With hotter preheats and higher peak temperatures, the classification of non-hermetic solid- state SMD components may change. Moisture pick-up can further aggravate component reliability with lead-free reflow soldering. Internal delamination, cracks, bond lifting, die lifting and in extreme cases pop-corning effects can occur.

The IPC/JEDEC J-STD-020C specifies the new reflow requirements for small to very large bodied components.

It is summarized in the following table.

Lead-free Temperature Profile for MSL Classification

Thickness/volume: Small components 350 mm3 or less, large 350-2000, very large 2000 mm3or more.

The flux chemistry used in lead-free pastes is designed to minimize several of the issues associated with higher reflow temperatures. Higher temperature issues can range from increased paste slump to charring of the residues. Solder paste manufacturers are using resins and gelling agents, which offer good hot slump resistance and good activators which are stable at higher preheats and higher peak reflow temperatures.

New activator packages, higher activation temperatures

New resins, low decomposition

New thermally stable gelling agents

Better surfactants

Additives to prevent oxidation

Alloy specific fluxes

Below are photographs of two lead-free solder pastes after hot slump testing at 185° C. Paste A has gelling agents incorporated within its flux system that do not prevent slumping. Paste B contains gelling agents, which prevent slumping at higher preheat temperatures; this paste will reduce the incidence of bridging, solder balls and mid-chip balling. Paste B has better hot slump behavior.

Solder pastes printed on inert ceramic and preheated at 185°C in air

Due to the lower wetting speeds associated with alternative lead-free solders, flux activation will be a critical factor in paste performance. No-clean and water washable solder pastes are being designed to not require nitrogen reflow and can produce reliable solder connections with good wetting in air. Water washable solder pastes with their higher concentration of activators will solder most metal finishes adequately. No-clean solder pastes will require careful selection of finishes to be soldered but also careful selection of a paste’s attributes. Some lead-free no- clean pastes are designed to solder adequately a variety of metal finishes; others have difficulty with second pass bare copper boards due to their lower activity. Some no-clean pastes require lower peak temperatures and others can withstand higher temperatures without charring or polymerization of the resins.

Nitrogen will impact solder joint cosmetics as seen in the photographs below. Solder paste reflow in nitrogen will offer brighter and more uniform solder joint surfaces. Nitrogen reflow also will enhance wetting with lead-free solders, especially on bare copper OSP surfaces. It must be noted that the vast majority of assemblers seek a solder paste that can be reflowed in air; so many lead-free paste chemistries are being developed with this in mind.

The test below was achieved by printing lead-free solder paste on a white ceramic substrate then reflowing one in air the other in nitrogen. Although the surfaces look different, this is only a surface reaction.

If the paste is selected carefully and the SMT process optimized to suit the chosen chemistry, the lead-free transition will be achievable without jeopardizing reliability and product yields.

Sometimes a solder paste is selected which does not fit the process, high speed printing may not be a necessity, a paste which will perform at lower peak temperatures not required. Solder paste formulation is all about creating certain attributes at the expense of others.

Common defects associated with lead-free are:

Mid-chip solder balling

Tomb-stoning

Bridging (shorts) on fine pitch QFP leads

Open joints

Non-Wetting

De-Wetting

Cold solder joints

Voids

Excessive dullness or surface cracks

Voids, flux by-products trapped within joints

These defects can be avoided in a properly optimized process with a lead-free solder paste designed to give adequate wetting, low slumping, and low voiding with lead-free alloys. Good activity at higher reflow temperatures will be had by choosing solder pastes with activator packages, which do not decompose before at 217 °C.

Lead-free solders due to their reduced wetting speeds will at times not completely solder the pads. This will be impacted by the flux activity, thermal profile used, board finish and the reflow atmosphere.

However some assemblers are modifying the stencil to have less aperture reduction. This requires a solder paste with excellent hot slump properties to avoid bridging, solder balls and mid-chip balling.

Other assemblers are seeking modifications to pad design. In most cases this is not considered an issue.

Exposed copper, OSP Board, SAC Alloy in air

In reference to solderability, SAC alloys will wet the metallization at substantially reduced rates when compared to 63/37. Solderability is impacted at both the speed of wetting and the degree of spread.

Pure tin finishes are the easiest to solder with SAC alloys. While bare copper OSP usually gives the poorest results especially on assemblies that have seen a previous thermal process. Silver Imm. and NiAu finishes give values between tin and copper, and are popular lead -free choices.

Silver Immersion Board and SAC solder

In reference to component finish compatibility and the use of leaded component terminations with lead-free solders, studies are still ongoing. Some OEM’S have found little impact to their products reliability when leaded terminations are soldered with SAC alloys. The Gintic Consortium, recent work on lead-free wave assembly, did report that the best results were obtained with lead-free components such as pure tin and copper tin. These finishes had fewer failures during flex testing using various thermal profiles and resulted in less solder cracks.

Bismuth containing paste and SnPb lead Close-up of fracture after 1000 hours at 150°C

 Solder joints containing bismuth such as using SnAgBiCu lead-free paste to solder lead bearing terminations is not recommended. High temperature storage at 1000 hours at 150°C resulted in more solder joint cracks. Typical failures are noticeable in the above micro-sectional photos of a gull-wing component.

As lead-free is slowly implemented in an organization, both leaded and lead-free solders will be used. It is important to maintain good traceability of components, boards and solder products. To assist the industry, component suppliers have started to adopt the new standard, JESD97, “Marking, Symbols, and Labels for the Identification of Lead-free Assemblies, Components and Devices”.

To avoid confusion with solder products, some manufacturers have adopted different colours and shapes for the packaging of lead-free soldering materials.

Training of all personnel will be required not only at the final

inspection level but also at incoming inspection, inventory Lead-free Packaging

control and at BOM levels. Good traceability of all components and boards, including solder materials will be required.

Other processes requiring attention are the hand soldering and rework processes. Although the lead-free solder spools may be packaged with different colours, the soldering irons used to solder lead-free parts must also be identified as to avoid cross contamination with leaded solder, which may also be used in close proximity. A simple way to do this is to label the soldering iron with a label stating “lead-free” and also applying fluorescent green tap on the handle.