fig1 illustrates a testing system 100 configured for detecting faults in holes of printed circuit boards . the testing system 100 includes a picking and dropping mechanism 12 , a resistance measuring device 14 , a storage means 16 and a processor 18 . the picking and dropping mechanism 12 is an apparatus capable of sucking a printed circuit board , and usually includes a robot arm , a vacuum pad and a vacuum plate . the resistance measuring device 14 can be flying probe test system and in - circuit testing ( ict ) system . the storing means 16 can be hard disk drive , soft disk drive , compact disk drive , flash memory , etc . the picking and dropping mechanism 12 and the resistance measuring device 14 are electrically coupled to the processor 18 . the processor 18 is capable of sending instructions to the picking and dropping mechanism 12 and the resistance measuring device 14 , thereby controlling the picking and dropping mechanism 12 and the resistance measuring device 14 . the storing means 16 has data stored therein . the data includes location information and standard resistance of electric trace segments in the printed circuit board , and criteria for determining whether the electric trace segment includes a certain type of fault based on a difference between the actual resistance and the standard resistance of electric trace segment . the criteria includes parameter related to resistance difference between a standard resistance value and a measured value . for example , an electric trace segment has a standard resistance of x and a measured resistance y , if a numerical value of ( y − x )/ x exceeds 20 %, then the electric trace segment has a certain type of fault , e . g ., a fault of scrap remaining in through holes or via holes . the criteria may be varied with different types of faults . a method of getting such a criteria will be described in detail in following . fig2 illustrates a method of establishing a criteria describing a resistance difference caused by a certain type of fault , the method will be described in detail as following : in step 1 , standard resistances of a printed circuit board is measured . fig3 illustrates a printed circuit board 100 having a number of electric trace segments s 1 , s 2 , s 3 , . . . sn formed therein . each electric trace segment includes a conductive transmission line 122 and one or more conductive holes 124 defined therein . two testing points 14 are defined at two ends of the conductive transmission line 122 with the one or more conductive holes 124 . in order to measure standard resistance of each electric trace segment , a number of printed circuit boards 10 , e . g ., 50 to 100 pieces , are provided . there are no faults in conductive holes 124 of these printed circuit boards 10 . preferably , the printed circuit boards 10 are fabricated in a same production line . resistance of each of the electric trace segment 122 of the printed circuit boards 10 is measured . an average resistance of the electric trace segments of the printed circuit board 10 is used as the standard resistance of the corresponding electric trace segment 122 . similarly , a number of standard resistances corresponding to a number of electric trace segments can be obtained . the standard resistances can be marked as x 1 , x 2 , . . . , xn . a four line flying probe test system can be used to measure the resistance of the electric trace segments . referring to fig4 , a fixture 200 can be used to hold the printed circuit board 10 . the fixture 200 includes a bottom plate 20 and a top plate 22 . the top plate 22 includes a number of through holes 24 defined therein . a location of each through hole 24 is corresponds to each of the testing points 14 . referring to fig5 , the printed circuit boar 10 is placed and sandwiched between the bottom plate 20 and the top plate 22 . the flying probe ( not shown ) of the flying probe test system can be inserted into the through holes 24 to contact with the testing points 14 . the electric trace segments 122 in the printed circuit board 10 may have different standard resistances . these standard resistances distribute in a certain range , e . g ., from 0 . 01 to 2 ohms ( ω ). five or more electric trace segments in the printed circuit board 10 , whose resistances are well - distributed in the range , can be selected as sample electric trace segments . in step 2 , m pieces of printed circuit boards , labeled as l 1 , l 2 , . . . , lm , are provided . printed circuit boards l 1 , l 2 , . . . , lm have same structure to the printed circuit board 100 , but may contain faults in conductive holes . these printed circuit boards are used to establish a relation between resistance difference and certain type of fault . in step 3 , in order to establish such a relation , 22 electric trace segments s 1 , s 2 , s 3 , . . . , s 21 , s 22 , whose standard resistance are 0 . 01ω , 0 . 05ω , 0 . 1ω , 0 . 2ω , 0 . 3ω , . . . , 1 . 9ω , 2ω respectively , in printed circuit boards l 1 , l 2 , . . . , lm are selected as sample electric trace segments . it is to be understood that these standard resistance are illustrative examples ; the resistance may vary according to a certain printed circuit board . for example , there is no electric trace segment whose standard resistance is 0 . 1ω but there is a electric trace segment whose standard resistance is 0 . 