The acceptance of surface mounting in the electronics industry has been slowed by problems with component availability, electrical testing, and inspection. Industry suppliers and users have been working to solve these problems, and they are easing. Component manufacturers are offering substantially more devices in surface mountable packages. Automated test equipment vendors are offering test fixtures for surface mount circuit boards. And just recently, solder connection inspection has been automated through a partership effort with a major electronics manufacturer. The results achieved in this effort are presented

Juha, Michael

English

NASA Technical Reports Server

INTRODUCTION 
Mi c hael Juha 
IRT Corporation 
San Diego, CA 
The acceptance of sur face  mounting i n  the e l e c t r o n i c s  indus t ry  has  been slewed 
by problems with component a v a i l a b i l i t y ,  e l e c t r i c a l  t e s t i n g ,  and inspec t ion .  
Indus t ry  s u p p l i e r s  and use r s  have been working t o  so lve  these  problems, and 
these  problems a r e  easing. 
more devices  i n  sur face  mountable packages. Automated t e s t  equipment vendors 
are of fer ing  tes t  f i x t u r e s  f o r  sur face  mount c i r c u i t  boards. 
r ecen t ly ,  s o l d e r  connection in spec t ion  has been automated through a 
"par tnership" e f f o r t  w i th  a major e l ec t ron ic s  manufacturer. The r e s u l t  achieved 
i n  t h i s  success fu l  first i n s t a l l a t i o n  i s  t h e  sub jec t  of t h i s  paper, 
Component manufacturers a r e  o f f e r ing  s u b s t a n t i a l l y  
And j u s t  
THE NEED FOR SOLDER QUALITY INSPECTION 
Surface-mounted devices  a r e  held i n  place on t h e  c i r c u i t  board by t h e i r  s o l d e r  
connections.  
However, with sur face  mounting, t h e r e  i s  no s o l d e r  plug surrounding a p i n  t h r u  
a hole i n  t h e  c i r c u i t  board t o  give t h e  connection added s t rength .  
t h e  s o l d e r  a lone  bonds t h e  device t o  the  c i r c u i t  board, as shown in figure 1. 
The e l e c t r i c a l  i n t e g r i t y  of t h e  c i r c u i t  board is t o t a l l y  dependent upon t h e  
s t r u c t u r a l  i n t e g r i t y  of t h e  s o l d e r  connection. This i s s u e  of s t r u c t u r a l  
i n t e g r i t y ,  p l u s  t h e  large numbers of solder  connections on each c i r c u i t  board,  
is the impetus for automating so lder  connection inspection. 
Surface mounting makes more e l e c t r o n i c  products man-portable o r  mobile i n  
veh ic l e s .  With t h i s  p o r t a b i l i t y  or  vehicular  mobil i ty  comes shock, v i b r a t i o n ,  
and extremes of temperature.  Shock, v ibra t ion ,  and temperature approach o r  
exceed l e v e l s  t h a t  were previously associated only with m i l i t a r y  e l ec t ron ic s .  
These higher  s t r e s s  l e v e l s  must be sustained by the  smal le r  so lde r  connect ions 
c h a r a c t e r i s t i c  of sur face  mounting. 
mounted s o l d e r  connections are disposed t o  f a t i g u e  and creep f a i l u r e .  
The same s i t u a t i o n  occurs on plated-thru-hole c i r c u i t  boards.  
In s t ead ,  
I 
Numerous expe r t s  have c i t e d  how su r face  
I STRESS AND ELECTRICAL TESTING 
The s t r u c t u r a l  i n t e g r i t y  of  s o l d e r  connections can be assessed t o  some degree 
with "shake and bake" stresg t e s t i n g .  However, t h i s  type of t e s t i n g  confronts  
t h e  r i s k  of wearing out  t h e  product before it ever  reaches t h e  customer, 
Fur ther ,  t h e  electrical  t e s t i n g  that i s  used with "shake and bake", as wel l  as 
t h a t  u sua l ly  performed i n  r egu la r  manufacturing q u a l i t y  assurance,  d e t e c t s  only 
t h e  "open" o r  "short"  condi t ions.  S t ress  testing t y p i c a l l y  does not expose t h e  
s t r u c t u r a l l y  marginal connections t h a t . s t i l 1  conduct, bu t  are long term 
candida tes  f o r  f a i l u r e ,  such as: 
o i n s u f f i c i e n t  so lde r  
o poor wet t ing  
o excess  s o l d e r  o r  l ead  p ro jec t ion  
o device  o r  l ead  o f f  p o s i t i o n  
o unwanted so lde r  b a l l s  o r  sp lashes  
o device  t i l t e d  r e l a t i v e  t o  t h e  board 
o po ros i ty  i n  the  s o l d e r  connection 
65 I PRfCEDffi PAGE B U N K  NOT FUED 
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VISUAL INSPECTION 
Visual inspec t ion  can d e t e c t  g ross  d e f e c t s ,  such as missing devices ,  b r idges  
ou t s ide  t h e  devices,  t h e  absence of s o l d e r  f i l l e t s ,  and non-wetting. 
