Resolution of solder voids in pin-in-hole product
Variable + -
PCB supplier with all other PCB manu-
facturing steps being done at the factory
PTH diameter Normal (29 mil); H-O Reduced (-2); H-R(-2)
which produced the suspect PCBs. The
Desmear Original desmear With filtration; D-B
primary difference between the two per-
w/o filtration; D-O
manganate desmear processes is that while
both processes recirculated the permangan-
OSP H
2
O
2
based micro-etchant; O Cleaner + persulphate based
ate desmear bath, only one incorporated
micro-etchant; CPS
a filtration system to remove the residue
Bake Prior to Wave Solder w/o bake; W/O Bake 105˚C for 6 hours; W generated by the process. The wave solder-
Conveyor Speed Original speed, Faster speed,
ing results shown in Table 5 demonstrate
95 cm/min; 95 120 cm/min; 120
that the dppm can be effectively supressed
by the use of a filtration system to remove
Table 1. List of variables and variable levels.
residue generated by the permanganate
desmear process.
Variable Setting
Hole Size H-O H-R(-2)
root cause
The results of the last experiment clearly
Desmear D-O
demonstrated that residue from the
OSP O CPS O CPS
permanganate desmear process resulted
in nodule formation underneath the PTH
Baking W/O
copper barrel which then out-gassed to
Conveyor 95 form solder voids. It is assumed the core
Speed of the nodules had excess solvent from
the laminate sweller bath which was the
Sample Size 5 20 5 180
contaminate which was out-gassing.
Board Defect 80% 40% 40% 22% Of course, one set of data is not
Hole dppm 2,292 781 417 307
enough to prove the validity of this conclu-
sion. Hence, five lots of PCBs including
Table 2. Experimental results 1.
the original part as well as two other
production parts, B and C, were fabricated
Variable Setting with the modified process to verify the
Hole Size H-O H-R(-2)
proposed solution. A total of 780 PCBs
were run through the modified PCB
Desmear D-O
manufacturing process, wave soldered and
OSP O
examined by x-ray inspection.
The results of this larger verification
Baking W/O
test, in Table 6, show that the modified
Conveyor 95 120 95 120
process is capable of reducing the board
Speed
defect rate to less than 4% and the hole
dppm to 80. This effectively validates the
Sample Size 25
conclusion that residue from the perman-
Board Defect 72% 72% 64% 72% ganate desmear process led to solder void
Hole dppm 9,833 5,958 3,000 1,583
formation in the wave solder process.
Table 2. Experimental results 2.
Nodules from permanganate desmearing:
Without an effective filtration system,
Variable Setting Variable Setting residue in the permanganate desmear-
ing bath may adhere to the side-walls of
Hole Size H-O Hole Size H-O
PTHs resulting in nodules. The volatiles
Desmear D-O Desmear D-O D-B
entrapped by the nodules are vaporized
OSP O OSP CPS
during wave soldering. The vapor breaks
the thin copper surface of the nodules
Baking W/O W Baking W/O
but the vapors are trapped as the molten
Conveyor 95 Conveyor 95
solder solidifies leaving a solder void. The
Speed Speed
fact that the solder voids were either not
there or present and large would suggest a
Sample Size 40 10 Sample Size 20 30
minimum threshold of energy provided by
Board 53% 10% Board 40% 0% the solder wave was necessary to volatilize
Defect Defect and release solvent in the form of a vapor
Hole dppm 3,047 1,458 Hole dppm 781 0
resulting in solder void formation.
Table 4. Results of bake prior to wave solder. Table 5. Results of desmear experiment, with and problem resolution
without filtration.
After defining the root cause, correspond-
26 – Global SMT & Packaging – February 2009
www.globalsmt.net
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