|Ask the Experts|
September 2, 2020
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?
|Expert Panel Responses|
There are two possible causes for the rough surface of solder joints on the smaller components:
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.
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:
Heller Industries Inc.
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.
Austin American Technology
There are a few ways to combat graping.
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.
Senior Applications Chemist
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):
MK Electron Co. Ltd
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.
Technical Sales Manager
BLT Circuit Services Ltd
|Submit A Comment|
Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.
Insert Your Email Address