copper
solution movement, etc. The coupon parts are run through the direct
metallization machine, through the dry film developer, and a standard
electroplate cycle. The number of holes propagated during the normal
electroplate time is easily counted. Again, the number is recorded for SPC
analysis.
The whole effort is to determine a process drift before scrap product is
produced. A useful paper is "Developing Reliable Process Control Methods for
BlackholeTM Direct Metallization Technology" Presented at IPC Expo in 1995
by Tim Spencer. I can send copies if I get your mail/fax address. Maybe I
even have a software version here - we're getting into the electronics age.
Let me know.
Denny Fritz
MacDermid, Inc
Waterbury, CT 06702
203-575-5740
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