Models for Bridging Defects Test and Diagnosis - Models in Hardware Testing - page 58

which have failed on the tester, but are not predicted by the fault model, are called
Nonpredictions . The ranking criterion is based on the Intersection value; the higher
the Intersection , the better the diagnosis.
Fault dictionaries are feasible when the diagnosis is performed repeatedly for a
given design. However, their main drawback is the storage space. A circuit with

n number of nets has 2 possible bridging faults. Thus, considering every possi-
ble bridging fault is infeasible. Physical layout information is usually considered to
eliminate bridges between nets that are extremely unlikely to be bridged together
due to their physical location ( Aitken and Maxwell 1995 ; Lavo et al. 1998 ). If the
two nets are farther than some minimum distance or if there is another net between
them (that would also be involved in the bridge), the corresponding bridging fault
is discarded. However, there are also some techniques to reduce the number of can-
didates without using layout information, as the two techniques reported by Lavo
et al. ( 1997 ). The first technique uses the SA fault diagnosis to identify one of the
bridged nets. If this is accomplished, assuming a circuit with n nets, knowing the d
net candidates to be one of the nets involved in the bridge, the number of bridged
pairs is then reduced to n d . The second technique identifies the candidates that can
have an intersection with the behaviour observed on the tester. Candidates with no
intersection are then discarded.
Zou et al. ( 2005 ) proposed a diagnosis methodology based on dictionaries which
take the bridge resistance into account. The methodology is divided into two steps.
The first step consists in a logic diagnosis to find the potential candidates that can
explain the faulty behaviour. In the second step, layout information as well as the
resistive bridging fault model using the concept of critical resistance are used to
prune the candidates list. The intersection between resistive intervals is utilized to
discard bridging candidates. As an example, consider the bridged outputs (net A
and B)oftwogates(G1 and G2), as depicted in Fig. 2.21 . They drive, in turn,
gates G3 and G4, respectively. Assume that test patterns TP 1 and TP 2 cause G3
and G4 to fail, respectively, whereas TP 3 passes although it also activates the
bridge. Gate G3 should have failed in this case. In the fault free case, consider
that TP 1 and TP 3 set net A and net B to logic 1 and 0 respectively. On the con-
trary, TP 2 sets them to logic 0 and logic 1, respectively. As test patterns TP 1 and
TP 2 make the circuit to fail, the bridge resistance should be lower than the mini-
mum of the two critical resistances R c . TP 1 ;G3)andR c . TP 2 ;G4/. Nevertheless,
for passing pattern TP 3 the bridge resistance should be higher than the critical re-
sistance R c . TP 3 ;G3/. Thus, it must be accomplished that the bridge resistance is
R c . TP 3 ;G3/ < R b < min .R c . TP 1 ;G3/;R c .TP 2 ;G4//. On the other hand, in
case that R c . TP 3 ;G3/ > min .R c . TP 1 ;G3/, R c . TP 2 ;G4/, there is no bridge

A

G3

G1

R b

Fig. 2.21 Resistive bridging
fault diagnosis

G2 G4

B