v i s u a l  inspect ion is  q u a l i t a t i v e  r a t h e r  t h a n  quant i ta t ive- - - i t  does  no t  measure 
t h e  ex ten t  t o  which a d e f e c t  e x i s t s .  Also, v i s u a l  i n spec t ion  r e l i e s  upon the  
e x t e r n a l  appearance of t h e  s o l d e r  connection t o  i n f e r  i t s  i n t e r n a l  s t r u c t u r a l  
i n t e g r i t y ,  
f u l l y  underneath the  devices ,  making v i s u a l  i n spec t ion  imprac t i ca l ,  
However, 
And with  su r face  mounting, t h e  s o l d e r  connections a r e  p a r t i a l l y  o r  
STRUCTURAL INSPECTION 
S t r u c t u r a l  inspect ion is not  a new problem. For yea r s ,  aerospace and c a s t i n g  
manufacturers have used X-ray in spec t ion  t o  examine t h e  s t r u c t u r a l  i n t e g r i t y  of 
c a s t i n g s  f o r  a i r f rames ,  engines,  and t ransmiss ions .  Since s o l d e r  connect ions 
are a "casting" formed by t h e  sur face  t e n s i o n  of t h e  molten s o l d e r  and t h e  
s u r f a c e s  of the device and t h e  c i r c u i t  board,  X-ray techniques w i l l  work f o r  
s o l d e r  connections. The keys t o  making X-ray techniques v i ab le  f o r  s o l d e r  
connect ion inspect ion a r e  t o :  
1) speed up t h e  X-ray imaging process  
2 )  r ad ia t ion  from damaging e l e c t r o n i c  components 
3) improve X-ray imaging t o  r e so lve  0.001 inch  f e a t u r e s  
3) automate inspec t ion  t o  achieve fast ,  accu ra t e  r e s u l t s  
4 )  make t h e  techniques usable  i n  t h e  product ion l i n e  
DEVELOPING A SOLDER QUALITY INSPECTION MACHINE 
I n  1984, w e  became aware of t h e  need f o r  s t r u c t u r a l  i n spec t ion  of s o l d e r  
connect ions i n  surface-mounted e l e c t r o n i c s  t h r u  a customer, and developed t h e  
machine shown in figure 2 specifically f o r  solder connection inspection. This 
machine took " f l a sh"  X-ray images of each device on t h e  c i r c u i t  board t o  keep 
r a d i a t i o n  from damaging t h e  device,  and au tomat ica l ly  inspec ted  t h e  " s t r u c t u r e "  
of each solder  connection according t o  a " r u l e  set" t h a t  took i n t o  
considerat ion:  
o 
o 
o 
o 
o 
o 
t h e  type of device (PLCC, SOT, LCC, etc . )  
t h e  shape of t h e  pad on t h e  c i r c u i t  board 
t h e  amount of misalignment allowed between t h e  
device and t h e  c i r c u i t  board 
t h e  range of s o l d e r  connection t h i c k n e s s  allowed 
t h e  range of s o l d e r  connection shape allowed 
t h e  amount of po ros i ty  allowed 
This  first machine adminis tered a maximum dose of 5 RAD(Si) t o  each c i r c u i t  
board,  w a s  designed t o  trseet' f e a t u r e s  as small as 0.002 inch i n  s i z e ,  and used 
images like t h e  one shown in figwe 3 to inspect each solder connection. A 
schematic for this first machine is shown in figure 4 .  In operation, the 
machine used a n  e l e c t r i c a l  X-ray source t o  p r o j e c t  a col l imated beam of X-rays 
up through a 1" by 1" area of t h e  c i r c u i t  board. 
