After nearly 20 years of rapid development, China SMT from small to large, has now become the world's largest SMT application countries. Now SMT not only takes cell phone products, consumer electronic products, PC peripheral products as the leading production, but also automobile electronics, medical equipment and telecommunication devices, and more military products are put into production.
At the beginning only the most simple devices to now can mount a variety of packages of resistors and capacitors, various sizes of BGA, chips and integrated modules, etc.. Of course, the high-end SMT technology is gradually refined with the integration of materials and package changes, so the maturity of SMT technology is established on the basis of high-quality materials, the two are complementary to each other. In summary, material inspection is essential for SMT welding technology, is an essential process in the production, the following China SMT factory SysPCB talk about material inspection from several aspects.
First, the PCB
PCB is the overall substrate of the product, the quality of PCB directly affects the quality of welding and the product's range of application, life. The following we use visual inspection with five aspects of PCB, warping and twisting, solderability, appearance, gold fingers and special materials, using calipers and other tools to introduce PCB pre-solder inspection. Need to emphasize a premise, as long as the SMT materials related to the contact. SMT factory workers are required to bring anti-static gloves and wristbands to avoid damage to the device, or contamination of the PCB etc.
1, warping, twisting
PCB warping, twisting for many reasons, in addition to the design of the reasons, may be due to the preservation of a humid environment or placed in a position that does not meet the horizontal requirements, according to the provisions of the acceptable range should be controlled in the diagonal length of the PCB board 0. 5% or less, of course, for the complexity of the board the range should also have a floating margin, for example, the PCB with the large number of BGA, high integration, the board warpage should be controlled more strictly, the same if the PCB only with some small chips and resistive parts, low integration, the range can be appropriately relaxed. At present, the electronic assembly plant allows the warpage, regardless of double-sided or multi-layer. 1.6mm thickness, usually 0.70 ~ 0.75%, many SMT, BGA boards, the requirement is 0.5%.
Some electronic factories are encouraging to raise the warpage standard to 0.3%, and the method of testing warpage follows GB4677.5-84 or IPC-TM-6126.96.36.199B. The warpage of the board can be calculated by putting the board on a certified platform and inserting a test pin into the area with the greatest warpage degree.
PCB pads in the air for a long time exposure is easy to be oxidized, if the pads are oxidized after continuing to weld will cause poor pad wetting resulting in a series of problems such as pseudo soldering, so the PCB must be solderability tested before welding. Detection methods generally use visual inspection, for the generation of suspicion PCB in edge dipping test. Visual inspection can be directly concerned about the brightness of the pad.
General tinned pads or gold-plated pads are oxidized to appear darker; for the indeterminate can be swabbed with an eraser on a certain part, with the previous comparison is also a simple means of detecting PCB oxidation.
There is also flash gold this special pads, flash gold plate color compared with the normal color of gold-plated board is much lighter, this pad plating layer is very thin, Au layer of the thinnest place only 0.05um, Ni layer is easy to oxidation, very bad solderability, resulting in many welding defects.
SMT Processing plant encountered this PCB, incoming inspection must be carried out before mounting, can not be exposed to air for a long time, open the vacuum packaging immediately start SMT, otherwise adhere to the principle of no mounting no open.
PCB appearance for the direct sale of finished PCB customers is very important, of course, may also have a direct impact on the function of the product, so the appearance of the damage can simply be divided into the following two cases by visual inspection.
(1), only affect the appearance of the board, does not have an impact on the use of the board
① Knock corner, ② Halo ring
③ friction injury ④ scratches (no damage to the solder resist layer and no obvious depth)
⑤ exposed copper (no obvious depth, to ensure that the wire does not have any damage, you can fill the solder resist oil)
The above five cases such as customers do not sell the bare board or appearance of special requirements can continue to use, does not affect the performance of the product itself, but if the customer will be sold directly to the finished product, the above situation is not acceptable.
(2) changes in appearance may lead to the destruction of the PCB as a whole
① blistering / delamination (will affect the PCB internal circuitry)
② scratches (broken ring solder resist layer, there is a certain depth, may cause cuts to the PCB lines)
encounter these two irreparable problems, you can directly request a replacement PCB, because the consequences of these two situations are unknown and not can be used blindly.
4, gold fingers
Gold finger for the PCB is also relatively special, it is the connection pins of PCB and other devices such as motherboards, chassis and other electrical, so its quality is very important for the entire product, so the incoming inspection should be relatively strict. General inspection needs to pay attention to the following aspects.
1), scratches or dents in the middle 3/5 region of the gold finger, mainly in deeper wounds, resulting in copper leakage, depression area of more than 6mil, or a whole row of gold fingers on more than 30% of the pressure wound
2), Gold finger oxidation, mainly in the color darkening or reddening.
3), Plating stripping, need to do peel test after the plating stripping or buckling.
4), Gold finger contamination, gold finger
tin, paint, glue or other contaminants.
5), Gold fingers not beveled or beveled edge length is not good
6), gold finger cutting edge bad, manifested in the glass bundle protruding, missing corners, cutting edge is not straight, there are hairy head. If the above problems, we need to promptly feedback with the customer, requesting replacement of PCB.