This technical paper explains soldering process is optimized through selective soldering technology to avoid voids in thru-hole connectors and components. Chances of defects from PCB design and manufacturing, soldering consumables and machine parameters are analysed and corrected to resolve the void problem using system engineering approach. Test vehicle is assembled with optimized soldering parameters and tested in X-Ray machine. Test report shows that even with 2.4 mm PCB and thick pin components, void doesn’t occur in the PTH. Finally design for manufacturability (DFM) guideline is proposed for PCB design to eliminate the void issue caused by PCB design process. A test vehicle is assembled with optimized soldering parameters and tested in X-Ray machine. Test report shows that even with 2.4 mm PCB and thick pin components, void doesn’t occur in the PTH. Finally design for manufacturability (DFM) guideline is proposed for PCB design to eliminate the void issue caused by PCB design process. This technical paper is published during the fulfillment of my post graduate degree in electronics System Design Engineering at Coventry University, in sept 2016.

1. Design and Development of Optimized Selective Soldering
Process for Thru-hole Aerospace Connectors and
Components.
Shiju Jacob1
, Dr. Hariharan Ramasangu2
, Ugra Mohan Roy3
1- (Engg) student, 2-Professor and Dean, 3- Asst. Professor (EEE)
Electronic Systems Design Engineering
M.S. Ramaiah School of Advanced Studies, Bangalore
Abstract
Selective wave soldering process is an essential method in aerospace and defence applications,
due to high mix of packages. However selective soldering technology is badly hit by the airholes
generated inside the plated thru-hole (PTH) area, which is called as void. Void problem spoils
lot of time and money in the manufacturing industry and the research conducted so far could
provide only partial solutions. An optimized solution for void problem in selective soldering is
proposed here.
Chances of defects from PCB design and manufacturing, soldering consumables and
machine parameters are analysed and corrected to resolve the void problem using system
engineering approach. Operational deficiencies of PCB design parameters and soldering
consumables have been analysed. Functional process flow of void issue resolution is defined
with enhancement in PCB design, selection of consumables and machine parameters design. A
test vehicle is also designed with the recommendations from the concept definition. Test vehicle
is assembled with optimized soldering parameters and tested in X-Ray machine. Test report
shows that even with 2.4 mm PCB and thick pin components, void doesn’t occur in the PTH.
Finally design for manufacturability (DFM) guideline is proposed for PCB design to eliminate
the void issue caused by PCB design process.
1. Introduction
Form factor and interfacing constraints
in commercial, industrial and defence
products mandate assembling of all types of
component packages on same PCB.
Selective Wave Soldering is used for
soldering thru-hole components, when both
are to be assembled on the same board.
Selective soldering selects specific
component and solders only those
components with melted solder wave from a
tiny nozzle moving around below the PCB
which is already placed with SMT devices.
Void means the holes (missing of
solder) produced inside the soldering
volume of PTH components. Figure 1
shows example of void identified in X-Ray
test. Aerospace and defense products
undergo tremendous vibration. Component
pins covered with solder vibrates in different
frequency than component without solder.
Thus at a maximum magnitude pins crack
and circuits get disconnected.
Military & Aerospace board
manufacturing industry spends lot of money
and time due to this problem in critical
electronic boards manufacturing.

