Soldering SIMS 4 chips: technology, tools and cautions

If after replacing the control unit SIMATIC S7-1200 or S7-1500 (often referred to as SIMs 4 in the documentation) the car gives an error 0x8123 (loss of connection with the module), the problem in 80% of cases lies in the cold soldering of the microcircuit EMMC or power controller. This defect does not appear immediately: the symptoms are periodic freezing of the diagnostic scanner, loss of signal from the sensors, or complete failure of the unit after heating under the hood. Soldering a microcircuit requires precise adherence to temperature conditions (maximum 260°C for BGA chips) and the use of lead-free solder with the addition of silver, otherwise the risk of repeated detachment of contacts reaches 95%.

Unlike soldering connectors or wires, working with SIMs 4 chips involves the use infrared soldering station (for example, Quicko TH-301) or a hot air dryer with an accuracy of ±5°C. A regular 60 W soldering iron is useless here: it will not provide uniform heating of all 256 contacts of the BGA package. In addition, before soldering it is mandatory ultrasonic cleaning boards from old flux residues - even microscopic particles can cause a short circuit after installing the chip.

1. Diagnosis of fault before soldering

Before you take up the soldering iron, make sure that the problem is in the microcircuit and not in the firmware or mechanical damage to the board. Typical symptoms of malfunction SIMs 4:

• 🔴 Error 0x8123 or 0xA400 when scanning VCDS/ODIS.
• 🔴 The control unit reboots when heating above 60°C (for example, after 20 minutes of engine operation).
• 🔴 There is no connection with the CAN bus, although there is power on the unit (checked with a multimeter on the contacts 30 and 15).
• 🔴 Cracks on the soldering of the microcircuit or blackening around it are visually noticeable.

For an accurate diagnosis:

1. Connect the oscilloscope to the line CLK microcircuits (pin 17 on S7-1200). If the signal is intermittent, the problem is in the contacts.
2. Ring the power circuit (VCC and GND) for a break: the resistance should be no more than 0.5 Ohm.
3. Check the integrity of the tracks under the chip using a ×10 magnifying glass: they are often damaged during previous repair attempts.

⚠️ Attention: If the control unit has been previously repaired and traces of overheating are visible on the board (darkened areas of FR-4), soldering may not help - the entire board will need to be replaced. Microcircuits SIMs 4 sensitive to repeated heating: after 3–4 soldering cycles, their crystal degrades.

2. Tools and materials: what you need

For high-quality soldering of microcircuits SIMATIC S7-1200/S7-1500 specialized equipment will be required. Saving on tools will lead to chip peeling again in 1–3 months. Minimum set:

Tool/material	Requirements	Model example
Soldering station	Infrared or hot air, temperature adjustable up to 300°C	Quicko TH-301, Aoyue 968A+
Flux	No-clean, for BGA (for example, Amtech NC-559-V2)	RMA-223, Kester 951
Solder	Lead-free, with the addition of silver (Sn96.5/Ag3.0/Cu0.5), diameter 0.3 mm	Kester EP256
Soldering stencil	For uniform application of solder on BGA contacts	Laser, thickness 0.1 mm
Ultrasonic bath	To clean the board from flux and oxides	Branson 2510

Additionally you will need:

• 🔧 Tweezers with antistatic coating (for example, ESD-12).
• 🔧 Magnifier with backlight (magnification ×10–×20) for monitoring the quality of soldering.
• 🔧 Thermocouple for measuring the temperature on the board (error no more than ±2°C).
• 🔧 Kapton electrical tape to protect adjacent components from overheating.

3. Preparing the board and microcircuit for soldering

The mistake most beginners make is trying to solder the chip SIMs 4 without prior preparation. This leads to cold contacts and repeated failure of the unit. Step by step process:

1. Removing the old microcircuit:
   • Cover adjacent components foil or Kapton tape.
   • Heat the board with a hairdryer to 180°C (chip surface temperature).
   • After the solder has softened, carefully pry the chip from four sides with tweezers.
2. Cleaning pads:
   • Remove solder residue braid for soldering with flux.
   • Treat the sites alcohol solution (isopropyl alcohol 99%).
   • Check the integrity of the tracks under a microscope.
3. Preparing a new chip:
   • Apply flux to the chip contacts brush.
   • If you are using BGA balls, place them on the stencil and heat them to 220°C to fix them.

⚠️ Attention: Do not use acetone to clean circuit boards SIMATIC - it destroys the protective coating of the paths. Also avoid mechanical cleaning with metal tools: even micro-scratches on the contact pads will lead to unreliable soldering.

Remove old braided solder|Clean the board with ultrasound (5 minutes in alcohol)|Apply flux to the contacts of the microcircuit|Check the stencil for compliance with the BGA pitch (0.8 mm)|Fix the board in the holder to prevent displacement-->

4. Soldering technology: step-by-step instructions

Soldering the microcircuit SIMs 4 requires strict adherence to the temperature profile. A deviation of even 10°C can result in overheating of the die or insufficient solder fusion. Optimal profile:

Stage	Temperature, °C	Time, sec	Notes
Preheat	150–180	60–90	Uniform heating of the entire board
Flux activation	180–200	30	The flux should spread evenly
Melting solder	240–260	20–30	Maximum temperature for BGA
Cooling	Reduction to 100	120	Natural cooling, without blowing

Step by step process:

1. Secure the board in the holder to prevent movement.
2. Apply flux to the contact pads of the board and microcircuit.
3. Place the chip in place, aligning the marks (usually a triangle or dot on the body).
4. Heat the board according to the profile (see table). To control, use a thermocouple attached to the edge of the chip.
5. After cooling, check the soldering x-ray (if possible) or a multimeter in dial mode.

