Meeting green challenges in miniaturisation
12 May 2008
The world is moving rapidly toward green manufacturing. The miniaturisation trend of the electronics industry is constantly imposing new criteria, making the selection of ideal Pb-free alloys a moving target. Dr. Ning-Cheng Lee looks at the challenges poses by miniaturisation to both solders and fluxes.
Miniaturisation not only poses challenges to the solder market, flux chemistry is also complicated due to requirements for no-clean applications and the increasing amount of oxides. But first the considerations for solder alloys.
Future Pb-free solder alloys
1. Process Temperature Challenge
Reducing the size of microvia in pads will inevitably result in a greater sensitivity toward CTE mismatch. In addition, reducing the pitch dimension will significantly increase the chance of a circuit short caused by conductive anodic filament. To avoid the problems described above, a lower soldering temperature is highly desirable. Alloys of this type can be achieved with the use of Bi or In, such as 58Bi42Sn or 52In48Sn, and can be further improved by adding a small amount of additives, such as 57Bi42Sn1Ag.
2. Wetting Spread Challenge
Miniaturisation often results in reduced wetting. Solder alloys with lower viscosity should help both solder spread and hole-filling directly. On the other hand, alloys with lower surface tension elements, such as Bi, P, or Sb, often improve wetting considerably.
3. Voiding Challenge
A reduced joint size is also more prone to void formation, mainly due to a greater difficulty in wetting, and can be reduced by increasing the liquid solder wetting. Approaches for improving the solder wetting will also reduce voiding.
4. Wetting Speed Challenge
Decreasing discrete component size will inevitably result in an increase in vulnerability toward tombstoning, skewing, or billboarding. The most effective way to reduce those defects is by decreasing the solder wetting speed at the onset of solder melting. This can be accomplished by an increase in liquid solder paste behavior. Fig. 1 shows the decreasing tombstoning rate with an increase in solid content of solder at the onset of the melting temperature.
5. Fragility Challenge
Reduction in the size of the solder joint increases vulnerability toward shock cracking. Reducing the Ag or Cu content will reduce the hardness and fragility somewhat; however this can compromise the thermal cycling performance. Fragility can be reduced by adding a small amount of elements such as Mn, Ti, Ce, Bi, Y, Ni, Co, Pt.
This fragility could also be reduced by dopants, which can reduce Kirkendall void formation, such as Ni, In, high Cu, or by employing a high Cu, which reduces intermetallic compound spalling phenomenon and converts the solder to a more ductile material.
Refining the grain size by using dopants appears to be a feasible approach to nullifying the unfavorable orientation effect of anisotropic Sn crystals, thus reduce the chance of cracking.
6. Intermetallic Compound Plate Challenge
SAC Alloys (Sn/Ag/Cu) tend to form an Ag3Sn intermetallic compound (IMC) plate upon extended heating or slow cooling. This IMC plate threat increases with further miniaturization. Reducing the Ag content will decrease the probability of IMC plate formation, but likely at the expense of creep resistance. Additives that suppress this IMC plate formation may be feasible by a reduction in undercooling of the joints.
7. Corrosion Challenge
Pb-free solders are more prone to corrosion than Sn/Pb solders. This corrosion is attributed to galvanic corrosion induced by the presence of Ag. To minimize this issue, future alloys should avoid compositions with propensity toward galvanic corrosion, such as Ni or Co.
8. Oxidation Challenge
With air reflow soldering, oxidation of fine solder powder will increase as particle sizes decrease. This is due to the increasing exposed surface area of powder. (see Fig. 1).
The addition of elements such as Ge or P may be beneficial in minimising oxidation.
Future fluxes
In the course of advancing toward miniaturisation, the flux technology has to advance as well, in order to cope with the increasing demand on performance.
1. Reduced Emission of Volatiles
The major drivers for this feature are: (1) environmental considerations, (2) lowering the cost for oven flux management considerations, and (3) reducing the tendency toward voiding.
2. Halide-Free
This issue is driven by the questionable perception of halides being corrosive, as well as the convenience of passing the RoHS qualification inspection.
3. Greater Fluxing Capacity
The demand for greater fluxing capacity is due to unbalanced scale-down at miniaturisation. While the flux volume deposited may reduce in proportion to decreasing pitch and pad dimension, the oxide thickness of parts, pads, and powder does not reduce in parallel.
4. Higher Residue Resistivity
With reducing pitch, the resistivity of flux residue has to be elevated in order to prevent a decreasing resistance between electrodes.
5. More Resistant to Oxidation and Charring
With decreasing pitch, the rate of flux oxidation and charring under air will increase due to an increase in surface area per unit volume of flux.
6. More Efficient Oxidation Barrier
For a smaller flux/solder paste dot, oxidation of powder, pads, and parts will become more significant due to a shorter oxygen diffusion path. Therefore, fluxes with more efficient oxidation barrier capability will be needed.
7. Lower Activation Temperature
Fluxes with a low activation temperature are required if a low melting alloy is to be used.
8. Slower Wetting Speed When Solder Begins To Melt
This is critical for suppressing tombstoning, wicking, and walking parts caused by reduced component size.
9. Less Spattering
This can be achieved by employing a non-hygroscopic flux combined with a slow fluxing speed.
10. Higher Probe Penetrability
With solder dots getting smaller and smaller, the flux residue is more likely to harden due to more oxidation.
11. Capability of Inducing Nucleation of Solder Upon Cooling
Solder with a refined grain size can also be accomplished by using fluxes which could induce nucleation of the solder upon cooling.
12. Greater Resistance Against Slump
Solder paste with a finer powder is more prone to slumping, and requires the use of fluxes with a higher molten viscosity, or molecules with a higher molecular weight.
Dr. Ning-Cheng Lee works for the Indium Corporation
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