SMT & Inspection | April 30, 2010

Special Flux Characteristics

Continuation of two previous papers: ‘3 Steps to Successful Solder Paste Selection’ and ‘Flux Type Selection’
The last area to consider when finalizing solder paste selection is any other special characteristics. Two flux formulas can differ greatly in performance despite having the same QQ-S-571E and J-STD-004 classifications. Solder pastes with specific characteristics can be used to solve technical assembly problems that other forms of solder do not. The following are examples of flux characteristics that modify how a solder paste performs.

Reduced Slump: The reduction of solder paste spread after deposition, resulting in loss of definition. This feature is important when pads are close together and bridging is a risk.

Restricted Residue: The flux residue remains either on or very close to the fillet after reflow. This feature is most important with NC formulations where the joint is visible or the spread of flux to surrounding areas can cause a problem.

Halide-free: Halide-free solder pastes have an IPC J-STD-004A rating of 0 for the fourth character. Example: ROL0. Halides may be found in some flux activators. They assist in oxide removal due to their high energy state. Halides are materials that contain a halogen: Chloride, Bromide, Fluoride, or Iodide.

Low Residue: The quantity of flux residue left over after reflow is less than with normal solder pastes. Either there is less flux to begin with, or a larger percentage evaporates as part of the reflow process.

Difficult to Solder Surfaces:
Difficult to wet metals and oxidized surfaces can both require flux with higher activity or different activators that work better with the metal involved. Aged components, Alloy42 leads and the like are a special consideration for flux selection.

Gap Filling and/or Vertical Surfaces: The flux is designed to hold the alloy in place until liquidus is reached. These formulas are suited to bridging gaps, filling holes, and soldering joints on vertical surfaces.
Note: They are not normally successful in forcing a bridge between two adjacent, empty, pads on a circuit board in place of a zero ohm resistor. Surface tension of molten solder alloy is too high and the bridge breaks in the absence of vertical surfaces to stretch between.

Rapid Reflow: A term used to describe the heating of solder paste in under 5 seconds. A rapid reflow solder paste will not spatter when heated as quickly as ¼ of a second. Typical reflow methods that achieve rapid reflow include laser, solder iron, hot bar, and induction.

UV Traceable: A fluorescent dye is added to aid in the inspection of the solder deposit using UV lighting. Un-reflowed solder paste can be inspected with appropriate vision systems. Post reflow inspection can be conducted both with vision and human inspection under black light.

Pin Transfer or Dipping: An application technique where the solder is applied by dipping a component or pin into the solder paste. A thin, consistent layer of solder paste sticks to the component. This technique is useful in applying solder to products that do not lend themselves to printing or dispensing, such as pin arrays. Dipping paste is a useful way to apply additional solder paste to a through hole component when the amount of paste that can be printed for paste-in-hole is insufficient due to stencil thickness.

Low Void: IPC-7097A is the Specification for the Design and Assembly Process Implementation for BGAs. The inspection criteria for Ball Grid Array (BGA) and MicroBGA often call for voiding under 20% of area and some under 9% or even 4% depending on the location in the ball. A low-void solder paste is required to meet the very low voiding limits for Class 3 assemblies.

This paper covers the most significant issues in solder paste selection. There are additional details of alloy and flux performance not covered here that can be very important in the solder paste selection process. It is always worth a call to your solder products vendor to review your requirements to ensure you are using the best solder paste for the job.

Author: John Vivari z EFD, Inc.
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