Ed Briggs

Example:

Figure 1: Example shown Indium8.9 flux with SAC lead-free alloy

The reason for approaching this subject is that often there has been some confusion in regards to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet.
























Figure 2: Max Slope

The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100°C or better. The change in temperature between ambient and the first zone then is a minimum of 75°C (assuming 25°C as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone

The focus of max slope is more from a component view point, to avoid thermal shock, usually 3°C/s is recommended as the upper limit

Figure 3: Ramp or Average Rate


The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. And is more practically used in a ramp to spike type profile

From the view point of the solder paste, the lower the ramp rate the better, usually 1-2°C/s. This is to drive off volatiles and help minimize solder defects such as solder balling, solder beading, and tombstoning. This rate becomes even more important as the solder paste deposit continually decreases in size, as we move to 0201’s and smaller discrete components and from 0.5mm pitch area array packages to 0.4mm and smaller. Due to this miniaturization, the observance of graping and head-in-pillow have become more common. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature.

I'd love to discuss this with you, if this topic is affecting your SMT process. If you'd like, feel free to contact me.

I just visited a customer that was converting from water soluble solder paste to no-clean. Not exactly a slam dunk transition as this customer found out.

During my visit, solder balls and solder beads were observed in the no-clean flux residue adjacent to discrete components (capacitor/resistors). These could potentially be a reliability concern…electrical shorts.

In water soluble processes, solder defects such as solder balling and beading can be washed away in the cleaning process…no worries. However, introducing a no-clean solder paste often requires that the process be “cleaned” up a bit. Here are some ways to do it:

STENCIL DESIGN:
My first step was to investigate the stencil design for these discrete components. Why? Because, since water soluble post-reflow residues (including solder balls & beads) are washed away, many customers will opt to place as much solder (1:1 ratio) as possible on the pads - to achieve a good solder joint. This is especially true for military or medical applications where a robust solder joint fillet is vital. However, because no-clean residues are typically not cleaned, the solder balls and solder beads remain in the flux residue and may produce electrical shorts.

When printing in a 1:1 ratio, especially if the stencil is thicker than average, solder paste is often pushed under the component and onto the solder mask during component placement. Upon reflow, the sub-component solder paste may not pull back into the solder joint. This is one way that solder balls/solder beads are produced.

No one wants to hear that they need to buy new stencils with reduced apertures, but I did recommend, in this case, that some aperture reduction be considered (generally down to 0402 components). Usually a 10-15% reduction, with home-plate or similar design, is common. Many stencil manufacturers are fully aware of the issue and can make suggestions on aperture designs.

REFLOW PROFILE:
Simultaneously, the reflow profile often needs to be adjusted. In the preheat portion of the typical reflow profile, the first few oven zones are used to drive off flux volatiles, making the paste less "mobile". A balance in the ramp rate is vital; too fast - and small “explosions” may cause paste to spatter into other areas; too slow - and two bad things happen: the flux will spread excessively and the flux activity can be exhausted.

Good Starting Points:

• Ramp rate (IMPORTANT: not max slope, see "Best Practices Reflow Profiling for Lead-Free SMT Assembly" for reference): 1°C/s
• Initial first zone setting: 100-110°C

Recently I was at a customer who reported that their SMT components were “blowing off” their PCB. In most of the instances the component was still on the PCB but completely off pad.

Further investigation showed that the solder paste print for the component was well defined and that the component, after pick-and-place, was not skewed but placed correctly within the paste deposit.

Inspection of the oven revealed there were no obstructions on the conveyor and that the conveyor did not vibrate or shake excessively during reflow. The static pressure (air flow within the oven) was set at a low pressure.

The reflow profile used was a ramp to peak type profile, peak and time above liquidus was well within paste spec limits, but the initial (first zone) was set at a low temperature, ~70°C.  After changing the first zone to 100°C the issue was resolved.

Solder paste flux chemistries are unique, performing a number of functions including printability and retention of the shape of the stencil aperture they are printed through. These flux chemistries include ingredients that you don’t find in a simple liquid flux. Because of this the reflow profile plays a very important roll. In the first heating zones the solvent in the flux chemistry needs to evaporate increasing the tack of the solder paste (ability of the paste to hold onto the component). At 70°C the solvent does not evaporate quickly enough, a minimum of 100-110°C is recommended.

