Design For The Envirorirnerit Project
                Releases Direct IMetallization
                         Performance  Resiolts
                      By BiH Birch, Susan Wlansilla, Katlty Hart and Deborah Bogef
          he U.S. Environmental Protection Agency
          (EPA) Design  for the Environment (DfE)
          Program has been working closely with the
          Institute for Interconnecting and Packaging
          Electronic Circuits (IPC), its member compa-
          nies and other interested parties to conduct a
          comparative study of the performance  of
          alternatives to the electroless copper process
          for making PWB holes conductive. PWB pap-
          els designed to represent industry "middle-ot-
          the-road" technology' were manufactured at
          one facility, run dirough individual "making
          holes conductive- (MHC) lines at 25 facih-
          ties, then electroplated at one facility. The
           panels were electrically pre-screened, fol-
           lowed by electrical stress testing and ffltfchan-
           ieal testing, in order to identify variability m
           performance of the MHC process. The test
           methods used to evaluate performance were
intended to indicate characteristics of a tech-
nology's performance', not to define parame-
ters of performance or to substitute for thor-
ough .on-site testing. This study was.ihtended
to be a "snapshot" of the technologies. This
' study was carried out as part of. the U.S. EPA-
IPC DfE PWB Project and was, conducted with
extensive input and participation from PWB
manufacturers, their suppliers, and PWB test-
ing laboratories,                      i
   The studv is now complete and the results
 suggest that-direct metallization (DM).techr
 nologies perform at least as well as electroless
 copper if operated according to specifica-
   The  electroless  copper  through-hole
  process is die technology most commonly
152' CIJCIITBEE •  J»««»IT •

 .v ;.  ' •:..•    ,.,.„:• i'UB  r.i-H
 . "IlilIU ".'.'' IJIl'M  '.) df( U'»K tie
 riu-.   r«-iinoli>irv  upicallv  einplovs
 tormaldehuir  as  a  copper-reducing
 agent, requires large  amounts of water
 and energy and is a significant source of
 hazardous waste. There are alternative
 technologies available,  most of which
 eliminate  the  use  of formaldehyde,
 reduce water and energy use and gener-
 ate less waste.
   The potential environmental and cost
 advantages of the alternatives are begin-
 ning  to become -apparent and have gen-
 erated strong interest on  the  part  of
 industry. However, minimal product per-
 formance information is publicly avail-
 able on these technologies. The project
 was conducted to address this data gap.-
 The technologies tested include electro-
 less copper, carbon, graphite, palladium,
 non-formaldehyde electroless and  con-
 ductive polymer.
   The  resulting information will be
 included  in a document titled Cleaner
 Technologies Substitutes  Assessment
 (CTSA), to be completed by early 1997.
•The CTSA will allow  business decision-
 makers to evaluate the alternatives  in
 terms of human health, environment,
 cost and performance.
   One hundred 8 layer test panels mea-
 suring 24"  x 18",  laminated to .062"
 thickness, were produced from B and C
 stage FR4 materials. Each test panel con-
 tained 54 test coupons consisting of 27
 Interconnect Stress Test  (1ST) coupons
 (used for electrical testing) and 27
 microsection coupons,  1ST  coupons
 measure 6.5" x 3/4" and contain  700
 interconnecting vias on a 7  row  by 100
 via 0.050" grid. This coupon contains two
 independent circuits: a post  circuit
 which contains 200 interconnects, used
 to measure post interconnect resistance
 degradation, and a PTH circuit  which
 contains 500 interconnects, used to mea-
 sure  PTH  (barrel)  interconnect resis-
 tance degradation: 1ST coupons  had
 either .013" or .018" holes (finished).
   The microsection coupon measures 2"
 x 2" and contains 100 interconnected
 vias on a 10 row by 10 via 0.100"  grid. It
 has internal pads, at the second an4 sev-
 enth layer and a daisy chain'interconnect
 between the wo surfaces of the coupon •
 through the via. Microsection coupons
 had either 0.013", 0.018"  or 0.036" holes
   For the purposes of this evaluation,
 the MHC process was defined as every-
 thing from the end of the desmear step
 through 0.1 mil of copper flash plating.
