Manual Soldering and Repair Challenges in
the Lead-free Soldering Era
Inge Schildermans
Alcatel Bell, Geel
Introduction
>
RoHS directive
>
Solder processes
•
Reflow soldering
•
Wave soldering
RoHS Gorinchem 23/11/05 — 2
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Content
1.
Manual Soldering
> Introduction
> Lead-free solder wire evaluation
> Impact of higher melting temperature:
a physics perspective
2.
Component replacement (repair)
> Introduction
> Convection heating limitations
> Radiative heating: overcoming the limitations
RoHS Gorinchem 23/11/05 — 3
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1. Manual soldering: Introduction
Manual soldering:
Create a solder joint between component lead and board pad or
PTH using flux-cored solder wire as solder supply and
conduction heating using a soldering iron as heat source.
RoHS Gorinchem 23/11/05 — 4
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1. Manual soldering: Introduction
Soldering stations:
RoHS Gorinchem 23/11/05 — 5
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1. Manual soldering: Introduction
Important parameters of soldering stations:
>
Power rating (W): heating power capacity of the soldering iron
>
Temperature control
•
•
>
Temperature controlled iron (power control circuitry)
Bimetallic soldering pins with specific temperature set point (Metcal)
Soldering bits or tips: different shapes and sizes
RoHS Gorinchem 23/11/05 — 6
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1. Manual soldering: Introduction
Solder
>
Flux-cored solder wire
>
Parameters:
•
•
•
•
>
Alloy
Type of flux
Flux amount
Diameter (0.25-1.5mm)
Lead-free:
•
•
higher melting temperature
reduced wetting
RoHS Gorinchem 23/11/05 — 7
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1. Manual soldering:
Lead-free solder wire evaluation
>
Alloy choice: SnAg3-4%Cu0.5-0.7% is becoming the industries leadfree alloy choice.
>
Solder wiring evaluation:
•
•
•
Contrary to solder pastes and fluxes there is very little
standardisation around solder wire testing.
Define solder wire requirements
Define solder wire tests:
Reliability evaluation: corrosion, SIR, Electro-Migration
Wire performance requirements
– Control of flux amount
– Tackiness of flux remainders
– Flux spitting: contamination of PBA, esthetics, ergonomics
– Solderability test
–
>
Flux-cored solder wires from different suppliers were evaluated.
RoHS Gorinchem 23/11/05 — 8
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1. Manual soldering:
Lead-free solder wire evaluation
Reliability requirements
>
Evaluation of corrosiveness of fluxes
•
•
>
SIR (Surface Insulation Resistance), electromigration evaluation
according to telecommunication standard
•
>
J-STD-004 flux classification
acceptable classes: L0, L1, (M0)
Telcordia (Bellcore): GR-78-CORE standard
Test results supplied by solder material supplier
RoHS Gorinchem 23/11/05 — 9
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1. Manual soldering:
Lead-free solder wire evaluation
Performance requirements
Metal/flux content
•
•
•
The actual weight percentage of flux
contained in the wire is measured.
Goal: quality control of wire
Procedure
–
–
–
–
–
–
Weigh adequate amount (10g .. 50g) of
solder wire.
Melt wire in a beaker,
Remove flux residues with IPA
Dry
Weigh left-over metal
Determine flux and metal content
RoHS Gorinchem 23/11/05 — 10
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Extracted metal
1. Manual soldering:
Lead-free solder wire evaluation
Performance requirements
Flux remainder tackiness
>
>
Flux residues should not be tacky to avoid contaminating particles to
accumulate between solder joints which may cause SIR issues
Test:
• Reflow solder wire on a copper surface
• Cover with chalk powder
• Should be possible to wipe off the powder easily with a soft brush
RoHS Gorinchem 23/11/05 — 11
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1. Manual soldering:
