Solution for Grape Effect

We are processing lead-free boards with large components that require high peak temperatures and slow conveyor speeds in are flow oven that does not use nitrogen.

Our smaller parts are showing the 'grape' effect. What is the best solution for this since we cannot increase the conveyor speed or lower temperatures?

A.M.

Expert Panel Responses

There are two possible causes for the rough surface of solder joints on the smaller components:

The flux is being exhausted prior to the solder being completely reflowed
The solder is picking up a lot of metallic contamination from the PWBs and/or the component terminations

Both of these causes may be contributing, but since you describe the "grape effect" I suspect that (1) dominates, and the surface you are seeing is incompletely-coalesced solder spheres.

Working on this assumption, there are two approaches. First, you can work to shorten the pre-reflow heat exposure. Whether this is possible depends on the capability of the reflow system and the mass density at the locations of the largest, heaviest components. Shortening the pre-reflow soak or ramp is the best approach. If you have an incapable reflow system, however, this option may not be open to you.

If you cannot gain ground by profile optimization, it may still be possible to improve your lot in life. You can look for a solder paste that maintains its activity at the extended times and temperatures you are experiencing. Talk to solder paste suppliers and discuss your issue in detail.

Fritz Byle
Process Engineer
Astronautics

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

See a solution offered by D. Ron Lasky from indium.

4. Likely solutions:
a. Use a square aperture. An square aperture provides more volume, and it has better transfer efficiency. (Transfer efficiency is the volume of the solder paste deposit divided by the volume of the aperture times 100.) The result would be > 30% more solder paste. The more solder paste, the less likely to experience graping.

b. The solder paste they were using was notbest of breed re: graping resistance. She recommended another one, which sheknew performed well in all respects - and minimized graping. This solder paste's flux was robust and designed to minimize defects like graping.

See more at:
http://blogs.indium.com/blog/an-interview-with-the-professor/patty-tackles-graping-in-china

Marc Peo
President
Heller Industries Inc.

Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

Switching solder paste formulas is your best bet. Ask your solder paste suppliers for a formula with a high temperature oxidation barrier. This is the component of solder paste that prevents the re-oxidation of the solder surface once the solder connection is formed.

Fluxes with high temperature synthetic resins are better than formulas with natural plant sourced rosin. You may also want to consider increasing the flux % in your paste.

Steve Stach
President
Austin American Technology

Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.

There are a few ways to combat graping.

Use a solder paste with higher activity. Graping can be caused by oxides on the solder powder surface. The activity level in the solder paste may not be high enough to deal with the oxide present on the solder powder. High temperature profiles, especially with long soak, can cause more oxide to form than the solder paste flux can deal with. A solder paste with a higher activity level can help with this.
Modify the reflow profile to eliminate a soak if one is used. A linear ramp to peak profile will help to minimize oxidation of the solder powder during the profile. This allows the solder paste flux to do it's job with maximum effectiveness.
Increase the solder paste volume on the pads that show graping. Graping becomes more common as solder paste is printed through smaller apertures, typically with area ratios below 0.6. The transfer efficiency decreases with decreasing area ratio. Low solder paste volumes can lead to graping. Changing the stencil aperture design increases solder paste volume. Use of a suitable nano-coating, like our NanoSlic Gold also increases transfer efficiency.

Answer submitted by Mike Scimeca and Tony Lentz

Mike Scimeca
President
FCT Assembly

Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

Your situation leaves little room for maneuver, the problem is flux exhaustion due to the high surface area to volume ratio of the small deposits and the extended preheats that you are probably using. This being said you need to remove heat from the front of the profile so look at shortening the preheat time or increasing ramp rates.

This will result in a larger Delta T during reflow but you will need to decide if you can live with this.Once the solder has coalesced further flux exhaustion will not impact the joint appearance. If these changes are not possible the you will need to look for anew solder paste with extended reflow performance, this is an issue of flux longevity more than pure activity.

Neil Poole
Senior Applications Chemist
Henkel Electronics

Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.

Graping is due to un-reflowed solder particles atop the solder mass. This results from higher lead-free reflow temperatures,the decrease in volume of the printed paste deposit, and finer powder particle sized solder pastes.

Following are some recommended steps (other than increasing conveyor speed or lower temperatures or use N2 gas):

Use RTP reflow profile instead of soak profile.
Shorten the TAL to 40-60 seconds.
Use no-clean resin based flux in place of water soluble paste for better oxidation protection.
Use square aperture stencil design for relatively higher paste volume and transfer efficiency.

(Source:Indium Corporation)

Santosh Kumar
R&D Manager
MK Electron Co. Ltd

Santosh Kumar is R&D Manager at the MK Electron Co. Ltd., Korea and engaged in the electronic interconnect materials development and technical marketing. His key focus is novel lead-free solder materials, electronics packaging, wire bonding materials and process.

Flux has exhausted is most likely the issue, try an alternative Paste or ask the Paste manufacturer to increase the flux content to last the higher heat and duration. Typically 12% flux should be sufficient.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

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