When a flux is required to facilitate a bond in an engineered solder application, the flux type depends on the alloys involved, the temperature range and the surface you're dealing with. Herbert Ludowieg is one of our manufacturing engineers involved with engineered solders and flux coated preforms, he has this advice;
For those of us who grew up in colder climates, snowball fights were a right of passage. As a child I fell victim to the misconception that a really big snowball would hasten the victory I desired. I realized quickly that too much snow was hard to throw and basically useless. Needless to say I was pummeled into oblivion by my adversaries. It didn't take long for me to realize that there had to be "just enough" for optimal performance. When it comes to flux coating your engineered solder preform, the same rules apply.
"Since most surfaces involved are in good shape, starting with a low percentage of flux is best. Larger quantities can result in excessive cleanup after reflow and can change the dimensions on through holes by reducing the size of an opening. Ultimately, more is not always better. We have several customers who have reduced their flux percentage from more than 2% to 0.5% with excellent results, the parts are easier to handle and have a better overall experience."
These issues along with possible voiding can really make your flux coated preform experience a bad one. For further support and information, please use the following links:(Flux Coatings for Preforms) and (World Class Tech Support)