The EMPF is conducting a program to determine optimized gas mixtures for lead-free soldering.  The experiment is aimed at enhancing the manufacturability of lead-free products manufactured using no-clean pastes, reflow ovens, and processing atmospheres used on the production line.  

Electronics Manufacturing of Test Samples

Test Vehicle
The PCB assembly used throughout this experiment was a glass-epoxy (FR-4) substrate (P/N EMPF010, Figure 3-1) that was assembled using standard automated processing equipment.  The test vehicle incorporates an industry standardized Surface Insulation Resistance (SIR), pattern within a convenient snap-off design.  The test vehicle has flip chip features and SMT devices as well. 

The IPC-B-36 Standard Test Board and Assembly is defined within the IPC-9201 Surface Insulation Resistance Handbook.  (Often, IPC-A-36 is used to designate the artwork, and IPC-B-36 is used to designate the board.)  This board design is an accepted test vehicle for MIL-STD-2000A and J-STD-001 and is also used in cleaning studies.  For qualification, the substrate metallization is bare copper.  The board has four isolated quadrants with top side interdigitated patterns that are routed to the edge card contacts through vias and circuitry on the bottom side of the board.  There are ten surface insulation resistance (SIR) patterns on the board.  There are four relatively large comb patterns that have 0.006 in. lines and spaces, and an area of 3500 squares.  There are two patterns that utilize the SMT pads and have 0.025 inch spaces.  There are also inner and outer perimeter patterns that have 0.006 inch lines and spaces referred to as Y-patterns.  The PLCC28-TR-DC-Sn components placed on the SIR pattern and the 5 mil standoff height, presents a cleaning challenge.

For evaluation of a no-clean flux, the most important patterns are those on the SMT patterns.  Flux is not expected to migrate to the comb pattern since there is no turbulent cleaning process.  For evaluation of a cleaning process, particularly a water soluble flux, the low stand off height of the component poses a cleaning challenge and may cause entrapment of residues.   The four large comb patterns are appropriate for this type of evaluation.

Board Finsh: ENIG versus OSP/Cu
Boards are being evaluated with electroless nickel followed by immersion gold (ENIG) plating and bare copper with an organic solder preservative (OSP) finish.  The OSP finish approximates processes primarily used by users of quality gases, and since the surface is the most difficult to solder, these samples are expected to show differences within the range of the experimental design.  IPC 9201 states that the contact fingers are normally gold plated for compatibility with edge card connectors.  The remaining metallization is normally bare copper or can be any other surface finish.  Discussions with the IPC SIR committee have shed some light on the upcoming release of IPC 9201a, and they offered the following points:

• The B-36 board was originally designed to be bare copper because that was the cleanest finish back when the original work was conducted (circa 1988)
  
  
• J-STD-001 took a looser approach, allowing and encouraging the B-36 board to be closer to what was used in production.
  
  
• Since six mil lines and spaces have become routine, any of the plated finishes (ENIG, immersion tin/silver/PdAg), can be manufactured with good yield.
  
  
• OSP may provide an advantage since the copper etching can be controlled a little more precisely and has fewer plating steps than the other plated finishes.  The uniformity on the tight 6 mil patterns might be slightly better.
  
  
• Lastly, the main suggestion by the committee was to approximate the process being used.  If an ENIG board is being used in manufacturing, the SIR board should also have an ENIG finish.

Manufacturing Equipment
A reflow oven processed the boards using five specific processing atmospheres that varied oxygen (O2) content.  This allows a response surface plot to be created.  The processing atmosphere oxygen content levels ranged from 100ppm to dry air.  The partial pressure of oxygen was monitored from the reflow area in order to correlate for a given paste, the effects of O2 level and temperature on surface insulation resistance, wetting properties, visual appearance, and cleanliness.  The oven has manual flow meters that control the main gas input and an Air Doping Leak Valve to control the oxygen ppm-levels.  The gas flow rates for each of the zones were set for zones 1-6 at 130, 70, 100, 180, 80, and 50 liters per minute (LPM) respectively.  The highest flow rate was in the reflow zone.

The external liquid nitrogen source pressure was increased in order to increase the flow rate through the oven.  The main pressure was regulated at the machine to 80 psi.  The pressure entering the oven was 60 psi, to overcome the back pressure, the nitrogen for the air doping was regulated to 70 psi.  The external nitrogen tank was monitored so that its capacity was not depleted.

A Samsung SM3320 pick and place machine was used to accurately and quickly assemble the boards.  Three types of Pb-free no-clean solder pastes were used, and a no-clean tacky flux was also used with the flip-chip components.

Reflow
A typical reflow profile has an initial ramp, a soak, a spike to reflow, a reflow period, and a cool down zone.  Profiles are classified either as a soak profile, where the assembly is subjected to the same temperature for a period of time, or as a continuous ramp profile where the temperature is steadily ramped.  The melting point for SAC305 (Tin 96.5%, Silver 3.0%, Copper 0.5%) solder is 217°C, and the time above reflow (TAF) is referenced from there.  Figure 3-2 shows an example of one of the profiles used.  It shows that there is not much difference in peak temperatures between the air (above) and nitrogen (below) runs (nitrogen peaked about 1 degree lower on all settings).

Preliminary Results
A battery of tests is being conducted to examine wetting characteristics, solder voiding levels, and relationships between the surface insulation resistances.  Preliminary results show there are benefits to using specialty gas mixtures for solder reflow of lead-free components; the final results will quantify the process windows for obtaining maximum benefits.  For more information on how specialty gas mixtures may be used in an electronics manufacturing environment, please contact the EMPF.