IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

Novel Method for PTH Soldering Driven by the

Thermal Restrictions of a PTH LCD Device

Cliff Chen, Michk Huang – IBM, Taiwan

Theron Lewis—IBM, US

ABSTRACT

The approaching deadline for the RoHS Server exemption expiration provides incentive to the server

manufacturing industry to accelerate the Pb-free transition for complex manufacturing processes. The Pb-

free soldering process requires a higher reflow temperature than standard eutectic SnPb solder to achieve

proper solder joint formation, but introduces more thermal stress to thermally/temperature sensitive

components (TSC), especially for the pin through hole (PTH) components attached by surface mount

soldering/intrusive soldering or legacy wave soldering processes. Hand soldering and solder fountain

(solder pot or selective wave) are two methods used as replacements for legacy wave-soldering to avoid

thermal damage to TSC’s. There are some challenges with hand soldering that include achieving the 50%

hole fill requirements and consistently performing a manual operation during high volume mass production.

Solder fountain has become the most reliable method for thermal sensitive PTH components to ensure

temperature limitations. In one application that it was required to use solder fountain was the soldering of a

Liquid Crystal Display (LCD) PTH component. This particular LCD component had very severe

temperature limitations restricting its exposure to 90~100℃. The method presented in this publication is to

reduce the thermal exposure of this specific TSC PTH component, a process utilizing a solder fountain with

some additional tools to reduce the heat transfer. In order to further control heat transfer via conduction,

convection, or radiation, the idea of applying a controllable air flow between the component body and

heated pins was conceived. The air flow between the heat source and component body formed an air wall

thermally isolating the component without causing a disturbance with the soldering process. A design of

experiments (DOE) using manufacturing parameters that influence component body temperature was

completed and provided optimized settings for the “air wall’s” effect. This paper documents the

experiment varying contact cycles, contact time and air flow that generated an effective air wall process

and discusses how the air wall successfully reduces heat transfer to the component body. It will compare

the thermal impact on the LCD component body of hand soldering, solder fountain, and solder fountain

with the “air wall” mechanism.

BACKGROUND

While working on thermal sensitive plated through hole (PTH) type components, there are commonly two

methods available for typical computer card assembly lines as a replacement for wave-soldering to avoid

thermal damage to the component. The two methods are hand soldering and solder fountain. Its very

challenging to reach IPC solder hole fill requirements with hand soldering and even a larger challenge with

multiple hand soldering operators in mass production. Solder fountain has become the most reliable

method for plated through hole component assembly when wave soldering is not acceptable/available.

Solder fountain provides more flexibility to control the heat transfer by varying its contact cycle and

dipping time. However in some cases, especially working on lead-free assembly, component may still be

damaged by the heat transferred to the component body during the assembly process.

In order to further enhance heat transfer via conduction, convection, or radiation, an idea of applying a

controllable air flow between the component body and heated pins to isolate heat transferring to thermal

sensitive component’s body was raised. Normally thermal sensitive component’s temperature limitation is

around 90~100℃. The air flow between the heat source and component body is to form an air wall which

can help reduce the heat transfer to component without causing soldering disturbance during dipping.

222

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

This paper will study the thermal impact on the component body of a PTH component by using hand

soldering, solder fountain, and solder fountain with “air wall” mechanism. The variables to be monitored

were contact cycles, contact time, and air flow in the case when air wall was applied.

Hand Soldering:

Hand soldering applies a peak of high temperature through solder iron to PTH. Normally several contact

cycles with certain contact time is applied. Thermal profiling as shown in Figure 1 illustrates the common

temperature changes on a component pin when hand soldering is applied. When the soldering-iron touched

the component pin, the temperature rises. Also shown in Figure 1, the peak temperature at each cycle has

some deviation, which demonstrates that inability of providing a repeatable process and adding to the

concerns with hand soldering.

Figure 1 Component pin peak temperature cycle in hand soldering

(Green line shows pin temperature. Purple line shows the component body.)

In addition, there are several common defects caused by heat transfer using hand soldering, but the most common is

PTH/Pad fillet lifting. Pad lifting, fillet lifting and fillet tearing are effects resulting in part from the differences in

thermal expansion coefficients of the PCB base material, the epoxy/glass laminate, the copper barrels, and copper

tracks on the PCB. During contact with solder iron, a rapid temperature increase resulted in a relative large thermal

expansion of the board material in the Z-direction. This expansion triggers a deformation at the joint pad and gives a

conical shape to the pad. This is caused by the difference in thermal coefficient between the epoxy and the copper

hole-wall metal. Even after the joint has passed the select wave or dipped into the solder of the nozzle, board

expansion continues because much of the solidification heat has been transmitted to the adjacent board material [refer

1].

Pad lifted is a potential process defect indicated in IPC-610. If the pad lifted height is over one pad thickness, it is a

defect. Although pad lifted is acceptable based on this rule, it is still a good practice to eliminate this symptom through

process improvement.