2. Figure 1 X-Ray Image of Void
2. Selective Soldering ; a debrief
Selective soldering is done with a
programmable machine. PCB is fitted
horizontally on a fixture with the
components placed. Soldering is done with
a nozzle which is moving under the PCB
and chooses the components based on the
program. Machine has a solder bath from
which melted solder is pumped to the nozzle
tip with nitrogen gas. Machine has fluxing
and preheating feature to enhance the
soldering performance. Figure 2 shows
soldering chamber with nozzle under the
PCB just before it starts the soldering. All
operations in the soldering chamber is
captured by a camera and displayed on the
computer screen to help the calibration and
alignment of the nozzle and fluxer.
Figure 2 Soldering Nozzle
Materials used for soldering are solder
alloy and flux. Compound of Tin (Sn) and
Lead (Pb) at a ratio of 63:37 is used for
leaded process. Either solder wire or solder
bar is used but purity of the compound is the
important factor. Flux is used to remove
oxides from the PCB surface and component
leads. It also prevents re-oxidation prior to
soldering. Importances of flux are reducing
surface tension which allows solder to flow
easier and create a better bond between the
solder and soldering surfaces.
Soldered joint is tested in X-Ray
machine to verify quality is good in thru-
hole. X-Ray test is very useful to detect
void.
3. Proposed Solution for Void Problem
Application engineers from machine
vendors agree that quality issues in selective
soldering are contributed not only by the
process but from row material also. Reiner
Z. and Christian O. concludes their
research mentioning with implementation
of some basic PCB design rules cost
intensive and time consuming thru-hole
rework can be avoided (Reiner Z. and
Christian O., n.d.).Various concepts are
explored and concluded that corrections
required from all factors involved in the
process. A process flow is drafted
considering correction of parameters in PCB
design & manufacturing, selection of
consumables and machine parameters
setting. Figure 3 shows the flowchart of
proposed solution. Concept includes
enhancements from all aspects studied in the
concept exploration. A test vehicle is
designed in which all challenges are
incorporated. This test vehicle contains
Thru-hole connectors with different pin
thickness, no of rows, pitches.
Thick PCB having more than 2.2 mm
thickness.

3. 2 oz copper layer for duplication of
internal drain
Specific profile is developed on the machine
with all machine parameters are set
according to the test vehicle components and
PCB. Test vehicle is then assembled with
the specific assembly profile to verify the
concept.
Start of
Mfg. Engg.
DFM Check on
PCB
Verification of
Components
Design
Optimization
Selection of
Consumables
Soldering Trials
Production Start
Machine
Programming
Figure 3 Process Flow of Proposed
Solution
4. PCB Design Optimization and DFM
Major parameters of PCB which
generate void are PTH plating thickness,
component pins to PTH wall clearance and
no of internal ground layers and overall
thickness of the PCB.
Figure 4 PTH Hole Dia and Plating
Thickness
Figure 4 shows cross section of PCB
with PTH thickness and hole dia. Industry
standard for the hole diameter is
W = Pin diameter + 0.6 mm
But the pins become tight in the hole in
many thick pin components. So each
componnet pin thickness assesed and extra
dimaetr of 0.2 mm is provided for thik pins
connectors. Plating thickness of 70 microns
is used, which is sufficient to avoid the
outgassing of PCB during soldering.
Figure 5 PTH Design with Hole
Clearance
Figure 5 shows snapshot of PTH
designed in Altium design software. Pin to
wall air gap is the major parameter which is
controlled in design. This is proposed as
DFM guideline in the PCB design.
Requirement of internal ground plane is
unavoidable in power supply board 2.6 mm
stack is used to simulate a thick PCB used in
industry. Also de-burring is highly preferred
in PCB manufacturing to remove the burrs
after drilling and plating.
5. Consumable Selection
Solder and soldering flux are major
consumables used in selective soldering.
Purity of solder is very important. Selection
of solder is done based on chemical test
conducted or based on the laboratory report
from the manufacturer. Flux is very
important for the process and it selection is
WT