s o l d e r  connections was pro jec ted  onto a f l u o r e s c e n t  sc reen  j u s t  above t h e  
c i r c u i t  board. 
image, which was viewed by a high-resolution video camera through a first 
su r face  mir ror  t o  keep t h e  camera and o p t i c s  out  of t h e  X-ray beam, 
The X-ray shadow image of the  
This  sc reen  converted t h e  X-ray image i n t o  a v i s i b l e  l i g h t  
The video 
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image of t h e  s o l d e r  connec t ions  xas  input  t o  a d i g i t a l  image processor  t h a t  
performed t h e  ac tua l  i n s p e c t i o n  under t h e  d i r e c t i o n  of  a s e t  of  programs i n  a n  
IBM Personal  Computer. 
RESULTS ACHIEVED WITH THE FIRST MACHINE 
I n  mid-1985, t h i s  first machine was i n s t a l l e d  i n  a s u r f a c e  mount product ion 
l i n e  t o  i n s p e c t  1000 c i r c u i t  boards per  day with J-leaded s u r f a c e  mounted I C s .  
Each c i r c u i t  board had 252 s o l d e r  connections,  and i n s p e c t i o n  t ime p e r  c i r c u i t  
board was 30 seconds.  
p e r  week,- t h i s  first machine and i ts  software have in spec ted  more than  200,000 
c i r c u i t  boards  263 X 106 joints. This  l a rge  product ion  volume has  forced  us t o  
make our i n s p e c t i o n  programs e f f e c t i v e  f o r  the  wide v a r i a t i o n s  found i n  pro-  
duc t ion  s o l d e r  connect ions made wi th  vapor phase ref low.  
Since t h e  customer's  product ion l i n e  opera ted  seven days  
The defects i d e n t i f i e d  by t h i s  first machine have been (and are today):. 
o 
o i n s u f f i c i e n t  s o l d e r  ( inc lud ing  po ros i ty )  
o 
o 
o s o l d e r  b r i d g e  
o 
o device  miss ing  from board 
void (absence of  s o l d e r  j o i n i n g  l e a d  t o  pad) 
bent  lead ( o f f  p o s i t i o n  from pad) 
l e a d s  touch ing  (producing a s h o r t  without a b r idge )  
device  off p o s i t i o n  (skewed o r  s h i f t e d )  
These d e f e c t s  are i d e n t i f i e d  accu ra t e ly  and repea tab ly ,  and ou r  customer i s  
p leased  wi th  t h e  performance of t h e  machine ( p a r t i c u l a r l y  s i n c e  it has  a l r e a d y  
pa id  f o r  i t s e l f ) .  However, t h e r e  are some cavea ts  on i n s p e c t i o n  accu rac i e s .  
Solder  b r i d g e s  and miss ing  devices  are p r a c t i c a l l y  always found, s i n c e  they  
r ep resen t  extreme cond i t ions .  I n s u f f i c i e n t s ,  bent  leads, vo ids ,  and o f f  
p o s i t i o n s  are q u e s t i o n s  of t h e  degree t o  which t h e  d e f e c t  i s  p resen t .  Through 
manual re -screening  of au tomat i ca l ly  inspected c i r c u i t  boards,  we have l ea rned  
t h a t  t h e s e  d e f e c t s  are found roughly 95% t o  99% of t h e  time. 
95% t o  99% is  l a r g e l y  a t t r i b u t a b l e  t o  t h e  v a r i a b i l i t y  of  t h e  human i n s p e c t o r s  
used t o  perform t h e  re-screening. 
A more impor tan t  facet i s  t h e  r e l a t i o n s h i p  we found between increased  d e f e c t  
d e t e c t i o n  and i n c r e a s e d  "false rejects",  product r e j e c t e d  as bad when it i s  
t r u l y  good. 
The range from 
Inspec tors  do make "bad" calls on occasion.  
