SMT007 Magazine

SMT007-July2021

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50 SMT007 MAGAZINE I JULY 2021 e original intention of a "no clean" solder was to eliminate the post PCB assembly clean- ing process while still not risking any perfor- mance or long-term reliability degradation. Some industry surveys indicate that about one- half of assemblers using no-clean flux chemis- tries clean the PCB aer assembly. Many times, the first look or inspection of a reworked sol- der joint is the aesthetic appeal defined by the cleanliness inspection criteria found in IPC- A-610 (Section 10.6). Due to uneven heating from the rework process, corrosive no-clean flux activators may not be completely encap- sulated with activated flux. is means the activators are on the exposed circuits and are likely to cause corrosion and dendritic growth, thereby negatively impacting reliability. In PCB rework, a no-clean flux (Figure 1) is active when spread out on the board and the only way that that residue becomes safe to leave on the board is when it is properly activated via a temperature reflow cycle. at requires all the flux, even that spilling over to neighboring devices and not being subject to a reflow cycle, to be cleaned off the PCB. Hand solder and rework methods that do not sufficiently con- trol flux volume are going to leave dangerous residues such as chlorides and weak organic acids which may, in turn, impact the perfor- mance or reliability of the solder joints. ere are several ways to mitigate the prob- lems associated with rework and the associ- ated flux residues le behind: 1. Limit the volume of flux applied to the rework area. More flux means that there is a greater likelihood of non-benign residue being le on the board. 2. Proper training on flux and flux residue removal for any hand assembly or rework technician. is training should include the method of cleaning, the method of rinsing, as well as the disposal of any remnant solution. e training on "when to use" what cleaning solution is also important. 3. Proper training on the importance of activating the flux during rework. Cleaning the flux post rework is defined in Procedure 2.2.1 of the IPC 7711/7721 "Rework and Repair of Printed Circuit Assemblies." e procedure defines that up to 10 ml of clean- Knocking Down the Bone Pile Feature Column by Bob Wettermann, BEST INC. Cleaning of 'No Clean' Fluxes in PCB Rework Figure 1: No clean flux residue around through-hole pad.

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