Lead-free solder wire evaluation
Performance requirements
Flux spitting
>
Rapid heating of the flux cored wire lead flux spattering on
PCB and operators hands. Should be minimal.
>
Test
•
•
•
•
Melt specific amount of solder wire
under controlled conditions
Collect droplets of flux that are ejected
Determine the weight of the collected flux
Calculate percentage of ejected flux
RoHS Gorinchem 23/11/05 — 12
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1. Manual soldering:
Lead-free solder wire evaluation
Performance requirements
Solderability (solder spread)
•
•
A solder wire ring with an inner diameter of 3mm,
is allowed to wet a preconditioned copper foil.
Visual evaluation of solder spread
++
0
--
RoHS Gorinchem 23/11/05 — 13
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1. Manual soldering:
Lead-free solder wire evaluation
Performance evaluation results:
Spread factor:
++
RoHS Gorinchem 23/11/05 — 14
0
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1. Manual soldering:
Lead-free solder wire evaluation
Not all commonly available solder wires are fulfilling the no-clean
reliability requirements!
Be careful with those that wet well!
RoHS Gorinchem 23/11/05 — 15
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1. Manual soldering:
Lead-free solder wire evaluation
Reliability evaluation results:
SW1 supplier A
-
Telecom
requirement
fullfilled?
-
SW2 supplier B
ROM1
-
SW3 supplier C
ROM1
-
SW4 supplier D
ROM0
-
SW5 supplier D
ROM1
-
SW6 supplier D
ROM1
-
SW7 supplier D
ORL0
?
SW8 supplier E
ORL0
-
SW9 supplier E
ROL1
+
SW10 supplier F
REL0
+
SW11 supplier G
ROL1
+
Flux Act. Class.
J-STD-004
RoHS Gorinchem 23/11/05 — 16
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Selected
wire
1. Manual soldering:
Impact of higher melting temperature
How to define the manual soldering process parameters?
Parameters on which the temperature of the soldering area depends:
•
•
•
PBA/PCB: thickness, build-up, land/PTH-size, material (thermal
conductivity, thermal capacity), component to be soldered,...
Soldering iron: set temperature, power, thermal capacity, temperature
control, tip properties (dimensions, shape, material,...),...
Soldering method: solder supply, board preheating temperature, iron
position, contact time,...
What approach should we take?
>
Leave it to the (chinese) operator to find out?
>
Set-up a 10+ parameter full-factorial Design-of-Experiments?
Let’s have a look at some basic thermal physics.
RoHS Gorinchem 23/11/05 — 17
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1. Manual soldering:
Impact of higher melting temperature
Basic thermal elements to
take into account
iron
PCB
RoHS Gorinchem 23/11/05 — 18
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1. Manual soldering:
Impact of higher melting temperature
Basic physical model:
TB
•
T: solder joint temperature
•
TB: soldering iron temperature
•
T0: base temperature of the PBA
T
T0
∂T − T0 TB − T T − T0
=
−
Cp
∂t
r + rc
R
Power steering conditions: P
∂TB − T0
TB − T
= P−
CB
∂t
r + rc
or
∂T − T0 TB − T T − T0
Cp
=
−
r + rc
R
∂t
At temperature control around TBset
∂T − T
T −T
CB B 0 = − B
→ TB ≥ TBset
∂t
r + rc
RoHS Gorinchem 23/11/05 — 19
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1. Manual soldering:
Impact of higher melting temperature
How to reach a higher temperature?
The stationary case:
(
R
TBset − T0
T − T0 =
R + r + rc
)
Method 1: Increase iron temperature
•
Too high a temperature can damage the component or PCB
Method 2: Minimize resistance r+rc of the iron and the contact area
Good soldering practice
•
Select a solder tip that
matches the soldering area
•
Take care of proper tinning of the solder tip to reduce contact
resistance
RoHS Gorinchem 23/11/05 — 20
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1. Manual soldering:
Impact of higher melting temperature
Dynamical analysis: solve the set of differential equations
 t' 
 t' 
T − T0
RP