 156 • CUCIITHE • JIII1IT •  1117
          The potential
      environmental  and
      cost advantages of
        the alternatives
        are beginning to
       become apparent
In order to minimize differences in per-
formance due to processes outside this
defined MHC function, the panels used
for testing  were all manufactured and
drilled at one facility.
  After drilling, three panels were sealed
in a  plastic  bag with  desiccant  and
shipped to each*of 25 test sites to be
processed through the site's MHC line:.
All bags containing panels remained
sealed until the day of processing.
  The facilities that were used as test
sites were identified by suppliers' of the
technologies being tested and included
production  facilities, testing' facilities
(beta sites), and supplier testing facilities.
  An on-site observer was present at each
site to confirm that all  processing was
completed according to instructions and
to record  observations. Each test site's
process was completed within one day; it
took slightly longer  than one month to
complete all MHC processing.
  After the MHC processing, the panels
were put into sealed bags with desiccant
and shipped to a single facility, where
they remained until all the panels were
collected. At this facility the panels were
electroplated with 1.0 mil of copper fol-
lowed by a tin-lead etch, resist,  etched,
stripped of tin-lead, solder mask coated
and finished with hot air solder leveling
   After HASL, the microsection coupons
were routed out of the panels and sent to
Robisan Laboratory Inc. for mechanical
testing. The  1ST coupons were left in
panel format. The panels containing the
coupons were passed twice -through an
IR reflow to simulate assembly stress. The
panels with the 1ST coupons were then
sent. to Digital Equipment of Canada
  DEC i  for electrical pre^creehing ancL
 electrical testing.
   Electrical testing was completed using
 the 1ST technology. 1ST is an accelerated
 stress test method used for evaluating the
 failure   modes  of PWB  interconnect •
 which uses DC current to create  the
 required temperatures  within the inter-
 connect. The three principal types of
 information generated  from the 1ST are
 initial resistance variability, cycles to fail-
 ure  (barrel integrity, and  post separa-
 tion/degradation (post interconnect).
   Electrical pre-screening, used to deter-
 mine initial  resistance variability,  was
 completed on the two circuits in the  1ST
 coupons. The resistance value  for  the
 first internal circuit" (PTH circuit)  for
'each coupon was determined; it gives an
 indication  of the resistance variability
 (plating thickness)  between coupons
 and between panels. • ,
   The cycles to  failure indicate how
 much stress the individual coupons can
 withstand  before failing to function
 (measuring  barrel   integrity).  1ST
 coupons contain  a second  internal  cir-
 cuit (post circuit), used to monitor  the
 resistance degradation of the post inter-
 connect The level of electrical degrada-
 tion in conjunction with the number of
 cycles completed is used to determine
 the presence and level of post separa-
 tion.  The  relative performance  of  the
 internal  circuits  indicates which  of  the
 two internal circuits, the Post circuit or
 the PTH circuit, has the dominant fail-
 ure mechanism.
 .  For mechanical testing, the coupons
 were  sent  to Robisan  Laboratory  Inc.
 Mechanical testing'evaluated metallurgi-
 cal microsections of  plated  through
 holes in the as-produced condition and
 after thermal stress. One test coupon of
 each hole size, from each panel, was sec-
 tioned.  The coupons  were microsec-
 tipned to produce the  most inner-layer
 circuitry connections  in, the microsec-
 tions.                  •       .
   Microsections  were stressed per  IFC-
 TM-650, method 2.6.8. The  plated
 through holes were evaluated for com-
 pliance  to the requirements found in
 IPC-RB-276.  Microsections  were exam-
 ined after final polish, prior to metallur-
 gical microetch and after microetch.
   Coupon selection was four .013"  1ST
 coupons from each of  the 3 test panels
 from each test site. Test sites 3 and 4 had
 six coupons selected from the two avail-
 able panels. Three coupons from within
 six inches of die  1ST coupons selected
 were microsectioned  from  the same
 panels. In some cases, the desired micro-
 section coupons  exhibited  misregistra-

                        - :  ir-r ,.t>( att'ci
                        • i- '.He .panel as
 Testing Results     .
