Below is a classification of the sampling inspection methods for SMT chip processing:

　　Many unique sampling inspection plans and systems have been formed, which can be roughly classified according to the following aspects.

　 　1. Classification of quality indicators according to SMT production characteristics

　　The characteristic quantity used to measure product quality is called the product quality indicator. Quality indicators can be divided into two categories according to their measurement characteristics: measurement indicators and counting indicators. Measurement indicators refer to quantitative data indicators such as material purity, workpiece size, steel chemical composition, and product life. Counting indicators can also be divided into two categories: piece counting indicators and counting indicators. The former is measured by the number of non-conforming products, while the latter refers to the number of defects in a product, such as the number of appearance defects per square meter of fabric, and the number of bubbles and sand holes in a casting.

　　According to the classification of quality indicators, the sampling inspection methods for SMT product quality inspection are also divided into two types: sampling inspection and measurement sampling.

　　1. Measurement sampling method is a method to extract a certain number of samples from a batch of products, inspect the quality of each sample, and then compare it with the specified standard value or technical requirements to determine whether the batch of products is qualified.

　　2. Counting sampling method is a method to extract a certain number of samples from a batch of products, inspect the quality of each sample in the sample, determine whether it is qualified or not, and then count the number of qualified products and compare it with the specified "qualified judgment number" to determine whether the batch of products is qualified.

　　Sometimes, the counting sampling inspection method and the quantity sampling inspection method can be used in combination. For example, select one or a few quality parameters of the product for measurement sampling, and use the counting sampling method for the rest of the quality parameters to reduce the amount of calculation and obtain the required quality information.

　　2. Classification by sampling method type

　　Sampling inspection methods can be divided into two categories: adjustment type and non-adjustment type.

　　The rescue type is to connect several different sampling schemes to the transfer end, forming a complete sampling inspection system. Then, according to the quality situation of each batch of products, the sampling inspection scheme is adjusted according to the transfer end, and the conversion of normal, strict, or relaxed sampling inspection schemes is carried out. ISO2859, ISO3951, and GB2828 standards are examples of this. The rescue type sampling inspection method is used for quality inspection of continuous batches and consistent quality products.

　　Non-salvage single sampling search scheme does not consider the quality history of product batches, and there is no transfer at the terminal, so it is easy for quality inspectors to control, but it is only suitable for the quality inspection of Gua Ao batches.

　　Regardless of the sampling method, they all have the following three common characteristics:

　　1. Products must appear in the form of an "inspection batch" (referred to as "batch"). Inspection batches are divided into continuous batches and isolated batches. Continuous batches refer to batches that are closely related to product quality or are continuously produced and continuously submitted for acceptance. For example:

　　Product design, structure, process, and materials have not changed;

　　The manufacturing location has not changed;

　　The intermediate shutdown time should not exceed one month.

　　Individual submitted inspection batches or batches to be delivered cannot use the quality information provided by the most recently inspected batch. This batch is called an isolated batch.

　　2. A qualified batch does not mean that every product in the batch is qualified, and a non-qualified batch does not mean that every product in the batch is not qualified. Sampling inspection only guarantees the overall quality of the product, not the quality of each product. That is to say, there may be two "errors" or "risks" in sampling inspection. One is the error of misjudging a qualified batch as a non-qualified batch, also known as "producer risk", often denoted as α, and the α value is generally controlled at 1%, 5%, or 10%. The other is to misjudge a non-qualified batch as a qualified batch, also known as "user risk", often denoted as β, and β is generally controlled at 5% or 10%.

　　3. The non-conforming rate of SMT samples is not equal to the non-conforming rate of the submitted batch.

　　Samples are randomly selected from the submitted batch. Random sampling means that each time a sample is taken, all unit products in the batch have an equal chance of being selected, and are not affected by anyone's will. The sample extraction time can be during the batching process or after the batching. The random sample size can be determined according to the random number table, or according to standards such as GB10111.

　 　3. Classification by inspection quantity

　　According to the number of sampling inspections, it can be divided into single sampling inspection, double sampling inspection, multiple sampling inspection, and sequential sampling inspection methods.

　　1. Single sampling method. This method is simple and only needs to sample one sample to determine whether the batch of products is qualified.

　　2. Double sampling method is to take a batch of samples for inspection. If the batch of products can be judged as qualified or not, the inspection is terminated. If it cannot be judged, another batch of samples needs to be taken for re-inspection to determine whether it is qualified.

　　3. Multiple sampling method has the same principle as the double sampling method, and the sample size of each sampling is the same, i.e., n1=n2=n3…=n7. However, the number of sampling inspections is large, and the number of qualified judgment and non-qualified judgment is also large. The ISO2859 standard specifies seven sampling inspection schemes. China's GB2828 and GB2829 implement five sampling inspection schemes.

　　4. Sequential sampling method is equivalent to the limit of multiple sampling methods. Each time, only one unit SMT product is randomly sampled for inspection. When the batch of products can be determined to be qualified or unqualified, the inspection is terminated.