Note that setting the first zone too high (>130°C) can cause solder defects such as solder balling and solder beading

Included in the Product Data Sheet, among other things, are parameters which guide the customer in designing an SMT reflow profile. The data sheet gives general recommendations, for time above liquidus, peak temperature, and ramp rate.

The reason for addressing this subject is that, often, there has been some confusion in regard to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet.

The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100°C or better. The change in temperature between ambient and the first zone then is a minimum of 75°C (assuming 25°C as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone

The focus of max slope is more from a component view point, to avoid thermal shock, usually 3°C/s is recommended as the upper limit

The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. And is more practically used in a ramp-to-spike type profile

From the view point of the solder paste, a low ramp rate is desired, usually 1-2°C/s. This  gently evaporates volatiles and helps minimize solder defects such as solder balling, solder beading, and tombstoning. This rate becomes even more important as the solder paste deposit continually decreases in size - as we move to 0201’s and smaller and from 0.5mm pitch BGA’s. Due to this miniaturization, the emergence of a defect known as "graping" has also become fairly well known. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature.

Graping is a phenomenon which appears as un-reflowed solder particles, typically seen on the surface of the solder joint.  



             Cross-section of “graped” solder joint





The graping phenomena has become more common due to some of the following issues:

1. Reduction of the stencil aperture to accommodate smaller and smaller discrete and passive components (i.e. move from 0603”s to 0402”s to 0201’s)
2. The use of finer particle size solder pastes to accommodate fine feature printing (move from Type 3 to Type 4 to now to Type 5)
3. Higher reflow characteristics for Pb-free soldering
4. The use of water-soluble vs. no-clean solder pastes. No-clean chemistries generally protect the solder powder particles and the metallized surfaces from oxidation during the heating process (after the activator package removes existing oxides). (so how does water-soluble fit into this?)

A combination of any of these factors may exhaust the capability of the solder paste flux to remove surface oxides. This depletes the flux and exposes solder paste particles to oxidation, which means the solder particles do not coalesce into the solder joint.

To avoid the graping phenomenon, use the following tips in setting up your reflow profile. The intent here is to decrease the amount of heat the solder paste experiences during the reflow process.

1. A ramp to peak profile is better than a soak profile   
2. Decrease total time in oven by adjusting the belt speed. A ramp rate of 1°C/ second from ambient to peak is recommended
3. Use a lower peak temperature - 235°-240°C
4. Shorten the TAL to 40-60seconds

Reflow profiling can be broken down into several phases. I generally use the following;

Preheat

Pre-reflow

Reflow

Cooling

Preheat Phase preconditions the PCB assembly prior to actual reflow, removes flux volatiles, and reduces thermal shock to the PCB assembly. Because the preheat phase is often the longest of phases the ramp rate (rate/rise of time vs. temperature) is often established in this phase.

Pre-reflow Phase involves flux activation to remove surface oxides (on mating surfaces as well as the solder paste particles themselves), further pre-conditions the PCB assembly before reflow, and can be utilized for the soak portion of the profile, if needed. A soak profile may be suggested to diminish any delta T between components if there are both very small and very large components or the physical size of the PCB assembly is very large in and of itself. A soak profile is also often suggested to reduce voiding in area array type packages, though with Pb-free chemistries, this is often not as effective as with SnPb.

Reflow Phase is where the mechanical/electrical connection is made through the formation of intermetallics. Peak temperature and TAL (time above liquidus) help define the actual reflow portion of the profile. Peak temperature 20-40°C above liquidus and TAL of 30-90s is common.

Cooling Phase determines the grain structure when solidified and is defined as the solder cools from the peak temperature to solidus. A fast cooling rate is desired to create a fine grain structure (most mechanically sound) but is limited by the differences in CTE (coefficient of thermal expansion) of the joining surfaces. If excessive, stress can be exerted on the solder joint or component, fracturing or tearing can occur. Cooling rate of 4°C/s is commonly suggested.

Ramp to Peak profile depicted

For more please see “Best Practices Reflow Profiling For Pb-free SMT Assembly"