THE IMPORTANCE OF ACCEPT/REJECT THRESHOLDS 
Figure  5 shows two overlapping d i s t r i b u t i o n s  that h e l p  exp la in  t h i s  
r e l a t i o n s h i p .  
composite o f  many measurements of t h e  s ize  and t h i c k n e s s  of each s o l d e r  
connection. The ver t ical  a x i s  i s  t h e  number of s o l d e r  connec t ions  wi th  t h a t  
measure of  q u a l i t y  i n  a ba tch  of boards. The a c c e p t / r e j e c t  t h re sho ld  
determines whether a s o l d e r  connection i s  accepted as good o r  r e j e c t e d  as bad. 
Solder  connec t ions  t o  t h e  l e f t  of t h e  threshhold are rejected as bad. Solder  
connect ions t o  t h e  r i g h t  of t h e  threshhold  are accepted as good. 
The h o r i z o n t a l  ax i s  is  a "measure of  q u a l i t y "  t h a t  is a 
This  r e l a t i o n s h i p  between defec t  d e t e c t i o n  and false re jec ts  can be seen i n  
F igure  5. A s  the accep t / r e j ec t  t h re sho ld  i s  moved t o  t h e  r i g h t ,  more and 
67 
more de fec t s  are de tec t ed  u n t i l  p r a c t i c a l l y  none escapes inspec t ion .  However, 
as d e f e c t  de t ec t ion  grows i n  e f f ec t iveness ,  s o  does t h e  number of good 
connections ( t h e  l e f t  "tail" of t h e  "good" d i s t r i b u t i o n )  t h a t  w i l l  be f a l s e l y  
r e j e c t e d  as defec t ive .  This  r e s u l t s  from t h e  overlap of t h e  "good" and "bad" 
s o l d e r  connection d i s t r i b u t i o n s ,  and r e f l e c t s  r e a l i t y .  
c h a r a c t e r i s t i c s  of marginal ly  good s o l d e r  connections s i g n i f i c a n t l y  over lap  
those  of marginally bad s o l d e r  connections.  
A s  a r e s u l t  of t h e  r e l a t i o n s h i p  between t h e  accep t / r e j ec t  th resholds  and the 
economics of our  customer, t h e  accep t / r e j ec t  th resholds  f o r  t h e  first machine 
have been s e t  up t o  d e t e c t  roughly 97% of a l l  de fec t s  while  making fewer than  
5% false r e j e c t s .  
achievable  with in spec t ion  personnel.  And, i n spec t ion  by our  machine i s  done 
before  e l e c t r i c a l  t e s t i n g  t o  inc rease  t h e  e f f e c t i v e n e s s  of e lectr ical  t es t s .  
For o t h e r  customers wi th  a d i f f e r e n t  manufacturing process ,  and d i f f e r e n t  c o s t s  
f o r  inspec t ion ,  rework, s c rap ,  and escape of de fec t s ,  d i f f e r e n t  accep t / r e j ec t  
threshholds  would be necessary t o  achieve t h e  b e s t  economic r e t u r n  f o r  t h e i r  
circumstances. 
We have found t h a t  t h e  
These performance l e v e l s  a r e  far b e t t e r  than  those  
THE "STRUCTURAL" SOLDER QUALITY STANDARDS PROBLEM 
A s  an  a s i d e  comment, t h e  requirement f o r  f l e x i b l e  a c c e p t / r e j e c t  th resholds ,  
when coupled with t h e  wide v a r i a t i o n s  found i n  product ion s o l d e r  connect ions,  
and t h e  absence of complete information about  what a good s o l d e r  connection 
"looked l i k e " ,  almost prevented u s  from d e l i v e r i n g  a s a t i s f a c t o r y  working 
machine. For tuna te ly ,  our  customer was w i l l i n g  t o  spend cons iderable  time and 
money developing t h e i r  own s t r u c t u r a l  s tandards  f o r  what made s o l d e r  
connections good versus  bad ,  This  requi red  stress cyc l ing  hundreds of c i r c u i t  
boards,  analyzing each f a i l u r e  t o  e s t a b l i s h  causes,  and then  proceeding with 
production while monitoring product ion items f o r  i n - the - f i e ld  f a i l u r e s  on an 
on-going bas is .  A l l  t h i s  work e n t a i l e d  considerable  investment ,  and r e s u l t e d  
i n  s tandards t h a t  are not  c o n s i s t e n t  with present  v i s u a l  i n spec t ion  s tandards .  