09ω . the electric trace segment can be selected as sample electric trace segment . in step 4 , a resistance of each sample electric trace segment in each of printed circuit boards l 1 , l 2 , . . . , lm is measured , and the index of the printed circuit board , the resistance difference of the electric trace segment whose resistance difference exceed a predetermined range are recorded . the resistance difference represents a difference between a measured resistance and the standard resistance of a certain electric trace segment . for example , a certain sample electric trace segment has a standard resistance xn , a measured resistance of the certain sample electric trace segment in the printed circuit board labeled as l 1 is yn , the resistance difference δyn equal to yn − xn . the predetermined range varies in accordance with a certain type of fault in the conductive holes , such as impurity , over - etching and disconnection . the range can be obtained from test results . for example , with respect to flexible printed circuit boards , scrap of adhesive layer remain in conductive through holes can increase a resistance of the electric trace segment more than 20 %. if a numerical value of δyn / xn is larger than the predetermined range ( e . g ., 20 %), then the index of the printed circuit board and the resistance difference of the electric trace segment are recorded in table 1 . for example , sample electric trace segment s 1 in printed circuit board l 1 has a standard resistance 0 . 01ω , a measured resistance is 0 . 014ω , δyn / xn thereof equals 40 %, and therefore the index of the printed circuit board l 1 and the resistance difference 0 . 004ω are recorded in table 1 . adequate printed circuit boards must be provided such that each cell in column 3 of table includes at least five or more records . in step 5 , the electric trace segments with resistance differences exceed a predetermined range are verified , the verified result ( i . e ., whether or not the electric trace segments includes the certain type of fault after the resistance differences ) are recorded . in this embodiment , micro section analysis is performed on each of the electric trace segments and the inner portions of conductive holes are observed via a microscope . a certain type of fault , i . e ., scrap of adhesive layer remaining in the through hole , analyzed and the resistance difference is recorded in table 2 . a count of records in each cell of column 3 of table 2 can be different . for example , in the cell corresponding to s 1 , there are 5 records , and the cell corresponding to s 2 there are 6 records . in order to find a resistance difference range corresponds to each sample electric trace segment , a maximum value and a minimum value of resistance difference in each cell of column 3 of table 2 are calculated and recorded . in this embodiment , the results are recorded in table 3 . in step 6 , regression analysis is performed on the data stored in columns 2 and 3 of table 3 thereby obtain a function of δy max = f max ( x ), which represents a relation between the maximum resistance difference caused by a certain type of fault and the standard resistance . regression analysis is performed on the data stored in columns 2 and 4 of table 3 thereby obtain a function of δy min = f min ( x ), which represents a relation between the minimum resistance difference caused by a certain type of fault and the standard resistance . it is understood that difference between δy max = f max ( x ) and δy min = f min ( x ) represent resistance difference caused by other type of fault can also be obtained in a similar way . standard resistances , δy max = f max ( x ), δy max = f max ( x ) can be stored in a storing means , for example , storing means 16 in the testing system 100 . preferably , the standard resistances of the sample electric trace segments can be divided into several sections . above mentioned regression analysis can be performed in each section . for example , 0 . 01ω - 2ω can be divided into 4 sections : 0 . 01ω - 0 . 1ω , 0 . 1ω - 0 . 5ω , 0 . 5ω - 1ω , 1ω - 2ω . δy max = f max ( x ) and δy min = f min ( x ) are calculated in each section . fig5 illustrates a method of detecting faults in holes of printed circuit boards using the testing system 100 , the method will be described in detail with the following embodiment . firstly , the data stored in the storing means 16 is read into the processor 18 . the data describes location information and standard resistance of at least one electric trace segment in the printed circuit board , and a criteria of whether the electric trace segment includes a certain type of fault based on a difference between the actual resistance and the standard resistance of electric trace segment ( i . e . the aforementioned functions δy max = f max ( x ) and δy min = f min ( x )). secondly , the processor 18 controls the resistance measuring device 14 to measure resistance of electric trace segments according to the location information . thirdly , judging whether the electric trace segment including the certain type of fault according to a measured resistance , standard resistance of electric trace segment and the definition . for example , if a electric trace segment in the printed circuit board has a standard resistance x , a measured resistance is y . δy max and δy min are calculated according to δy max = f max ( x ) and δy min = f min ( x ) respectively . if y is between δy max and δy min then the electric trace segment is recorded as having the certain type of fault represented by δy max = f max ( x ) and δy min = f min ( x ). if the electric trace segment includes the certain type of fault , the processor 18 controls the picking and dropping mechanism 12 to picking the printed circuit boards including the certain type of fault and drop the printed circuit board to a given location . finally , it is to be understood that the above - described embodiments are intended to illustrate rather than limit the invention . variations may be made to the embodiments without departing from the spirit of the invention as claimed . the above - described embodiments illustrate the scope of the invention but do not restrict the scope of the invention .