Clean “no-clean,” or use a water wash solder paste?
Saponifier detergent: 20%
Cleaning time: 12 minutes Cleaning temp: 60˚C
Rinses: 6 rinses with Di-water
Dry time: 12 minutes Dry temperature: 65˚C Table 2. Cleaning conditions batch washer.
left on the assembly are defined in IPC J-STD-001E: • Class 1 assemblies less than 200 mg/cm²
• Class 2 assemblies less than 100 mg/cm²
• Class 3 assemblies less than 40 mg/cm²
Saponifier detergent: 20%
Cleaning time: 12 minutes Cleaning temps 60˚C Frequency: 30 kHz
Rinses: 4 rinses with Di-water
Dry time: 8 minutes Dry temperature: 65˚C Table 3. Ultrasonic cleaning conditions.
The spray in air test was conducted in a batch washing machine using an aqueous cleaner and the parameters shown in Table 2.
Ultrasonic cleaning of printed circuit
assemblies has been under discussion for 50 years. According to IPC-STD001E, ultrasonic cleaning is permissible:
Cleaning time
Components 20 min 40 min 60 min MELF 0402 0603 1206
-
X X X
0 - -
X
0 0 0 -
Table 4. 0 = all residues gone, - = some residues left, X = residues still there.
1. On bare boards or assemblies, provided only terminals or connectors without internal electronics are present.
2. On electronic assemblies with electrical components, provided the manufacturer has documentation available for review showing that the use of ultrasonic does not damage the mechanical or electrical performance of the product or components being cleaned.3
Modern ultrasonic cleaners have variable frequencies to prevent the build up of potentially damaging harmonics. The bare test board was cleaned in a single tank ultrasonic cleaning system. Visual inspection of the boards after
No-clean SnPb paste—before cleaning No-clean SnPb paste—after cleaning
cleaning showed that all flux residues were removed and the solder joints appeared cosmetically clean. The test boards were tested for ionic
residues. Results for the three different alloys and two washing methods are listed in the Figure 4.
No-clean SAC 305 paste—before cleaning No-clean SAC305 paste—after cleaning
Small components with low stand-off Potential risk of contamination Electrochemical migration may occur between adjacent conductors under an applied electric field in a humid environment. Metal at the anode dissolves and the resulting metal ions (cat ions) migrate towards the cathode where they are reduced, forming a dendrite structure that grows back towards the anode, ultimately risking short circuit. Even when this does not occur, the electrochemical cell that is formed between the conductors will exhibit reduced surface insulation resistance. Both phenomena are potentially detrimental to circuit integrity, especially fine-pitch. In particular, reliability concerns result
No-clean SN100C paste—before cleaning No-clean SN100C paste—after cleaning Figure 3. No-clean solder paste test boards before and after cleaning.
from highly active organic acid and/or halide/halogenated flux residues trapped under low stand-off components and not removed during post-soldering cleaning.
12 – Global SMT & Packaging – Celebrating 10 Years – December 2010
www.globalsmt.net
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