Critical error: Do not allow overheating above 260°C - this will destroy the internal connections of the microcircuit. If the solder does not melt, add flux and repeat the heating rather than increasing the temperature.

Soldering hair dryer|Infrared station|Soldering iron with nozzle|I send it to service-->

5. Soldering quality check and testing

Even if the soldering visually looks good, this does not guarantee the functionality of the unit. SIMs 4 sensitive to microcracks that are not visible without magnifying devices. Test methods:

• 🔍 Visual inspection: All contacts should be shiny, without voids. Dull solder color indicates cold soldering.
• 📊 Testing with a multimeter: Check the resistance between VCC and GND - it must be at least 10 kOhm (a lower value indicates a short circuit).
• 🔬 Microscopic analysis: At ×20 magnification there should be no visible gaps between the BGA balls and the pads.
• 🚗 Functional test: Connect the unit to the car and check for errors 0x8123, 0xA400 within 30 minutes of engine operation.

If the unit is unstable after soldering:

• Check track integrity under the microcircuit - they are often damaged during dismantling.
• Make sure all capacitors around the microcircuit are not swollen (especially C1206 and C1207).
• Repeat soldering with additional flux, if cold contacts are detected.

What should I do if the unit does not turn on after soldering?

1. Check availability 5V at pin 32 microcircuits (core power supply).
2. Make sure quartz resonator (usually 16 MHz) is not damaged.
3. Ring the chain RESET (pin 14) to a break.
4. If everything is in order, but the unit does not respond, the microcircuit could be damaged by static electricity. Try replacing it with a known good one.

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6. Common mistakes and how to avoid them

Around 70% of unsuccessful repairs SIMs 4 associated with repeated errors. Here are the most common ones

Error	Consequences	How to avoid
Using Lead Solder	Contact corrosion after 3–6 months	Apply only Sn-Ag-Cu (SAC305)
Heating over 270°C	Detachment of the crystal inside the chip	Control the temperature with a thermocouple
Insufficient board cleaning	Short circuit due to flux residue	Use an ultrasonic bath with isopropyl alcohol
Displacement of the microcircuit during soldering	Closing adjacent contacts	Fix the board in the holder and use the stencil

Another common problem is ignoring static electricity. Microcircuits SIMATIC sensitive to discharges even at 10 V. Always use antistatic wrist strap and work on a grounded table.

⚠️ Attention: If after soldering the unit works, but after 1–2 weeks the error returns, the reason is microcracks due to uneven heating. The solution is re-soldering with pre-heating the board to 120°C for 5 minutes.

7. Alternative repair methods

If soldering doesn't work or you don't have the necessary equipment, consider alternatives:

• 🔧 Replacement of the entire board. Suitable for blocks S7-1200, where the chip is soldered to a removable module. The cost of a new board is from 8,000 ₽.
• 🔧 Software recovery. Sometimes it's a mistake 0x8123 caused by a firmware failure, not a hardware failure. For this you need J2534 programmer and the original dump.
• 🔧 Bypass module. For blocks S7-1500 There are emulator boards that are connected instead of a faulty microcircuit. Cost - from 12,000 ₽.

If you decide to send the unit for service, choose a workshop with equipment for X-ray inspection of soldering. Without it, it is impossible to guarantee the quality of repair of BGA chips.

Frequently Asked Questions

Is it possible to solder a SIMs 4 chip with a regular soldering iron?

No. A regular soldering iron will not provide uniform heating of all 256 contacts of the BGA package. For high-quality soldering you need hot air dryer or infrared station with temperature control. When using a soldering iron, the risk of cold contacts exceeds 90%.

What soldering temperature is safe for SIMs 4?

Maximum temperature - 260°C. Optimal profile:

• Preheat: 150°C (60 sec).
• Soldering: 240–260°C (20–30 sec).
• Cooling: natural, without blowing.

Exceeding 270°C destroys the internal connections of the chip.

How to clean the board after soldering?

Use isopropyl alcohol 99% and ultrasonic bath. Acetone or acetone-based solvents will damage the protective coating of the tracks. After cleaning, dry the board with compressed air (no heat!).

How to check that the chip is soldered correctly?

Test methods:

1. Visual inspection under a ×20 magnifying glass: all contacts should be shiny, without voids.
2. Continuity test with a multimeter: resistance between VCC and GND - at least 10 kOhm.
3. Load test: Connect the unit to the car and check for errors for 30 minutes.

If you have an X-ray machine, take a picture to check the internal connections.

How much does it cost to repair SIMs 4 at the service?

The cost depends on the method:

• Soldering a microcircuit: 5,000–10,000 RUR.
• Board replacement: RUB 8,000–15,000.
• Software recovery: RUB 3,000–6,000.

On average, repairs cost less than buying a new unit (from RUB 20,000 for S7-1200).