INITIAL EFFORTS

Hand soldering with Pb-free solder is more complicated/difficult then legacy SnPb soldering. The higher solder iron

temperature applies thermal shock to the pin and PCB causing hole fillet lifted from PTH copper. Figure 2 shows a

typical pad lift caused by hand soldering. Another potential defect of pad lifted can be found at the left of Figure 2. A

hole fillet lifted happened due to the characteristic of lead-free solder alloy Sn-Ag-Cu.

Figure 2

223

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

Since there are several disadvantages to apply hand soldering for thermal sensitive PTH component, a more

advantageous technique is to use solder fountain process, or specifically mini wave soldering or selective

wave soldering. The heating/soldering reaction and process steps are quite different with mini-wave

soldering in comparions to hand soldering. Figure 3 illustrates the component body temperature alteration

during mini wave soldering process. The graph shows a much steady temperature change comparing to

hand soldering process. Maintaining the component body temperature to lower temperatures during the

process ensures the reliability of the component. In the case study of this paper, the component body

temperature needs to be under 100 ℃. Other than controlling the solder wave contact time with component

pin and contact cycle, a common method used to prevent component being damaged by massive heat is

using metal shielding to cover the top of component. The purpose of the metal shielding covers the

component and is suppose to reflect heat away from the component and increase the components mass, but

this method is not effective for thermal sensitive components due to often the heat shield will not be

sufficient to reflect/insulate the heat away.

Figure 3. Component temperature rising during mini wave soldering

In order to effectively control the temperature of the thermal sensitive PTH component during mini wave

soldering process, an air flow controlling mechanism has been invented. The concept used to prevent the

component from damage by heat is to limit the amount of heat reaching the component body. This concept

directs the air flow through the gap to form an air wall, and reduces the amount of heat energy transferring

to the component. What distinguished this idea from most others is that it is not designed to guide the air

directly blowing to the component to cool down the component body but to be applied across / parallel to

the component body and takes advantage of the gap between the PCB and the component. As shown in

Figure 4, the tool textured in blue is used to control air flow to form a flowing air barrier between the heat

Pad lifted from PCB

Hole fillet lifted from PTH

copper

224

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

source and the component body, like an air buffer. The air flow channel is forced only through the gap and

not blown directly on the soldering pins avoiding solder disturbances.

Figure 4 Thermal sensitive components –side view

Side view

In order to investigate how the air flow layer impacting mini wave soldering process, a design of

experiment (DOE) was performed to study the effect.

Design of experiment

To study the mini-wave soldering process utilizing the newly designed air flow control tool, three

parameters were chosen to be monitors: contact time, dipping cycle, and air flow. A three-level matrix

experiment was planned to assess the effect of each parameter and help establish the critical combination to

make a successful process. Table 1 below shows the parameters with their respective levels.

Table 1

Factors Level

Contact time(sec) 3 6 9

Cycle time(number) 3 6 9

Air flow(cm3/sec) 1 1.5 2

The design of experiment will assess the main effect on the change of component’s body temperature and

also the parameters’ interaction with each other. The design of experiment includes 27 combinations to

assess. After discovering the best combination of parameters, this combination will be used to run on the

product and PTH solder fillet, pad lifting, and hole fill will be monitored and evaluated.

Main Impact to Component Temperature.

From the thermal profiling of the TSC and going through the 27 trials, Figure 5 shows the relation between

component temperature changes against each of the three parameters individually. The increase in contact

PCB

Thermal sensitive component

tool to form air flow

PCB

Thermal sensitive component

Tool to form air flow

225

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

time and increase number of contact cycle gave a positive impact on raising the component temperature.

Rising solder wave contact time from 3 seconds to 9 seconds provides a relatively consistent increase in

component body temperature. Increasing contact cycle from 3 times to 6 times did not reveal much

difference in terms of component temperature change, but increasing contact cycle from 6 times to 9 times

generates a major reaction with a big slope showing significant impact to the raising of component

temperature. On the other hand, air flow presents a negative impact on raising the component body

temperature. The result also shows a stronger air flow tends to reduces the surrounding area temperature

and consequently reduces the component body temperature.

Figure 5

A3A2A1

100

95

90

85

80

B3B2B1

C3C2C1

100

95

90

85

80

contact time(sec) cycle(Times)

Air flow(Mpa)

Interaction between Three Factors

Examining the interactions of contact time, contact cycle and air flow was complex especially with the

three level defined. A cross analysis was done as shown in Figure 6. When studying contact cycle versus

contact time, it was observed that increasing contact cycle or increasing contact time both gives positive

impact to component body temperature increase. However, if contact cycle was relatively low, contact

time would become an indistinctive factor to component temperature change. When studying contact cycle

versus air flow, it was apparent that air flow dominates the impact on reducing component body

temperature. One result was that the impact of air flow was more evident at fewer contact cycles. In

addition, when the contact time increased, the air flow at 1.5cm3/sec or 2 cm3/sec gave a similar affect to

the component body temperature. This behavior can be expressed as the heat transfer was more efficient

through conduction than convection / radiation.

