Bad solder joints that require touch-up are a complex subject. First of all, we must judge it is caused by poor design, bad soldering technique, bad soldering materials, improper pretreatment or unsuitable equipment. In addition, technical and inspection standards often lead to unnecessary touch-up, but they are not included in our discussion since the soldering operation and quality standards required by each electronic industry are different, A lot of solder joints that are considered bad, in fact, are actually good. However, there are too many widely recognized inspection standards, which wrongly emphasize the beauty of solder joints and ignore their functions, thus resulting in a huge and unreasonable touchup cost in this industry. Remember that touch-up does not always improve quality.
Here, we assume that there is no problem with PCB's design, soldering materials selected and pre-treatment before soldering and only discuss technical problems during the soldering process. Special problems with soldering and suggested solutions will be discussed in this course. While many soldering problems may recur, problems faced by each electronics company are still not exactly the same, so there will be no so-called Standard Answer. Here we provide years of experience for customers' reference, but users still have to treat individual problems appropriately.
Trouble-shouting outline When problem occurs, the first thing that must be checked is the basic conditions of the manufacturing process. We summarize them as the following three factors.
1. 1 Bad materials
These materials include such chemicals for soldering as flux, oil, tin, cleaning materials, and PCB cladding materials such as anti-oxidation resin, temporary or permanent solder mask and printing ink.
1.2 Bad solder joints
This involves all solder joints’ surfaces, such as components (including surface-bonded parts / SMT parts), PCBs and electroplated PTHs, etc. must be taken into consideration.
1.3 Improper equipment
These include improper machines, equipment and maintenance and such external factors as temperature, conveyor belt speeds and angles, as well as the depths of immersion and so on which are variables directly related to machine. In addition, ventilation, air pressure, voltage and more factors must be analyzed. Every problem is different in its own way and should not be lumped under one head. Following is a series of standard inspection steps which can help you find out the root cause.
Step 1: When soldering, the smallest variable should be machines, so the first thing is to check them. In order to realize the correctness of your check, independent electronic instruments can be used as auxiliaries such as thermometers to detect temperatures and multi-meters to calibrate the parameters accurately. Try to find out the most suitable working conditions from the actual operations and records. Note: in any case, do not depend on adjusting the equipment to overcome temporary soldering problems because such adjustments may lead to bigger problems.
Step 2: Check all the soldering materials, such as flux's specific gravity, transparency, color, ion content and purity of tin-lead alloy. This is a continuous work accompanied by both regular inspection and random sampling. All these are helpful to ensure their quality.
Step 3: Poor solder joints of PCBs and components are the biggest factor causing soldering problems. To study the soldering problem of PCB, we must first fix or isolate the other variables which may occur, and then discuss them one by one. For example, if soldering defects occur on pins, other variables should be locked first, and only those pins with soldering defects can be thoroughly compared and analyzed. Through this way of tracking, the source of problem will be clear soon.
Step 4: Check the quality of the PTHs, punching, drilling and other defects. We may use
amplifying equipment to see if the PTH surface is smooth, clean or has any other
impurities or breaks or the thickness of electroplated layer is standard or not. In the
process of tracing soldering problems, the principle and concept should be correct. In
addition, steps are very important. How to find out the problem effectively by comparison
and analysis is the biggest problem for electronic engineers.