4. very critical. Properties of different types of
fluxes available in industry are as below.
Rosin-based fluxes, which use a base of
isopropyl isopropanol (referred as IPA). It
contains 5–20% of solids (rosin +
activators). These fluxes deposit residues on
the board after soldering which needs to be
removed as it contaminates the board. So
aggressive cleaning is mandatory after
soldering.
Low-solids fluxes have solids content in the
range 1–5%.They may be based on rosin or
synthetic colophony substitutes. Low-solids
fluxes are designed to avoid post-solder
cleaning.
Water-soluble fluxes contain active organic
acid components in an alcohol base. The
residue from these fluxes needs to be
removed from the circuit assembly with a
water wash. So boards are cleaned in
deionised water after soldering.
Water-based fluxes have active acidic
components which are blended with water.
Depending on the concentration of active
components in the mixture, these fluxes may
be classified either as ‘no-clean’ or ‘clean
with water’ types
Flux is selected based on solubility, solid
contents and activator presence.
Composition of soldering alloy, PCB
substrate, surface finish and PCB assembly
cleaning method are other factors which
influences the selection of flux. Flux with
activator gives good soldering result but
deposits lot of solids on the board. Water
soluble flux is preferred because it is easy to
clean. ALPHA NR 215 model is selected
which is a no clean flux with less solids and
surface tension. It has activator also.
6. Test Vehicle Design
A test vehicle is designed with custom
made PCB and specific thru-hole
components. Connectors are selected with
variation in pin diameter and pitch. Multi
row connectors are also included in test
vehicle, as shown in the table.
Bill of Material of Test Vehicle
Sl
No
Categor
y
Description Desig
nator
1 Connect
or
Power-D(M-Series),
Connector,PCBMount,5pin
,Male,40A, 1mOhm,Right
Angle, Thru-Hole
JS1
2 Connect
or
Power-D/Combo-D
Connector, PCB
Mount,3pin,Male,40A,
1mOhm,Right Angle, Thru-
Hole
JS2
3 Connect
or
Header,DIN41612,Double
Row, 64Positions,2.54 mm
Pitch, Type B,Class2,Right
Angle, 300V,3A
J1
4 Connect
or
Header, DIN 41612,Tripple
Row,96 Positions,2.54 mm
Pitch, Right
Angle,250VAC,1A
J2
5 Connect
or
Header, 3Rows,30
Positions,2.54 mm Pitch,
Type C/3, CL2,1.5A, Male,
Right Angle
J3
6 Connect
or
SMB PCB Jack Receptacle
without Standoffs,50 Ohm,
Vertical, Thru-Hole
JS3
7 PCB 160x100mm, 4 layer,
2.4mm, FR4
---

5. PCB is designed with 2.6 mm stack, 2oz
copper thickness in each layer. It has 4
layers and total dimension of 160x100 mm.
Both ENIG and HASL finish are
manufactured. PCB is designed with extra
clearance for thick pin connectors to avoid
the fillet problem during the soldering.
Figure 5 3D View of Test Vehicle
Figure 6 shows 3 dimensional view of
test vehicle designed in Altium designer
software. Thick pin and multi row
connectors are selected as components.
Thick pin connectors simulate temperature
absorption and multi row connectors
validate the challenge of dwell time
deficiencies during soldering.
7. Assembly and Testing
Various profiles has been analyzed.
Selective soldering profile is designed based
on the PCB parameters and types of
components. Figure 6 shows the soldering
profile used for test vehicle.
Test vehicle is assembled in Pillarhouse
JADE 200 MK II machine. Soldering is
conducted with specifically designed
machine profile of 120° C preheat for 65
seconds and dwell time of 3.5 seconds.
APLHA NR 215 flux and ALPHA 63:37
leaded solder wire is used as consumables.
Figure 6 Soldering Profile of Test
Vehicle
All components are tested under X-Ray
after assembly to verify the void. It is
observed that void doesn’t occur in any of
the components. Figure 6 shows the test
result of connector J3 in X-Ray.
Figure 7 X-Ray Result of Test Vehicle -
Connector J3.

6. 8. Conclusion
Void defects from PCB design and
manufacturing, soldering consumables and
machine parameters are analysed and
corrected to resolve the void problem using
system engineering approach. Operational
deficiencies of PCB design parameters and
soldering consumables have been analysed.
Functional process flow for resolving void
issue is defined with enhancement in PCB
design, selection of consumables and
machine parameters design. A test vehicle is
also designed with the recommendations
from the concept definition.
Test vehicle for void verification is
assembled with optimized soldering
parameters and tested in X-Ray machine.
Each solder joint is verified with different
machine settings and power level of X-Ray.
Test report shows that even with 2.6 mm
PCB and thick pin components, void doesn’t
occur in the PTH. Single spot of air hole of
less than one percentage is observed in one
component pin, which is negligible in X-
Ray test. Finally design for
manufacturability (DFM) guideline is
proposed for PCB design to eliminate the
void issue caused by PCB design process.
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