Figure 6

226

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

From conducting an ANOVA, Air flow had the greatest affect, followed by contact time, followed by

cycle time. ANOVA shows that all three variables play a significant part to controlling the component’s

temperature.

Table 2. Analysis of Variance for Thermal Sensitive Component Temperature

Source DF Seq SS Adj SS Adj MS F P

contact time(sec) 2 2453.63 2453.63 1226.81 45.33 0.000

cycle(Times) 2 812.07 812.07 406.04 15.00 0.002

Air flow(Mpa) 2 3090.74 3090.74 1545.37 57.10 0.000

contact time(sec)*cycle(Times) 4 281.93 281.93 70.48 2.60 0.116

contact time(sec)*Air flow(Mpa) 4 278.59 278.59 69.65 2.57 0.119

cycle(Times)*Air flow(Mpa) 4 134.15 134.15 33.54 1.24 0.368

Error 8 216.52 216.52 27.06

Total 26 7267.63

S = 5.20239 R-Sq = 97.02% R-Sq(adj) = 90.32%

For mini-wave soldering, longer contact time or more contact cycle can reduce the chance of having PTH

pad lifted and provide better solder hole fillet, but at the same time it can raise the component temperature

over its acceptable range. Choosing a suitable parameter sets is critical to not only prohibit damage to the

component but also provide ideal solder joint.

Figure 7, 8 and 9 illustrate contour plots of the temperature change of a thermal sensitive component versus

contact time and cycle times, temperature change of a thermal sensitive component versus air flow and

cycle count, and temperature change of a thermal sensitive component versus air flow and contact time

respectively. These three plots clearly show the parameter zone versus component temperature. Several

parameter sets, which best fit within the desired component temperature during mini-wave soldering

process, were chosen utilizing these contour plots.

Figur7 Figure 8

Figure 9

227

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

The Verification on Production Level Cards

Since the temperature limit for the thermal sensitive LCD module is 100℃, the resulting maximum

temperature was set at 100℃ to avoid any thermal damage to the component body. As a result, the running

parameters chosen on (contact time, cycle, air flow) was (6,6,1.5) , (6,9.2) and (9,6,2). More solder wave

cycle and contact time could have been achieved for better solder fill in the PTH’s, but was concerned with

the possible pad lifting due to potential temperature shock. Figure 10, Figure 11, and Figure 12 below

shows the x-ray result of the solder hole fill through the three process trials: (6,6,1.5) , (6,9.2) and (9,6,2)

respectively.

Figure 10

Contact time:6 sec ; cycle times: 6 : air flow 1.5

Figure11 Figure 12

Contact time:6 sec ; cycle times: 9 : air flow 2 Contact time:9 sec ; cycle times:

6 : air flow 2

As shown in the x-ray pictures above, the three chosen parameter sets can all fulfill the PTH solder fill

requirements stated in latest level of IPC-A-610. In order to reduce temperature shock generated by mini-

wave soldering contact cycle and contact time, a lower cycle and shorter contact time was selected:

(6,6,1.5).

To prove process stability/repeatability, multiple production cards were run and 5 sample cards were

randomly chosen to examine the fillet result through x-ray. Pad lifted was detected as shown in Figure 12.

Cross sectioning showed a slight pad lifting as observed in Figure 13; however, the PTH fillet was not

obvious compared to hand soldering.

Figure 12

228

IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

Conclusion

The goal of this case study was to assess how to prevent damage to thermal sensitive components by

assessing lead-free hand soldering, solder fountain, and solder fountain with “air wall” mechanism

processes. Experimental results of the hand soldering samples showed susceptibility to pad/PTH lifting and

incomplete solder hole fill. In addition hand soldering prove to be non-manufacturable for mass production.

Solder fountain/mini-wave introduced a much more controlled temperature gradient across the component,

but it did not meet the LCD component body temperature limitation even with a metal shielding. The most

successful technique was using mini-wave solder with “air wall” mechanism that ensured a consistent heat

source and can effectively reduce heat transfer to the component body during the process. The air flow

reduced the amount heat that transferred to the component body, and it was discovered that the air flow rate

can also improve the amount of dipping contact time and contact cycle to reduce copper dissolution during

soldering operation. A design of experiments (DOE) using manufacturing parameters that influence

component body temperature demonstrated that setting parameter of contact cycle (times), contact time

(seconds) and air flow (Mpa) to (6, 6, 1.5) generates the most effective air flow/barrier process to reduces

heat transfer to the component body to meet the temperature limits of the thermal sensitive LCD module

and fulfill the PTH hole fillet requirement stated in IPC-A-610.

229