PCBA Tech - SMT on the mixed assembly of lead and lead-free BGA

1 Introduction

The SMT lead-free process has been widely used, but the leaded process is still used in the military electronics manufacturing field, but the components cannot be bought with lead. There is a phenomenon of coexistence of lead and lead-free. At present, I have all lead/lead-free BGA devices are commonly used in joint assembly, and because the melting point of lead-free solder balls is different from that of lead-free solder balls, for example, the melting point of lead-free BGA using Sn-Ag-Cu alloy is 217 degree Celsius higher, while Sn63-37Pb The melting point of the solder ball of the alloy leaded BGA is 183 degree Celsius. If the temperature curve of Sn63-37Pb solder is used, the peak temperature is generally 210～230 degree Celsius. Assuming that the peak temperature of a PBGA is 220 degree Celsius, when the temperature rises to The Sn-37Pb solder paste printed on the pad starts to melt at 183 degree Celsius. At this time, the Sn-Ag-Cu solder balls of the lead-free PBGA have not melted; when the temperature rises to 220 degree Celsius, the temperature will start to drop according to the lead process The soldering is over, and the lead-free solder balls have just melted. Although the nominal melting point of the Sn-Ag-Cu alloy is 217 degree Celsius, in fact, the Sn-Ag-Cu alloy is not a true eutectic alloy. The temperature range of the liquidus line is 216～220 degree Celsius. Therefore, the temperature at the end of the lead process cooling and solidification is exactly when the lead-free Sn-Ag-Cu solder ball is just melted, and it is in a paste state where both solid and liquid phases coexist. When the solder ball melts, due to the gravity of the device, the solder ball begins to sink. During the sinking of the device, there is a slight vibration or slight deformation of the PCB, which destroys the original solder interface structure on the side of the PBGA component, and no new interface metal can be formed. The inter-alloy layer is likely to cause failure of the PBGA and one side of the solder joint. From the above, it can be seen that the temperature characteristics of the two solders should be taken into account in the lead/lead-free mixed assembly process, and the small process window is more difficult.

To obtain reliable welding products in mixed assembly, you should not only focus on the assembly and welding links, but should start from the front, middle and back of the assembly to strengthen the entire process control, treat them differently according to the specific situation, and timely modulate the assembly production plan to make it more Pertinence, so that reliable products can be obtained. The following takes a process test as an example, and respectively lists the points that should be paid attention to to strengthen control in each link.

2 Process test

2.1 Introduction to the test

There are two situations for the mixed assembly of lead/lead-free BGA. One is that the number of lead-free devices is the majority, or there are large-size lead-free BGA devices, and the compatible soldering process curve is used for soldering, which is the lead-free process curve. Reasonably increase the temperature on the basis of, so that the peak temperature is controlled within the range of 230-235 degree Celsius, so that the lead-free BGA device is not damaged while meeting the reflow temperature required for the lead-free BGA device, so as to achieve better soldering; the second is the number of leaded devices In most cases, and the lead-free BGA device is small in size, the lead-free device can be converted into a leaded device through the ball planting process, and then the leaded process curve can be used for soldering.

In this experiment, two kinds of test boards are used: PCB 2, PCB4, the size of the test board is 100*150 mm, the device uses Sn63-37Pb tin-lead and Sn-Ag-Cu solder ball material lead-free PBGA dummy sheet, the PCB thickness is 1.6 mm.

2.1.1 Selection of test board

2.1.2 Device situation

PCB2: D2, D3, D5 are lead-free dummy sheets, and D1 and D6 are lead-free dummy sheets for soldering after ball planting.

PCB4: D2, D3, D7 are lead-free dummy sheets, D1, D5, D6, and D8 are lead-free dummy sheets.

2.2 Preparation before assembly

The printed board and BGA device itself should be inspected before assembly. The specific inspection contents are as follows.

(1) Printed board: There should be no obvious warpage on the board surface; there should be no short circuit or open circuit on the pad; there should be no characters, solder mask and other pollution on the pad; for serious oxidation, poor processing quality, and serious surface pollution And printed circuit boards with other quality problems cannot be assembled and treated as unqualified products (as shown in Figure 2); for printed circuit boards with dirty surfaces, squeeze dry with absorbent cotton balls dipped in absolute ethanol before soldering After cleaning 2~3 times to ensure that the surface of the printed circuit board is clean and clean before soldering.

(2) BGA components: The BGA components to be assembled are distinguished from lead/lead-free, and the BGA solder balls are checked for oxidation and defects.

2.3 Assembling

(1) In order to eliminate the adverse effects caused by the evaporation of moisture in the welding high temperature, it is necessary to pre-bake the BGA devices and boards. The specific method is: put the BGA device in an oven at 120 degree Celsius for 48 h; printed board 110 degree Celsius, 4 h.

(2) When printing solder paste, the solder paste should cover more than 75% of the area of the pad, and the surface of the solder paste should be smooth, uniform, without voids, and not connected to short-circuit the adjacent pads, and not stick to the substrate around the pads. And the waiting time between the printing of the solder paste and the reflow soldering is controlled within 2 h.

(3) Set welding curve

It should be noted that the welding process curve parameters listed below are obtained during the test of the printed board used in this test. In production, it should be based on the actual board situation: such as board size, number of layers, device Types, number of components, distribution, etc. are considered comprehensively to adjust and set the welding curve.

For iPCB, the number of lead-free BGA devices on the board is more than the number of leaded devices, so consider setting the compatibility curve for soldering, and determine the parameters after multiple tests and adjustments to the curve