Bits on Lead free solder, not good for low current, high frequency circuits in most applications, (many formulas can self resonate at round 1.375 - 1.4MHz) and varies.
....Well this stuff does, had to drop the Buck down to 700K all because of this Lead free solder. Or could be incomplete joints or gaps.
Lead free has a higher than wanted melting point, usually around 432°F in typical range of greater than many IC's range. ..... using Kester 275 lead free 3.3%/275 (Diam 0.31").
Lead free solder can suspend particles of conductive material of the solder when not heated to the point of releasing a fume stream rise resulting in impedance resistance problems between pins! use caution with lead free on the temp and MOS territory of current ranges.
Component ideal.
Apply solder, let cool, reapply heat from heat source avg 30w iron as example, 3-4 seconds keeping at IC pin contact with board point (observing IC temp limit if datasheet states).
Overall RS may have deals, tho it has for some years been deals with a cost hidden in problems that tend to arise with the things they sell eventually depending on what it is and what it does on how it fails on what it was meant for and yet not sold for.