exp
exp
A
A
=
−
+
−
+
1
2
 τ 
 τ  T set − T
TBset − T0
 1
 2 B
0
 t' 
 t '  ( R + r + rc ) P
TB − T0


= A3 exp −  + A4 exp −  +
set
set
T
TB − T0
τ
τ
B − T0
 1
 2
 C B r + rc (r + rc ) P 
t

t' =
,
, set
⇒ A1 , A2 , A3 , A4 ,τ 1 ,τ 2 = f i 


C
R
T
T
RC p
−
p
B
0


Board temperature profile depends on a set of dimensionless ratios:
•
Iron power rating P versus iron temperature setting TBset
•
Thermal resistance of the iron r+rc versus PCB resistance R
•
Thermal capacity of iron CB versus local PBA capacitance Cp
RoHS Gorinchem 23/11/05 — 21
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1. Manual soldering:
Impact of higher melting temperature
Normalised temperature profile (power steering only):
TBset
Tmelt
soldering
domain
RoHS Gorinchem 23/11/05 — 22
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1. Manual soldering:
Impact of higher melting temperature
Conclusion
> For save soldering one needs to limit the soldering iron
temperature while still meeting the higher melting temperature
requirements of lead-free soldering.
> Soldering temperature conditions on the board should be
obtained in a pre-defined region of the temperature profile for
reproducible results.
> This should be done by carefully selecting soldering tips, irons
and the temperature/power setting in accordance with the
thermal properties of the PBA to be soldered. Thermal capacity,
thermal resistance and heating power are of major importance.
> Physical modelling gives a basic tool to master the large range
of parameters influencing manual soldering.
RoHS Gorinchem 23/11/05 — 23
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Content
1.
Manual Soldering
> Introduction
> Lead-free solder wire evaluation
> Impact of higher melting temperature:
a physics perspective
2.
Component replacement (repair)
> Introduction
> Convection heating limitations
> Radiative heating: overcoming the limitations
RoHS Gorinchem 23/11/05 — 24
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2. Component replacement: introduction
For components with
interconnections under the
body of the component,
e.g., BGA (Ball Grid Array)
and LLP (LeadLess Package)
removal and resoldering the
components using a soldering iron
is not possible. Dedicated repair
machines are required
Most of the machines are based on
forced convection heating. A
temperature profile approaching the
standard reflow profile is used to
replace these components.
RoHS Gorinchem 23/11/05 — 25
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2. Component replacement: introduction
Principle of forced convection:
RoHS Gorinchem 23/11/05 — 26
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2. Component replacement: introduction
Increased melt temperature results in increased operating
temperature and smaller process window for lead-free rework.
PBGA
260,00
240,00
220,00
200,00
160,00
°C
Temperature
profiling
is critical:
180,00
140,00
120,00
100,00
80,00
60,00
Board Centre (max 230°)
40,00
20,00
0,00
0
50
100
150
200
250
300
350
400
Seconds
Depends on: temperature setting per heating period, number of
heating periods, air velocity, nozzle shape/dimension,
PCB Pre-heating,...
RoHS Gorinchem 23/11/05 — 27
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2. Component replacement:
Convection heating limitations
First order thermal model:
T − T0
∂T − T0
Cp
= H (Tair − T ) −
R
∂t
Maximum temperature: Tmax
R
(Tair − T0 )
− T0 =
R +1 H
How to increase the temperature?
Method 1: increase air temperature
•
•
Danger of local overheating of component
Equipment limitation
Method 2: increase the convection heat transfer coefficient H
•
H ~ area x air speed
•
problematic for small components: small area
air speed increase limited: risk of blowing away components.
•
RoHS Gorinchem 23/11/05 — 28
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2. Component replacement:
Convection heating limitations
Increased temperature difference across
the component
TEBGA
260
240
220
200
180
°C
160
140
120
Comp Centre (max 249°)
100
80
Comp Corner (max 261°)
o
∆T = 30 C
60
40
Board Centre (max 231°)
Board Corner (max 236°)
20
0
0
50
100
150
200
250
Seconds
RoHS Gorinchem 23/11/05 — 29
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300
350
400
450
500
2. Component replacement:
Radiative heating: overcoming the limitations
Is there a solution to overcome the convection heating limitations?
Yes: Radiative heating
∂T − T0
T − T0
C p
= α IA −
R
∂t
T max − T 0 = R α IA
I intensity, α absorption coefficient,
A surface area
>
No (physical) heat transfer limitation
>
No danger of component blow-off
>
Overcomes temperature limitation
RoHS Gorinchem 23/11/05 — 30
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2. Component replacement:
Radiative heating: overcoming the limitations
Temperature difference across the component
A uniform temperature across a component by local heating
requires heating the edges of the component more than its center.
(Note: principle demonstrating drawings only)
RoHS Gorinchem 23/11/05 — 31
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2. Component replacement:
Radiative heating: overcoming the limitations
Convection heating: use of nozzles with peripheral openings only
>
Reduction of air flow and heating area reduces
the maximum temperature
>
Only a limited heat input modulation is
possible
Radiative heating: Exploit the disadvantage of IR heating in mass
reflow soldering as an advantage for local heating.
>
Heat input control using optical modulation. Near 100%
modulation possible.
>
Add reflective tape (Al) to limit heat input
>
Add absorbing black tape to increase heat input
RoHS Gorinchem 23/11/05 — 32
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2. Component replacement:
Radiative heating: overcoming the limitations
Conclusions
>
Infrared radiative local heating technique is a very promising
candidate to overcome the limitations set by the conventional
local convection technique.
>
It is at least an important complementary technique to
convection heating in order to deal with the repair challenges
imposed by the switch to lead-free soldering.
>
To be considered also: laser repair
RoHS Gorinchem 23/11/05 — 33
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2. Component replacement:
modeling the repair convection process

rt 2 Ω  
a2
 − t 
T (t ) − T0 = (Tmax − T0 )1 − exp
  + 4 2 exp 2
/
C
H
a σ   4 rt + 4 Kt




(
)

 − a2  
− t 

 − 1 − exp
 − 1
 exp
2 

 C / H 

 4 * rt  
Tmax = Φ(Tair , nozzle, PCB properties)
Tair = Φ(Tset , t , flow, nozzle)
a
Qin
Quit
a/2
Quit
RoHS Gorinchem 23/11/05 — 34
Tlucht
Quit
A
r
Quit
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 

 
Thank you!
www.alcatel.be/EMS
Alcatel Bell Geel
Tel.: +32 14 572 142
Bell telephonelaan 3
Fax: +32 14 572 294
B-2440 Geel
sales.geel@alcatel.be
België
RoHS Gorinchem 23/11/05 — 35
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