   This performance demonstration was
 designed as  a snapshot. Because the .test .
 sites were not chosen randomly; the sam-
 ples inav ncit be representative of all PWB
 manufacturing  facilities . in  the U.S.
 (although there is no specific reason to
 believe that  they are not representative).
 Also, the'number of-test sites for. each
 metallization- type  ranged  from  ten  to
 one. Due to the smaller  number of test
 sites for some  metallization, results  for
 these metallizations could more easily be
 due to chance than the results from met-
 allizations with more test sites.- Statistical
 relevance cannot be determined.

 Bectrtoal prerscreening
 - " Sevens-four, of seventy-five test panels
 from  twenty-  five  test, facilities were
 returned.  One of them  had  missing
 inner-layers. • •
    From the 73 remaining panels, .1971
 coupons  received, two resistance irieaf
 surements  using  a 4rwire  resistance
 meter. The total number of holes tested.
 was 1.4 million. One percent  (19) of the
 coupons were found  to  be defective
 because of opens and shorts and unac-
 ceptable for 1ST testing.
  ,  At inspection of die defective coupons,
  the opens  were found to be caused by
  voiding, usually within a single via, Shorfe
  were caused by misregistration.. The type
  of metallization did not contribute tonhe
.  shorts.- '•    .                 .UT .":.;....
     To permit resistance: distributipiKinleSr
  surements,  the  ' layers/panefe .iveice
  processed   in the  same  orientation
  throughout manufacturing. The jesis-.
  tance distribution  for the post cireuifcwas
  determined and proved very- consistent.
  This  result  confirms  that  inner-layer
 . printing and,etching did  not contribute
  to overall resistance variability.
      TaWe 1. Defective Coupons Found at Prescrsemng.
      Site   Metallization   Opens     Shorts
      1      Electroiess              4
      3"   Hectroiess^  V"  J ^2
      11~ _ Graphite     ~~2
      1T   "Graphite"              5
      14    Palladium  	1
        i-  __Palladium      2
      20""  Paiiadium"  "" 2	  "."
            Percentage of Panels
            Exhibiting Defect
    Percentage of Panels MMing Drfec*
  "Tier Technology (avg. of all test sites)
                                                      Drill Smr.   ResRec.    Post Sep.    Drill Smr.   ResRec.    Post Sep.
   The resistance-! distribution far, -the
 PTH  circuit was aBo- dejtennineS and
 showed die electFohtfis'jcbppfer plating:
- increased m thickness fromi^ to both  .
 torn of each panel.  Copper thickness
 variability-W3S-.calculated to be .Q0&3^
 thicker at the? bottom Compared? toVishe;
•top of eachj>anet Resistance \ariaBiEfe'
 was also found from right to left ore the
 panels indicating inconsistent drill regis-
 tration or outer-layer etching to be the
 most probable cause.
    Copper plating distribution  at each
 site  was   good.  Plating  cells  and
 rack/panel locations did not create large
 variability that could affect the results of
 each test site. Because resistance (plating
  diickness)  distribution was also consis-
  tent between test sites (Std, Dev. for
  mean resistance was 10.6 for all test
  sites), relative comparisons between met-
  allization sites can be made.
     It was  determined during correlation
  that the variations  in hole wall plating
  thickness  indicated  at electrical pre-
  screening was due to the flash plate pro-
  vided by each test site and not the elec-
  trolytic plating.
   The only defects  reported from the.
 micrbsection evaluation werp  voids in
 hole watt copper, "drill smear, resin reces-
 sion and inner-layer separation. Average
 hole wall diickness was also reported for
 each  panel. Defects present, but not
 'included as part of this report because
 they were not believed to be a function of
 the metallization technology, are regis-
. tration, inner-layer foil cracks and cracks
 in flash plating at the knees  of the holes.
 The inner-layer foil cracks appear to be
 the result  of the drilling operation and
 not a result of z-axis expansion or defec-
  live'foil. None of the cracks in the flash
  plating  extended  into  the electrolytic
  plate in the "as received"  coupons or
  after  thermal  stress.  Therefore,  the
  integrity of die hole wall was not affected
  by diese small cracks.