When s t r u c t u r a l  s tandards  are developed f o r  o the r  products ,  such as av ion ic s ,  
we f e e l  these s tandards  w i l l  no t  agree  with e x i s t i n g  v i s u a l  i n spec t ion  
s tandards ,  Since a s u b s t a n t i a l  bene f i c i a ry  of  t hese  new s t r u c t u r a l  s tandards  
would be t h e  m i l i t a r y ,  funding f o r  s tandards  development should be a l l o c a t e d  as 
soon as poss ib le ,  p a r t i c u l a r l y  i n  view of t h e  inc reas ing  concerns over t h e  
s t r u c t u r a l  v i a b i l i t y  of l e a d l e s s  su r face  mounted devices .  
experience of our customer, where p r a c t i c a l l y  no f i e l d  f a i l u r e s  now occur ,  
t hese  new s tandards  would c l e a r l y  he lp  reduce in-service failures. 
customer's  product environment is 3+ G ' s  of shock and v i b r a t i o n ,  ambient 
temperature from -40 F t o  + l 2 j  F, and humidity from 0% t o  100%. 
Based on t h e  
And, our 
68 
CONCL'JS IONS 
It is d i f f i c u l t  t o  e x t r a p o l a t e  gene ra l  savings r u l e s  from a s i n g l e  
i n s t a l l a t i o n .  However, we have shown w i t h  ou r  first i n s t a l l a t i o n  t h a t  
automated X-ray i n s p e c t i o n  can dramat ica l ly  reduce: 
o t h e  c o s t s  of  i n s p e c t i o n  
o 
o 
o 
o 
o t h e  inc idence  of d e f e c t s  
t h e  inc idence  of unnecessary rework on good boards  
t h e  r e c y c l i n g  of boards t h r u  rework as a d d i t i o n a l  
d e f e c t s  are c i ted  
t h e  c o s t s  o f  s c r a p  by minimizing rework 
t h e  escape o f  d e f e c t i v e  boards 
Th i s  last p o i n t  i s  a n  o f t e n  overlooked major  b e n e f i t  area. With t h e  
q u a n t i t a t i v e  q u a l i t y  d a t a  that i s  a by-product of  automated X-ray in spec t ion ,  
you can c o n t r o l  your manufacturing process  t o  make a b e t t e r  product .  
machine was ins ta l led  i n  a new manufacturing l i n e  wi th  completely new 
equipment. 
h e l p  set  up t h e  s o l d e r  p a s t e  sc reen  p r i n t e r  and reduce t h e  inc idence  of vo ids ,  
bridges and i n s u f f i c i e n t s .  During production, our  machine cont inues  t o  monitor 
p a s t e  p r i n t e r  performance by no t ing  t h e  incidence of  b r i d g e s  and i n s u f f i c i e n t s .  
When o u r  defect r e p o r t s  show an increase i n  bridges o r  i n s u f f i c i e n t s ,  t h e  
customer 's  personnel  know how t o  a d j u s t  the  process  back i n t o  con t ro l .  
r e s u l t ,  our  customer has  been able t o  achieve a s i g n i f i c a n t  increase i n  y i e l d .  
Our first 
During process  s t a r t -up ,  it was d iscovered  t h a t  o u r  machine could 
As a 
I 
69 
70 
Figure 1. Visual image of PLCC on a circuit board. 
CONTROL MONITOR 
CAMERA BOX 
SHIELDED 
. INSPECTION 
CHAMBER 
Figure 2. Line drawing of first inspection machine. 
ORlGlNAL PAGE IS 
OF POOR QUALITY 
Figure 3.  X-ray image of PLCC. 
r--==------'---- -__ T------ 
INSPECTION 
CIRCUIT BOARD 
X-RAY SOURCE 
ENCLOSURE 
$'+ OPERATOR 
Figure 4 .  Schematic for first inspection machine. 
71 
# OF 
UNITS 
DEFECT SIGNAL- 
REJECTABLE ITE.MS r f 
A MEASURE OF QUALITY 
Figure 5. Distributions showing solder connection quality. 
72 


Micro-focus X-ray imaging

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