There are many situations in electronics manufacturing where it is necessary to use high temperature solder. The act of implementing lead free solders will require soldering temperatures that are higher than standard Sn/Pb eutectic solders. Manufacturers soldering at high temperatures need to be aware of a few simple rules to prevent catastrophic component or board failures.
Solderability Issues
Keeping the pad sites clean and free of finger oils and salts has always been a key to ensuring solderability. Flux will do little to remove these contaminates from pad sites. Beginning a high temperature process with dirty or oily boards will only exacerbate solderability issues that will arise due to the increased temperature.
With increased heat comes increased oxidation. Oxidation will occur much more rapidly under high temperature conditions, making the need for a good cleaning flux even more critical. Furthermore, flux has a tendency to break down or evaporate when heated. The type of flux selected for use in high temperature environments must be able to resist thermal breakdown and be present and active throughout the soldering process. When troubleshooting flux related wetting issues, a good rule of thumb is that in non-wetted solder joints the flux has evaporated before the solder entered reflow, and in de-wetted solder joints the flux has evaporated during the reflow stage.
One way to combat increased oxidation is to use nitrogen to displace the oxygen present while at increased temperatures. It is recommended to use a nitrogen atmosphere in the final reflow zones during high temperature reflow soldering. Some manufacturers purge the entire reflow oven (including pre-heat and soak zones) with nitrogen and have seen significant improvements in wetting angle and joint structure. A nitrogen atmosphere is also recommended for wave soldering, as it reduces the presence of dross in the solder bath while slowing oxidation at the pad sites.
Component Integrity
Ensure that components that are to be exposed to higher temperatures can withstand those temperatures. Components not rated for the increased temperature will have to be added after the high-temp cycle or replaced with a more robust component. Additionally, many components are moisture sensitive, which is magnified in a high temperature environment. Increasing reflow temperatures from 220oC to 260oC will cause plastic components to lose anywhere from one to two levels of moisture sensitivity. These components may now need to be baked in order to drive off moisture prior to exposure to high temperature environments where rapid out gassing could cause pop-corning and delamination.
Board Materials
Printed circuit boards expand when heated. The internal layers of woven fiberglass cloth provide some constraint to expansion in the X and Y directions. However, when processing temperatures exceed the glass transition temperature (Tg) of the board material, PCBs will exhibit significant expansion in the Z direction causing stress to plated thru holes and micro-vias. The thermal performance of epoxy resin boards can be enhanced by using additives. Phenolic novolac, trifunctional and tetrafunctional additives have been used to increase the Tg of epoxy resin from 120oC to about 135oC. The Tg can be increased further to about 190oC by using vinyl phenols. Vinyl phenols not only increase Tg but reduce the Z-axis expansion at temperatures above Tg.
Polyimide resins were developed for fiberglass reinforced laminates requiring a Tg above that of normal soldering temperatures. Though more expensive than epoxy resins, typical polyimide glass transition temperatures exceed 290oC and exhibit no Z-axis expansion at soldering temperatures.
Cleaning
When performing high temperature soldering, it will be necessary to use fluxes of medium and high activity levels to combat increased pad-site oxidation. Halides or solids may also be necessary to ensure that the flux is thermally stable and aggressive. These fluxes can not be left on the board as they are very corrosive and encourage dendritic growth. If cleaning is important, as it is with high temperature solders, it follows that cleanliness testing is important as well. Be sure to qualify the high temperature process including the cleaning system and maintain applicable objective evidence of cleanliness.
Inspection
Manufactures may have to develop inspection criteria specific to high temperature soldering processes. Solder joints may be dull, matte, or even grainy as a result of increased oxidation, slower cool down, solder chemistry, and board/ component finishes.
Rework and Manual Soldering
When using conductive soldering equipment to process high temperature solder, care must be taken to avoid damaging sensitive parts, plated thru holes, and pads. Use heat sinks to protect sensitive parts. As with board materials, PCBs will expand and become thicker in heated areas when Tg is exceeded. To avoid lifting a pad or cracking a plated thru hole, always use a light touch and minimum dwell time.  The pin-to-cheek method can be used to demonstrate a light touch. If soldering times exceed 3-5 seconds, it may be necessary to pre-heat the board to bring it to near reflow temperature before beginning rework. |