    There were no plating voids noted on
  any of the coupons evaluated. ,The elec-
  trolytic copper plating was continuous
  and'very even, with no indication of any
     Nailheading was present  on all panels,
  so it was  determined  that comparisons
 • were possible  since all test sites had
    158' CIHCUITREE •  JAHiMY • 1117

 •',   ,           »   .,,'  ti'.ii ••sr.tMi uiujlu
':r r\, t sN.'.i-.  . >• :,li,t:i'«l • 'u rai'h  site's
':vii::ui" pio
   Neither :i»[i-ii»un,i'i,;  :io.8^
                                                               Je&   Micro-
                                                               Site#  sectioH
1ST   affected
                                              ElSctoolessCSr •
                                                                                     Note: Y or N (yes or  no) denote
                                                                                   whether post separation was detected on
                                                                                   any coupon or panel from each test site.
                                                                                   The "panels, affected" column refers to
                                                                                   how many of the panels within each test
                                                                                   site exhibited post separation.
                                                                                     Test site 17 was the only site for which
                                                                                   post separation was found in the micro-
                                                                                   section, but not on 1ST.
                                                                This study was. a snapshot based on
                                                              products built with B and C stage FR4
                                                              materials and a specific board construc-
                                                              tion. The data can not necessarily be
                                                              extrapolated to other board materials or
                                                              constructions. However, the results indi-
                                                              cate that each MHC technology has the
                                                              capability to- achieve  comparable (or
                                                              superior)  levels of performance to elec-.
                                                              troless copper.
                                                                 Product performance for this study
                                                              was divided into two  functions:  PTH
                                                              cycles to failure and the integrity of the
                                                              bond between the internal lands (post),
                                                              and the PTH. The PTH  cycles to failure
                                                              observed  in this study is a function  of
                                                              both  electrolytic  plating  and  MHC
                                                              process.                 -   •
   162-  CllCHUm - J1IIJIT •  1117 .

                                                       [pcomirig Seminars
 i;lis;ill.    .   ,....
,mmsu%  It 'A.I- .ipp..u-nt ilut.ill MHC
tftinitil'lint's. :iu liuiui',' rlec troll's-; n>p-
|HM'. art- Mis.opubk- to thi-. 'MH' "' luil"
  The results of this stucK further sug-
gest tlut posi  separation ma\ occur 111
ditterem decrees. The level of post sep-
aration nun plav a role in determining-
product performance; however,  the
determination of levels of post separa-
 tion remain to  be discussed and con-
 firmed by the PWB industry.
   Fine line evaluations  in  microsec-
 tions have always been a point of con-
 tention within  the industry. Current
 microsection specifications  state  that
 anv indication  of separation between
 the hole wall plating and the inner-
 layer is sufficient grounds to reject the
 product. An indication of post separa-
 tion would be a black line on  the
 microsection thicker than  normally
 appears with electroless. copper tech-
 nology (normally 0.02-0.04 mils).
   Separation may also be determined
 bv a variation  in  the thickness of the
 line across the inner-layer connection,
 especialK  on electroless deposits  that
 are  very thin.  The rationale for these
  rejection criteria is that product  with
  post separation degrades with time and
  temperature cvcling.
    With traditional  electroless copper
  products where  post separation is pre-
  sent, it can  usually be  determined-
  where the separation occurs: between
   the electroless and foil, within the elec-
   troless or between the  electroless and
   the electrolytic  plating. This detenni-
 '  nation Often  helps in troubleshooting
   the plating process.,        .   .
                              PWB Project participants, includ-
                            ing IPC, industry, EPA, Microelec-
                            tronics and Computer Technology
                            Corporation, University of
                            Tennessee, and Silicon Valley
                            Toxics Coalition representatives, will
                            to present the results of the MHC
                            CTSA. Information on the health
                            and environmental risk, perfor-
                            mance and cost of the technologies
                            evaluated in the CTSA will be pre-
                            -.ented, along with guidance for
                            installing and implementing the
alternative technologies. The pre-
liminary schedule for .these one-day
seminars is as follows: ^January,
Orlando, FL, AESF/EPA Pollution
Prevention Conference; 10 March,
San Jose, CA, IPC Expo; 5 May,
Chicago, IL. Chicagoland Circuit
Board Association: <)-12June.
Boston. MA, Nepcon East: 15 May,
 Midwest Circuits Association; (5
-.October, Washington, J)C. IPC  . . .
 TechWorks: and  13 November.
 Phoenix, A/,, Ari/oiia Circuits
 Association meeting.
                            In this study, some of the direct met-
                          allizations resulted in no line at all after
                          microetching  the  microsections.  This
                          posed a problem  in  interpreung the
                          results. If traditional criteria were used,- •
                          accurate evaluations of "no lines" prod-
                          uct would not be possible. So, for. this
                          studv the criteria we used on "no line
                          products was: if tke sections exhibited
                          anv   line   of "-demarcation   after
                          microetch, the product was considered
                           to have inner-layer separation.
                             This issue is significant to die PWB
                           industry because there remains a ques-
                           tion about  the  relationship between
                           the appearance of a line on the micro-
                           section to the performance of a board.
                           Traditionally (with electroless copper
                           products), the appearance of a line
                           thicker than normal electroless  line is
                           considered  to be  post separation, and
For IVLore  Infbrxriatiori
,=„ „ _,rrh.rornDl«e    '  lei: 202-260-1023; fax: 202-2604)178:
        To obtain a copy of the complete
       technical paper for this study, includ-
       ing appendices, please contact Star
       Siimmerfield at IPC (Phone: 817-509-
       0700, ext, 347). For more information
       about the DfE Program or the DfE
       PWB Projecj^to obtain copies of docu-
       ments produced by the Project, or to
       he added to the project mailing list,
       contact EPA's Pollution Prevention
       Clearinghouse (PPIC), U.S. Environ-
       mental Protec-tion Agency, 401 "M" S
                            e-mail: You
                            may also obtain additional project
                            information by visiting thefPC

                               For additional background informa-
                             tion on the DIE performjfe demon-
                             stration, please refer to tff|Sept.ember
                             1995'issue of CircuiTm. tfore back-
                             ground on the DfE Program and the
                             DfE PWB Project can be found in the
        HieilUil Iiuitu-uuii iig^"^-;, —-  --  --•  ;            -            ._
        S.W., (3404), Washington, DC, 20460;   i February 1995 tssue of CmmTm
  the board is scrapped. However, there-
  are no criteria for how to evaluate  no
  line" products. In addition,  there are
 . no official means of determining when
  "a little separation" is significant- to the
  performance of the board.
    1ST is not a subjective test  and is not
  dependent  upon the  presence  or
 .absence  of a line in  a microsection
  after microetch. The test provides a rel-
  ative number of 1ST cycles necessary to
  cause a significant rise in resistance in
  the post interconnect. This number of
  cycles may be'used to predict intercon-
  nect  performance. Tests such as this,
  when correlated with  microsecuons,
  can be useful' in determining how «>
  interpret "no line" product characteris-
   tics.  In  addition, 1ST may  be  able  to
   determine levels of post separauon.
     Future industry studies must deter-
   mine the  relevance of these curves to
   performance, based  on  number  of
   cycles needed to raise the resistance as
   well as  the amount of change in resis-
    tance. Definitions for -marginal" and
    "gross" separations mav be tied to life-
    cycle testing and subsequently related
    to class of boards produced. C

      Bill  Birch  is  President  of  PWB
    Interconnect Solutions Inc.  (Nepean,
    Ontario)   Deborah   Boger  was  an
    Environmental Protection Specialist with the
    EPA   She  currently   attends  Harvard
     University, Kennedy School of Government
      Kathy  Hart  is  an Environmental
     Protection Specialist tilth EPA's Design for
     the Environment Staff (Washington. DC).
       Susan Mansilla is Technical Director of
     Robisan  Laboratory  Inc.   {Indianapolis,
    ' 164- CIRCIIUEE • JAKIUT •  1917